# List of Ph.D. Theses

 Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Mr. Archan Mukhopadhyay Sagar Chakraborty14109877 Nonconvergent outcomes in the evolutionary game theory: A case study of replicator dynamics 08th October 2021 (Friday) 11:00 AMOnline, Zoom Meetinghttps://iitk-ac-in.zoom.us/j/95670659204?pwd=R1hQemMrNmhYUk1XSE5sMGNGclpkdz09Meeting ID: 956 7065 9204Passcode: @PhD8 The strategic aspects of the game theory proves to be useful in modeling evolutionary conflicts. Thus, it is no surprise that the evolutionary game theory has gained popularity over time as a successful way to model the Darwinian evolution. Among the plethora of models, that capture the Darwinian selection, the replicator equation is, undoubtedly, the most diversely used model that has found successful applications in different fields like biology, chemistry, physics, sociology, economics, etc. The beauty, this model carries, is that the folk theorems of the evolutionary game theory connect the convergent outcomes of this dynamic to the inbuilt solution concept of noncooperative game theory like the Nash equilibrium. Additionally, an evolutionarily stable state, a refinement of the Nash equilibrium, turns out to be a stable fixed point of this dynamics. This provides a way for biologists to predict the dynamical outcomes just by studying the underlying payoff matrix that mimics the corresponding evolutionary conflict under study. However, this picture is not always rosy. The non-linearity hidden in such dynamics may pave a way for complex dynamical outcomes when one includes the complexities like more strategies, mutation, delay, etc. Additionally, it is known that for the time-discrete replicator dynamics, chaos may appear even for the simplest case of two-player--two-strategy games. However, the relevant literature of the discrete replicator dynamics is less rich. Similarly, a sound connection of the noncooperative game theory with the periodic orbits and chaos is rather unexplored. Moreover, the microscopic birth-death process that the discrete replicator dynamics captures, is also a relatively less studied problem. In the talk, we aim to address the above-mentioned issues. We will start with a discussion about how the complexities, like mutation and delay, are capable of inducing a limit cycle following a Hopf bifurcation, in continuous replicator dynamics for two-player--two-strategy games. This is important because otherwise one has to go in the four-strategy space for witnessing a complex dynamical outcome. Subsequently, we will discuss the dynamical attributes of the two commonly used versions of the replicator map, viz., the type I replicator map and the type II replicator map. It is interesting how the type I replicator map can lead to chaos in the simplest case of two-player--two-strategy games that the type II replicator map is unable to. The ability of the selection strength to govern the emergence of chaos is very fascinating. Subsequently, we will define new dynamic equilibrium concepts, namely, heterogeneity stable orbit (HSO) and heterogeneity orbit (HO) that are the extensions of the evolutionarily stable state and the Nash equilibrium state respectively. Using these new concepts of HSO and HO, we will decipher the game-theoretic meaning of periodic orbit and chaos while using the type I replicator map as a prototype model. Next, we will describe a selection-driven Wright-Fisher process that rightly captures the underlying phenomenon hidden in the discrete replicator dynamics. Finally, we will conclude with the main results of this talk and the possible future directions of research. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Ravinder Kumar Zakir Hossain Strain and interface-driven magnetic properties of epitaxial insulating iron garnet thin films 30th July 2021 (Friday) 11:00 AMOnline, Zoom Meetinghttps://iitk-ac-in.zoom.us/j/97165257894?pwd=Znhud3VCMDh2YzhXdlRuVHg3WU1SUT09 The development of spintronics into a viable technology requires novel magnetic materials and Insulating iron garnets (ferrimagnetic insulator) are one such class due to its extremely low precessional damping, high curie temperature and magneto-optical (MO) activity. Coupling of insulating iron garnets with heavy metals gives birth to diverse spin-dependent phenomena such as spin pumping (SP), spin Seebeck effect (SSE), spin hall magnetoresistance (SMR), photo-spin-voltaic (PSV) effect etc. Significant enhancement of MO activity and magnetocrystalline anisotropy (MCA) is also possible with Bismuth or Cerium-doping.The thin-film fabrication has been proven as a backbone for the miniaturization of modern device technology. Also, many of the inherent bulk properties vary significantly in thin-film media due to different film thicknesses, growth-induced strains, crystallographic orientation and film-substrate reactions at the interface. In this seminar, we shall discuss a few intriguing growth-induced phenomena in pure and Bismuth-doped yttrium iron garnet (YIG) insulating thin films. The epitaxial crystallization of YIG thin films requires post-deposition annealing. Two different post-deposition annealing protocols were employed to realize the epitaxial conversion. Thin films prepared using both the protocols are highly epitaxial and show excellent figure of merit for saturation magnetization. Our study suggests that optimization of post-deposition annealing protocols is necessary to achieve high quality thin epitaxial films with improved structural and magnetic properties of this technologically important material.Bismuth-doped YIG thin films known for large MO-activity with low losses stands unexplored for its magnetization dynamics. Radiofrequency magnetization dynamic was performed using cavity and broadband ferromagnetic resonance technique. We demonstrate a controlled tuning of magnetocrystalline anisotropy (MCA) using growth-induced strain.Interestingly, the enhancement in magnetoelastic coupling (MEC) doesn’t strongly affect the precessional damping. Coexistence of two mutually exclusive properties i.e. low precessional damping and strong MEC, combined with large MO activity and MCA may help BiYIG thin films to emerge as a possible material platform for photo-magnonics and other spintronics applications.Finally, we will discuss our finding of room temperature positive exchange-bias (EB) and hysteresis loop inversion in an epitaxially grown monolithic YIG thin film which is attributed to antiferromagnetic exchange coupling between growth-induced thin interfacial layer and bulk YIG layer.We observe a critical field value (~600 Oe) mandatory for hysteresis loop inversion. The addition of features like an unconventional room temperature positive EB and hysteresis loop inversion in a monolithic YIG thin film enhances its application potential further for modern approaches to spintronics. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Swayamshree Patra Prof. Debashish Chowdhury RULERS, TIMERS AND TRANSPORT FOR LENGTH CONTROL IN LONG CELL PROTRUSIONS: A CASE STUDY WITH EUKARYOTIC FLAGELLUM 13 July 2021 (Tuesday) 11:00 AMOnline platform Zoom A living cell uses its long cylindrical appendages for locomotion and sensory purposes. Hence, assembling and maintaining a protrusion of correct length is crucial for its survival and overall performance. Usually the protrusions lack the machinery for the synthesis of building blocks and imports them from the cell body. What are the unique features of the transport logistics which facilitate the exchange of these building blocks between the cell and the protrusion? What kind of ‘rulers’ and ‘timers’ does the cell use for constructing its appendages of correct length on time? How frequently do the fluctuations drive the length of these dynamic protrusions out of the acceptable bounds? How do the multiple appendages coordinate and communicate among themselves during different stages of their existence? I address some of these questions in the context of a specific cell appendage called eukaryotic flagellum (also called cilium). Motivated by real experiments, we frame minimal models for capturing these wide range of interesting phenomena in the context of flagellar length control. We analyse our models by using theoretical techniques from non equilibrium statistical mechanics and stochastic processes. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Mr. Rohitashwa Chattopadhyay Prof. Sagar Chakraborty14109271 A Hamiltonian approach to the van der Pol oscillator: Perturbation techniques and the Hannay angle June 28, 2021 (Friday) 10:30 a.mOnline platform (Zoom link: https://zoom.us/j/97445142037?pwd=UjlTNWNTRUdGOENGOENYa0VGMlUyQT09) One of the most paradigmatic nonlinear dynamical systems is the van der Pol oscillator that finds extensive applications in modeling a plethora of physical systems. An exact analytical solution of the nonlinear differential equation governing the van der Pol oscillator dynamics is elusive. Thus, to find its approximate solution analytically, one has to resort to the perturbation methods. Also, the van der Pol oscillator is a dissipative system, thereby making the formulation of a global time-independent Hamiltonian a challenging endeavour. In this talk, we plan to present a generalization of the equivalent linearization technique---a popular perturbation method---to find the frequency and amplitude corrections up to any order of nonlinearity. This overcomes the hitherto perceived barrier that the equivalent linearization technique could find frequency and amplitude corrections only up to the first order of nonlinearity. The method developed is very general, and we illustrate the technique using conservative anharmonic oscillators and the nonconservative van der Pol oscillator. Next, we find a time-independent global Hamiltonian for a dual van der Pol system, which comprises the van der Pol oscillator, suitably extended to increase the system's degrees of freedom by adding an auxiliary equation. We furthermore generalize this formalism to find the Hamiltonian for a general class of Lienard systems. Having formulated the Hamiltonian, we employ the canonical perturbation theory and the Lie transform Hamiltonian perturbation theory (both exclusively applicable to the Hamiltonian systems) for such a system. In the process, we ingeniously bypass the (in)famous problem of small denominators. Obtaining a Hamiltonian also enables us to find the action-angle variable for the van der Pol oscillator perturbatively, which in turn allows us to find the Hannay angle. A more general concept of the geometric phase with properties similar to the Hannay angle could be calculated for the dissipative systems with a limit cycle such as the van der Pol oscillator. We establish an equivalence between the geometric phase and the Hannay angle. Again our formalism is very general and, in principle, can be applied to any Lienard system. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Pankaj Singh Prof. Asima Pradhan10109067 Design and Testing of Spatially Resolved Reflectance Based Fiber Optic Probe, & Development of Frequency Domain Measurement System For Characterizing Epithelial Pre-cancers June 23, 2021 (Wednesday) 04:00 PMOnline platform (Zoom link: https://us02web.zoom.us/j/9974039693?pwd=SDdGMFNJa2tlVDZHL00za1JCanZ6QT09) Cervical cancer is a type of epithelial cancer that is the fourth most common cancer in women. It has no clear symptoms at the precancerous stage and one has to go for regular screening tests. The conventional screening techniques are invasive and time-consuming, hence an efficient, non-invasive and fast substitute is required. Optical properties of tissues, directly correlate with the morphological and biochemical changes that occur with cancer progression. Therefore, optical techniques can be used to extract the signature of abnormality present in the biological tissues. An epithelial tissue (cervix, oral and skin) is considered to be a two-layered medium with ~300 μm thin epithelium layer on top and stroma, the connective tissue underneath. Signatures of pre-cancer can be captured from the epithelium layer only. Besides the critical thickness of the epithelium layer, the another concern is to reach the internal organs such as the cervix and take measurements. Hence, it is a big challenge to recover the optical properties in vivo to characterize the epithelial tissues. Spatially Resolved Reflectance, a well-known optical technique that collects reflectance at various spatial positions, can be used to extract the optical properties of a turbid medium. In this technique, the light reflections need to be collected very close to the position of incidence to recieve photons mainly from the epithelium layer, and a computational model needed for the extraction of the optical parameters. For the collection of required observations, fiber-optic probes can be optimized to comply with our requirements. Monte-Carlo method-based computational study has been carried out to study the efficacy of three common probe geometries for spatially resolved reflectance measurement-based precancer detection of internal as well as external organs. Thus, based on the results obtained, a fiber probe, suitable for spatially resolved reflectance measurement from the cervix, has been designed and fabricated. Analytical methods with some modifications of source term were applied to provide close the source forward solution but this could not meet our purpose. So, a Monte-Carlo simulation model, considered as a gold standard model was modified according to configuration of fibers and utilized to provide the reflectances (forward solution) at very close to the source. Subsequently, a Monte-Carlo look-up table-based method has been developed for the extraction of optical parameters (Inverse solution) from close to the source observations. The fabricated prototype has been employed to carry out steady state spatially resolved reflectance measurements on cervical tissue-mimicking two-layered phantoms and optical parameters of the phantoms have been recovered using the Monte Carlo look-up table. To further increase the efficiency of our technique, a frequency-domain measurement system for optical tomography has been set-up and can be used in combination with the developed fiber probe to record both spatially resolved intensity as well as phase. A preliminary study using our system has been carried out to show the significance of the technique. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Sanjeev Kumar Maurya Prof. Sudeep Bhattacharjee14209862 Studies of noble gas plasma based focused ion beams: guiding, focusing and creation of high aspect ratio microstructures 7th April 2021 (Wednesday) 11:00 AMOnline platform (Zoom link: https://zoom.us/j/5922225867)Meeting ID: 592 222 5867 A focused ion beam (FIB) is an inevitable tool for research and applications in nanoscience and technology. Conventionally, liquid metal ion source (LMIS) based FIB is used for nanofabrication, however, volume milling at micrometer length scales is a major issue due to extremely small beam currents (1 pA – 10 nA). Moreover, embedded Ga ions introduce metallic surface contamination which is undesirable in many applications. Many emerging applications require rapid fabrication and processing such as in biomaterials and semiconductors where non-toxic inert gas focused ion beams are required. In order to serve these requirements, a versatile microwave plasma based multi-element focused ion beam (MEFIB) system has been developed in our laboratory that can deliver ions of a variety of gaseous elements such as Ar, Kr, and Ne of beam size ~600 nm and significantly large beam currents in the range 20 pA - 5 µA. This thesis work has demonstrated that the focused ion beam size can be controlled and reduced from microns to sub-micrometers in size by overcoming the beam space charge effects, and micro-structuring on metallic surfaces with a variety of noble gas ions can be carried out. From these studies, scaling laws have been established between beam parameters (such as beam energy, beam current and ionic mass) and milling aspect ratio. In parallel, guiding of plasma ion beams is studied through micro-glass capillaries which helps in further reduction of beam source size for demagnification, without loss of beam current. The effect of plasma and beam parameters on focal dimensions (focal length and beam size) have also been investigated. n order to achieve the above, microstructures are created on 50 nm Cu thin films, using 26 keV Ne, Ar, and Kr ion beams for 40 µm plasma electrode (PLE) aperture. High aspect ratio (AR) (line width/depth) microstructures in the range 100 – 1000 are obtained, which is significantly larger when compared to conventional Ga focused ion beams of similar energy where AR is found to be ~ 2 – 9 for a single beam scan. A theoretical model is developed to calculate the impact of the focused beam on the target sample by defining a parameter called “current normalized force”, which is the total momentum transferred per unit time and normalized with the beam current. The possibility of employing ions of variable masses allows a huge variation of momentum transferred to the substrate, so that size controlled microstructures with a wide variety of aspect ratio can be fabricated. To further reduce the beam size, guiding of high current density plasma ion beams (J ~600 Am^-2) through micro glass capillary has been demonstrated over a tilt angle of 5 degrees. Self focusing of the beam is observed (maximum beam size compression ~81%) which can be employed to reduce the object (source) size (PLE aperture) for further demagnification. The investigation reveals that inward radial forces of the charges that get smeared on the capillary inner wall dominate over the Coulomb repulsion of the beam’s space charge, and therefore the beam is self focused. Hysteresis in beam current with ion energy is observed and charge dissipation is evaluated theoretically during a hysteresis cycle. Particle in cell simulations are performed to interpret the experimental results. Further, the effect of plasma parameters, such as space potential, Bohm velocity and the associated initial kinetic energy, ion mass; and beam related parameters such as beam energy, plasma and beam limiter electrode aperture sizes, potential applied to electrostatic lenses, on the focal dimensions are investigated by conducting experiments and comparing them with results obtained from beam simulations and theoretical models. A remarkable feature of nonlinear demagnification in plasma based focused ion beams is observed with significant enhancement in the demagnification when the plasma electrode aperture size is reduced to below the Debye length of the plasma. Finally, submicron focusing (beam size ~ 600 nm) is achieved by minimizing the space charge effects and reducing the PLE aperture (source) size. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Amit Kumar Jash Satyajit Banerjee Effect on non-magnetic disorder on the conducting surface states in a three-dimensional Bi2Se3 topological insulator. 1st April 2021 (Thursday) 16:15Online platform (Zoom link: https://zoom.us/j/3623899301)Meeting ID: 362 389 9301 Topological insulators (TI) possesses a topologically protected bulk insulating state with a high conducting surface state. While the effect of time reversal symmetry breaking (TRSB) magnetic defects on TI is well known, the effect of non- magnetic defects on the TI are less clear. In this talk I will address the non-magnetic disorder effects in Bi2Se3 single crystal and thin film. We use bulk Bi2Se3 single crystals possessing Se vacancy defects as a prototype TI material for exploring the effect of nonmagnetic disorder. In this thesis I have developed and used a sensitive non-contact, mutual inductance-based measurement technique to distinguish between topological surface conductivity in the TI and conventional bulk contributions to electrical conductivity in Bi2Se3. At low temperatures topological surface state dominates conduction, while above 70 K, bulk electrical conductivity takes over and it shows thermally activated nature. Thermally activated conductivity relates to activation of charges doped in the bulk by Se vacancies. The high doping leads to an impurity band in the TI which is few meV below the bottom of the bulk conduction band. We model our results as an inhomogeneous TI surface state generated by bulk doping. This inhomogeneity results in an interesting coupling-decoupling phenomenon between the topological surface state in Bi2Se3 single crystal. I will also present my results on the magneto-optical self-field current imaging technique we have developed to image the current flow through the inhomogeneous TI surface state. The Bi2Se3 single crystal at low temperatures, shows uniform 2D topological surface current sheets of thickness 3.6 nm. In the inhomogeneous TI state above 70 K the current partially diverts into the crystal bulk and concomitantly the sheet breaks up into a patchy network of high and low current density regions resulting in the inhomogeneous TI state. We have shown above 70 K an almost critical like behaviour which coinciding with the onset of current distributing through the crystal bulk, the current flow transforms from a uniform 2D sheet current flow into patches of high and low current density. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Ashish Kumar Manoj K. Harbola 13109062 Obtaining exact exchange-correlation potential in density-functional theory: New methods and insights 11th March (Thursday), 2021 2:00 PMA.K. Raychaudhuri Seminar Room (FB382), Physics Department Density-functional theory (DFT) is the most widely used theory of electronic structure, leading to the ground-state electronic density directly bypassing the need to solve the many-electron Schrodinger equation. In its Kohn-Sham (KS) version, the true system is replaced by a system of non-interacting electrons of the same density moving in a local potential. This potential is the sum of the external, the Hartree and the exchange-correlation (XC) potentials, with the latter two calculated as the functional derivatives of the corresponding energy functionals. Of these, the XC functional is known only approximately. However, over the years the many accurate XC energy functionals have been developed. The way to test them is to compare their results with the exact ones, wherever possible. It is in this context that knowing the exact XC potential, wherever possible, is significant. In the past, many different methods have been proposed to construct the KS potential for different systems from their densities. In our work we show that all such density-based methods can be derived from one general algorithm. This then also paves the way of creating a method of one’s choice. As an extreme example of our work, we show how density-to-potential inversion can be carried out using random numbers. Furthermore, we also propose a general penalty method to obtain the exact XC potential for a density. When carrying out density-to-potential inversion, even slight deviation of the density from the exact one can cause the potential to deviate significantly from the exact one. This problem, however, does not arise when wavefunction based methods are employed with an approximate wavefunction. Therefore, lately the latter methods have become popular. In our work we provide an insight into why wavefunction-based methods are more accurate than their density-based counterparts. In the process we also propose how one can obtain accurate ground-state densities starting from approximate wavefunctions. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Shubhajyoti Mohapatra Avinash Singh 13109075 Three-orbital-model investigation of electronic structure, magnetic excitations, and anisotropy effects in strongly spin-orbit coupled iridates and osmates 15th February (Monday), 2021 3:00 PMOnline platform (Zoom link: https://zoom.us/j/91874230998?pwd=S3NueEdkMkVsd1hrMjVnTElCR3dqZz09) Since the discovery of the spin-orbit assisted J_eff=1/2 Mott insulator Sr2IrO4 in 2008, the 5d family of transition-metal oxides, such as iridates and osmates with a variety of two and three dimensional lattices, octahedral sharing geometry and varying degrees of frustration, have been intensively investigated for novel physical phenomena that arise from the combined influence of spin-orbit coupling (SOC), electronic correlations, and structural distortions. As the SOC entangles real space geometry with spin space magnetism, the resulting J = 1/2 and 3/2 spin-orbital isospins are very sensitive to structural distortion and play key role in allowing various anisotropic magnetic interactions such as pseudo-dipolar, Dzyaloshinski-Moriya, Kitaev, and spin-off-diagonal (SOD) interactions. As a result, subtle interplay of these on-site interactions and the electron kinetic energy related to the hopping integrals is expected to drive various exotic quantum phenomena such as superconductivity, topological insulators, spin-liquid state etc. Intensive studies have been undertaken recently in an effort to better understand the interplay of SOC, electronic correlations and structural distortions. In this thesis, material-specific itinerant-electron models are investigated with respect to electronic structures, magnetic ground states, excitations, and magnetic anisotropy effects in square-lattice perovskites (Sr2IrO4, Sr3Ir2O7), honeycomb-lattice systems (Na2IrO3, RuCl3), and cubic perovskite (NaOsO3). Compared to the existing phenomenological spin models, the microscopic three-orbital-model approach with realistic parameters provides a unified description of the electronic and novel magnetic properties exhibited by the strongly spin-orbit coupled systems by simultaneously incorporating: (i) finite U + finite SOC effects, (ii) structural distortion effects, (iii) Hund's coupling, and (iv) intra- and inter-orbital pseudo-spin dynamics involving J=1/2 and 3/2 sectors. More importantly, our non-perturbative approach provides insight into the magnetic anisotropy effects induced by mixing between the J=1/2 and 3/2 sectors, not considered in earlier works. The three-orbital-model approach provides a good account of the experimentally measured magnon energies, anisotropy effects, and high-energy spin-orbit exciton modes in both mono- and bi-layer iridates. Octahedral tilting-induced spin-dependent hoppings and associated anisotropic interactions in iridate heterostructures strongly suppress the magnon gap, driving the system to the verge of isospin reorientation transition. Bond directional anisotropic Kitaev and SOD interactions generated by the J=1/2 and 3/2 mixing stabilize novel magnetic orders in the honeycomb lattice iridates and ruthenium based compounds without Hund's coupling. Despite the nominally orbitally-quenched osmium ions in the 5d^3 compound NaOsO3, the SOC and octahedral rotations play crucial role in explaining electronic band structure, insulating behavior and magnetic anisotropy effects. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Purna Chandra Patra Prof. Y N Mohapatra 12209066 Thin Film Graphitic Carbon Nitride: Suitability for Device Applications 7th January (Thursday), 2021 3:30 PM Zoom online Polymeric graphitic carbon nitride (g-C3N4) is an emerging 2D n-type semiconductor consisting of fem atomic layers linked through sp2 hybridized C and N atoms which has attracted much attention as a catalyst over the years. Among the different allotropes of g-C3N4, heptazine based structural unit is considered to be most stable in ambient conditions which exhibits high chemical and thermal stability. However, the synthesis and its applications are limited to the bulk. The electrical properties of this material especially in thin film form is yet to be studied in detail. In this thesis, we seek to study g-C3N4 in the form of bulk, exfoliation and particularly focusing on the films keeping in view of its possible applications in opto-electronic devices. Photo luminescence (PL) is studied as an optical tool to emphasize on different forms of graphitic carbon nitride. The nature of the PL explains the similarity between exfoliated layers and thin films. In order to study the electrical properties of the film, we fabricate sandwich devices consisting of ITO/g-C3N4/Al. The J-V shows symmetric nature with low leakage current density. Impedance spectroscopy is deployed to characterize the dielectric nature of the film. The dielectric constant is measured from the linear behavior of the capacitance-voltage (C-V) characteristics. To further enhance its dielectric nature, ITO/Al2O3/g-C3N4/Al device is fabricated and characterized through temperature dependent J-V and C-V. However, Al2O3/g-C3N4 heterostructure shows relatively high permittivity as compared to their individual permittivity. In the last section, we studied temperature dependent C-V characteristics of a metal-insulator-semiconductor (MIS) device using a-IGZO where Al2O3 and g-C3N4/Al2O3 heterostructure are used as dielectrics. Both frequency and temperature dependent C-V characteristics are used to determine carrier concentration of a-IGZO using conventional Mott-Schottky equation in the depletion region. The respective permittivity of their dielectrics are measured as a function of temperature. The high permittivity of the heterostructure is openly makes a path for dielectrics in possible thin film transistor (TFT) devices. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Suvankar Paul Tapobrata Sarkar 15109275 Black holes vs Horizonless Compact Objects: Strong Gravitational Lensing and Accretion Disk Images 2nd December (Wednesday), 2020 04:00 PM Online platform (Zoom link: https://zoom.us/j/9150404484) Einstein’s theory of General Relativity (GR) has been formulated more than a century ago and is one of the most successful theories of gravity to date. It has passed several experimental tests so far, among which the bending of light in a gravitational field (known as Gravitational Lensing) is one of the most significant ones. Historically, this was the very first experimental test that was carried out to validate GR in 1919. Most of these early tests probe gravity in the weak field regime, mainly in Solar system observations. However, with the advancement of modern technology, it now seems possible to test gravity in the strong filed regime too, which is essential for a complete understanding of it. In this context, astrophysical ultra-compact objects (UCOs) such as black holes, wormholes etc., turn out to be excellent laboratories to test the strong gravity phenomena. Such UCOs can be broadly classified into two categories: objects having event horizons, i.e., black holes (BHs), and objects without event horizons, such as wormholes, naked singularities, etc. Studies of BHs in relation to strong gravitational lensing, shadows, images etc. are, by now, quite extensive. However, horizonless compact objects (HCOs) are becoming increasingly popular in recent years. An important reason for this is that they can produce strong lensing effects which completely mimic that of BHs. With the recent imaging of the centre of the M87 galaxy by the Event Horizon Telescope (EHT) collaboration and the detection of gravitational waves, we now have the tools of high-precision observational astronomy to probe the extreme gravity regions around such UCOs. In light of this, it becomes essential for us to study how to distinguish HCOs from BHs, based on their characteristic strong lensing signatures. In this talk, I shall discuss various characteristic features of HCOs which can possibly distinguish them from BHs. First, I shall consider the nature of strong gravitational lensing around such HCOs, especially Wormholes, and contrast them with that of BHs. It will be shown that there indeed exist many distinguishing properties of HCOs as compared to their BH counterparts. Then I shall focus on the accretion disk images produced by wormholes and show that they may be dramatically different form the corresponding images produced by BHs. Therefore, such novel, distinguishing strong lensing signatures of HCOs may open up a possibility for their detection in futuristic observations. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Prasenjit Sanyal Pankaj Jain 13109072 Phenomenological study of the scalar extension of the Standard Model 25 November (Wednesday), 2020 04:00 PM Online platform (Zoom link: https://zoom.us/j/95795691834?pwd=cmpPbFZ1YzhuT1c4MXVRVkQ0TlVuUT09) The Standard Model (SM) of particle physics is the most successful model to explain almost all experimental results. However SM fails to explain the existence of dark matter (DM), neutrino oscillation, matter-antimatter symmetry and also suffers from theoretical shortcomings. Not only that the Higgs-like scalar particle discovered at LHC raises the question whether the observed scalar is the one predicted by SM or it is a part of a scalar extended structure of a more fundamental theory. In this talk I will discuss the phenomenology of models where the scalar sector of SM is extended such that we have the observed 125 GeV Higgs boson as well as other scalar particles. I will first talk about one of the simplest yet most popular scalar extension of SM called the two Higgs doublet model (2HDM), where an additional Higgs doublet is introduced with the same quantum numbers as the first one. In a CP conserving framework, the scalar sector consists of two CP-even neutral scalars (h=125 GeV, H) and one CP-odd scalar (A) and two charged Higgs (H+-). The Yukawa sector can be divided into four possibilities, Type I, II, X and Y, to avoid Higgs mediated FCNC. I will discuss the exclusion regions of the charged Higgs parameter space coming from the latest CMS 13TeV results and compare the exclusions from the previous 8TeV results. For completeness I will also discuss the constraints from flavour physics. In the next part of the talk. I will give a brief overview on DM. I will discuss DM relic density in the context of freeze-out and freeze-in scenarios, DM direct and indirect detection experiments, astrophysical constraints on DM self interaction and the status of the scalar singlet DM model where a scalar singlet is added to the SM and a Z2 symmetry is used to stabilize the DM candidate. Next, I will discuss the collider, astrophysical and cosmological constraints on a DM sector of a conformal model within the framework of freeze-out as well as the freeze-in mechanisms. The model has a dark sector with strong self interactions. The dark sector couples weakly to the SM sector through messenger scalars. The lightest dark sector is a pion-like fermion antifermion bound state which acts as a DM candidate. The model successfully satisfies the constraints coming from the Higgs decay to the dark pions and visible sector. For direct detection constraints we have used the limits from the Xenon1T experiment. The model also satisfies the indirect detection constraints of gamma rays from the galactic center and dwarf spheroidal galaxies. In the freeze-in scenario, the model also satisfies the astrophysical constraints on DM self interaction. Finally, I will talk about 2HDM extended with additional U(1) gauge symmetry. The Type X structure of Yukawa interaction is realized by proper charge assignments of fields under U(1) symmetry. Additional charged leptons are to be introduced to cancel the gauge anomaly associated with the extra gauge symmetry. In addition, scalar fields are added with Z2 odd parity as DM candidates. I will discuss the phenomenology of the model such as constraints from scalar potential, muon g-2, collider physics associated with Z' of the U(1) symmetry and DM physics. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Annwesha Dutta Debashish Chowdhury (PHY) and Vivek Verma (MSE) 13109061 Stochastic Thermodynamics and Kinetics of Molecular Machines: Principles and Applications to Ribosome 06 October (Tuesday), 2020 02:00 PM Online platform (Zoom link: https://zoom.us/j/5081711480) Living cell is a micro-factory of sophisticated nanoscale molecular machines which coordinate and perform well-defined biological functions with remarkable precision in a noisy and viscous environment. These machines are driven far from equilibrium as they consume energy in the form of fuel and convert it into some form of work. To understand how these molecular machines function, one requires a statistical description that goes beyond well-established equilibrium statistical mechanics. The rapid development in the single molecule experimental techniques has given unprecedented access to probing and viewing of the individual bio-molecular machines and even quantifying their distance, forces, and velocities. Against this background it is imperative to pursue an integrative approach to understand the individual and collective behavior of these molecular machines. My thesis primarily focuses on understanding the physical principles underlying biological function of a remarkable molecular machines,Ribosome, with the aid of non-equilibrium statistical mechanics. We have developed theoretical models to describe their complex mechanisms, the errors they make and their coupling to the cellular environment. Ribosome is one of the most complex, efficient, and robust molecular machines inside the living cell, which is responsible for synthesis of protein in every organism. A major portion of our work is based on theoretical chemo-mechanical modelling of the ribosome kinetics during the elongation cycle. In each successful chemo-mechanical cycle during the elongation stage, the ribosome elongates the protein by a single subunit, called amino acid, and steps forward on the template mRNA by three nucleotides called a codon. We describe the kinetics of the ribosome in terms of a Markov network of observable mesoscopic states, using experimentally measured interstate transition rates. We develop a stochastic kinetic model that captures the possibilities of mis-sense error i.e., misreading of mRNA codon and prior mischarging of a tRNA. The corresponding exact analytical expression for the average rate of elongation of a nascent protein turned out to be a biologically motivated generalization of the Michaelis Menten (MM) formula for the average rate of enzymatic reactions. This formula displayed the interplay of four different branched pathways corresponding to selection of four different types of tRNA. Furthermore, this result stimulated us to understand when does the MM equation hold in general? It has already been found that MM like formula appears not just in enzyme biochemistry but also in gene regulation, mRNA transcription, protein translation and molecular motors; they involve not only the macroscopic, but also the single-molecule, level. We have analyzed the question of validity of MM equation for ribosome in the light of a graph-theory based linear framework for timescale separation. Next, we visualize the ribosome as a thermodynamically open system that is coupled to various reservoir potentials which include not only the thermal reservoir at a constant temperature, but also several chemical reservoirs maintained at the respective chemical potentials and a force reservoir describing a load force acting on the ribosome. We use stochastic thermodynamics framework based on a graph theoretic approach to obtain a clear understanding of the energy transduction processes during translation by decomposing the complex network of distinct states into its cycles, focusing on the energetics and thermodynamic picture in terms of fluxes, their conjugate affinities, and entropy change associated with every cycle. We thereby understand what chemical, mechanical, and chemo-mechanical cycles compete in the network. We also compute important motor properties like velocity and hydrolysis rate in terms of external parameters like concentration of the different particles which bind to the ribosome. The flux balance relations give us the expression for stall force and balanced potential. We identify the various possible modes of operation of a ribosome in terms of its average velocity and mean rate of GTP hydrolysis. We also compute entropy production as functions of the rates of the interstate transitions and the thermodynamic cost for accuracy of the translation process. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Meenaxi Sharma Dr. Krishnacharya 14109267 Aqueous Drops on Thin Lubricating Fluid Coated Slippery Surfaces: Statics and Dynamics 04 September 2020, Friday 03:00 PM Online Platform (Details of the online platform will be communicated in due course) In recent years, interfacial science has emerged as an important research topic due to its relevance in various technological applications to develop advanced materials and devices for the betterment of mankind. One may get fascinated with very common appearances of applications of wetting physics e.g. non-wetting behavior of leaves of various plants, rolling water drops on windshields of cars and window panes, drying of ink drops during printing, breaking of a liquid jet into drops and slippage of drops on oily surfaces, etc. It is interesting to note that on the one hand, water drops roll effortlessly on non-wetting leaves of Lotus and Brassica oleracea, on the other hand, similar water drops spread completely on leaves of Alocasia odora and Ruellia devosiana. These phenomena can be understood on the basis of the interaction of the two objects, and it depends on the physical and chemical nature of both the surfaces. The process involved in bringing a drop placed on a solid substrate into the equilibrium is mainly governed by the surface tension force. Over the past few decades, a considerable amount of research has been carried out to create liquid shedding surfaces based on solid-liquid interaction, and the requirement of such surface is fulfilled with the development of superhydrophobic and superoleophobic surfaces using micro/nano-textures with appropriate surface chemistry. However, when a thin liquid film is deposited on a solid substrate, overall wetting behavior becomes quite different due to the appearance of a new liquid-liquid interface. This technique provides an alternative approach to create liquid shedding surfaces, where the thin liquid film behaves as a lubricating layer, thus creating lubricating fluid infused slippery surfaces. In nature, Nepenthes pitcher plants also depict similar slippery characteristics based on thin lubricating aqueous fluid infused on the inner walls of the plant to provide a slippery interface to trapped insects. Due to the involvement of four different phases, slippery surfaces offer enormous scope for fundamental research. The word done in the present thesis deals with the static and dynamic behavior of aqueous drops on lubricant coated slippery surfaces (LCS) to understand the role of various interfacial energies and system parameters. I will first discuss the stability analysis of aqueous drops on lubricated surfaces with varying surface energies. Our investigations reveal that surface forces play a dominant role in determining the final state of drops on a slippery surface: whether stable (float) or unstable (sink). I will discuss in detail the dynamic of the sinking of aqueous drops with the effect of lubricating fluid viscosity and substrate wettability along with the underlying mechanism. Further, I will talk about the other case, for which drops are stable on LCS and do not sink. Here I will discuss drop mobility on LCSs and investigate the effect of lubricating film thickness, surface tension, and viscosity on the drop mobility together with the thickness optimization for the best slippery behavior. I have also developed a theoretical model to predict the drop velocity by comparing viscous dissipations in various regions of the two liquids. Subsequently, I will discuss a novel and very efficient approach of designing anisotropic slippery surfaces with directional drop motion using chemically heterogeneous surfaces, having a hydrophobic and hydrophilic stripe pattern together with thin films of lubricating oil. When aqueous drops are deposited on such surfaces, the lubricating fluid gets completely displaced (due to dewetting) from the hydrophilic regions to the neighboring hydrophobic regions to minimize the total energy of the system. This results in the formation of lubricating microchannels, which subsequently provides the anisotropic slippery motion to the aqueous drops. We want to emphasize that the formation of such lubricant microchannels without any surface topography is the novel aspect of this study and has not been reported to date. This system provides superior performance in terms of anisotropic and controlled slippery behavior compared to dry hydrophobic, topographically, or chemically patterned surfaces. We present a detailed investigation of anisotropic drop motion on such surfaces as a function of the area fraction of the wettability pattern and finally summarize all the results in the form of a phase diagram. Towards the end, I will present an application of lubricated slippery surfaces to control drop evaporation compared to dry hydrophilic or hydrophobic surfaces. I will demonstrate a detailed experimental investigation of the evaporation of pure water and binary mixture of water-ethanol drops on dry hydrophobic and lubricating fluid coated slippery (LCS) surfaces. This study is divided into three sections, with a focus on the evaporation of pure water drops, binary mixture drops, and the drop residual after complete evaporation. To understand the evaporation phenomenon on different surfaces, I will compare the experimental findings with a diffusion-controlled theoretical model. I will also talk about the drop residual at the end of the complete evaporation, which is very distinct on LCS surfaces compared to dry surfaces. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Manohar Kumar Sharma Mahendra K. Verma and Sagar Chakraborty 13109881 Statistical Properties of Rapidly Rotating Turbulence 10 August 2020, Monday 02:30 PM Online platform Turbulence is a ubiquitous phenomenon in nature. We can observe its manifestation at every scale, from a cup of tea being stirred to the galaxy formation. Turbulent flows are characterized by randomness in velocity fields, enhanced diffusivity, and strong nonlinearity in the flow. One of the most popular models to study the statistical properties, like the kinetic energy spectrum of the flow, was proposed by Kolmogorov in 1941. Kolmogorov, through his model, predicted the scaling of the kinetic energy spectrum as E(k) ~ k^(-5/3) (where k is wavenumber) in the inertial range at the high Reynolds number. This scaling is validated in numerical and experimental studies. The nonlinear interactions in the velocity field are responsible for the scale-to-scale transfer of the kinetic energy in the inertial range. In three dimensional homogeneous and isotropic hydrodynamic turbulence, a local and forward cascade of the kinetic energy is reported in literature. Though homogeneous and isotropic turbulence itself is an intricate problem, the effect of rotation makes it even more intricate due to the anisotropy in the flow. The Coriolis force, owing to rotation, does not contribute to the total energy of the system. However, it modifies the energy distribution in different scales of the system such that the flow behaves like quasi-two-dimensional at a very high rotation rate. As a result, the scaling of the kinetic energy spectrum changes and does not show the Kolmogorov power scaling. We study the statistical properties of the rotating turbulent flow numerically. We solve the governing equation of the system in a three-dimensional cubic periodic box of box-size (2π)^3. First we discuss the statistical behaviour of the decaying rotating turbulence and our proposed model. Here, we show that our proposed model is in good agreement with numerical results. We also show the most dominant energetic modes in the system are (±1,0,0) and (0,±1,0) in the Fourier space. Most of the energy is trapped in the large scales. Furthermore, we discuss the behaviour of the kinetic energy spectrum for the forced rotating turbulence. In this part of the talk, we show that the energy spectrum in the wavenumber region smaller than the forcing wavenumber shows Kunznestov—Zakharov—Kolmogorov scaling. Among the wavenumbers larger than the Kolmogorov dissipation wavenumber, the energy is distributed such that the suitably non-dimensionalized energy spectrum is E(k)~exp(-0.05k), where k also has been appropriately non-dimensionalized. We end the discussion by comparing the anisotropic energy transfers for the decaying and the forced rotating turbulent flows. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Sayid Mondal Gautam Sengupta 14209863 Aspects of Holographic Entanglement Negativity in AdS/CFT 03.08.2020 ( Monday) 12:30 PM Online platform Entanglement is a fundamental property which distinguishes quantum systems from classical ones. For bipartite pure quantum states, it is characterized by the von Neumann entropy of the reduced density matrix. However, it fails to be a valid entanglement measure for bipartite mixed quantum states as it receives contributions from irrelevant correlations. In this context, Entanglement Negativity is a computable measure for characterizing mixed state entanglement in quantum information theory which provides an upper bound on the distillable entanglement for a mixed quantum state. In this regard, we propose a covariant holographic entanglement negativity construction for time-dependent mixed states of adjacent intervals in (1+1)-dimensional conformal field theories ($CFT_{1+1}$) dual to non-static bulk anti-de Sitter (AdS) geometries through the $AdS_3/CFT_2$ correspondence. Subsequently, we explore the time evolution of the holographic entanglement negativity following a global quench for mixed states in $CFT_{1+1}$s dual to bulk eternal BTZ black holes sliced in half by an end of the world (ETW) brane. Finally, we describe a possible higher dimensional generalization of our construction and its application to an example of mixed states of adjacent subsystems in $CFT_d$s dual to bulk pure $AdS_{d+1}$ spacetimes and $AdS_{d+1}$-Schwarzschild black holes. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Krishanu Sadhukhan Amit Agarwal 14109878 A study of anisotropic plasmons in tilted Dirac materials 28th July 2020 (Tuesday) 8.30 AM Online platform Plasmonics in nano-materials is evolving at a breathtaking pace. The fundamental excitation for plasmonics is the plasmon, which is the collective density oscillation of the electronic charges. Coulomb interaction between electrons gives rise to this elementary excitation, which is long lived and evolves independently with a definite energy-momentum relation. The remarkable discovery of graphene in the last decade lead to huge interest in the field of topological Dirac materials, following which many 2D and 3D anisotropic Dirac materials have been discovered. In this work, we investigate the plasmonic properties of 2D and 3D anisotropic tilted Dirac materials. In terms of theoretical technique, we use a generalized classical hydrodynamic theory and rigorous quantum mechanical framework within the random phase approximation to explore plasmons in anisotropic Dirac materials. Two dimensional anisotropic Dirac fermions are realized in 8-Pmmn borophene, which exhibits anisotropic plasmon, dynamical screening and Friedel oscillation. 3D Dirac plasmon is experimentally observed in type-II Dirac semimetal (DSM) PtTe2 and our theoretical findings matches reasonably well with the experimental data. We show the existence of a novel undamped gapless plasmon mode in type-II DSM when the momentum is along the direction of the tilt. We also generalize the classical hydrodynamic theory of plasmon to hydrodynamic theory for multi-component electron liquid and apply this to known systems like spin polarized electron gasses, spatially separated 2DEG, tilted DSM etc. These classical predictions are in reasonable agreement with the quantum mechanical results. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Paramita Dasgupta Pankaj Jain 13109070 Reflection of Ultra High Energy Cosmic Ray Induced Radio Signals From Antarctic Ice 22 July 2020, Wednesday 5:00 PM Online platform Coming out of space and incident on the high atmosphere, there is a thin rain of charged particles known as the primary cosmic ray radiation” (Cecil Powell, Nobel Prize Lecture, 1950) Cosmic Rays are particles that come to Earth’s atmosphere from space. For over six decades, several experiments have been deployed to determine the primary energy, the elemental composition of the cosmic rays in the energy range from 10^14 eV to above 10^20 eV from the measured properties of the secondary particles produced in the EAS. By measuring the secondary particle properties, scientists extract the information of the primary energy, their arrival direction distribution, energy spectrum, elemental composition. Neutrinos, by the virtue of being chargeless, are immune to inter-galactic as well as Earth’s magnetic fields. The High Energy Neutrinos interact with matter extremely feebly and reach us unscathed from the edge of the Universe. Due to these advantages, neutrinos are preferred as Astronomical Messengers over the ultra high energy cosmic rays. However, owing to the small neutrino-nucleon cross section, neutrino detection is extremely challenging due to the remarkable ability of neutrinos to escape dense astrophysical environments and their ability to pass through detectors without much interaction. The NASA sponsored balloon-borne ANITA detector operating in Antarctica is designed to detect ultra high energy cosmic rays (UHECR) with energies exceeding 1 EeV (10^18 eV) by collecting the radio pulse generated through the interaction of the primary particle with Earth's atmosphere. The radio pulse is detected after reflection from the Antarctic ice surface. For calibration and measurement of surface reflectivity, the balloon-borne HiCal radio-frequency (RF) transmitter is used. Here we are interested in determining the mean value of reflection coefficient over the range of frequencies which are of interest in HiCal observations. At the beginning of this talk, I will discuss a general formalism that we developed to treat the reflection of spherical electromagnetic waves from a spherical surface. Our main objective is the interpretation of radio wave signals produced by cosmic ray interactions with Earth's atmosphere which are observed by Antarctica based ANITA detector after reflection off the ice surface. The incident wave is decomposed into plane waves and each plane wave is reflected off the surface using the standard Fresnel formalism. For each plane wave, the reflected wave is assumed to be locally a plane wave. This is a very reasonable assumption and there are no uncontrolled approximations in our treatment of the reflection phenomenon. The surface roughness effects are also included by using a simple model. We apply our formalism to the radiation produced by the balloon-borne HiCal radio-frequency (RF) transmitter. The final results for the reflected power are found to be in good agreement with data for all elevation angles. I will present the numerical results for the reflected power at the ANITA detector locations, and also the comparison with the HiCal experimental data. This theoretical framework is general and based on the First Principle Calculation, and this can be directly incorporated in the simulation techniques applied in ongoing and future cosmic ray detection experiments. I will discuss this in detail. ANITA detected some unusual events that are believed to be the signatures of Beyond Standard Model (BSM) Physics. We apply our formalism to unfold the mystery of these anomalous events by studying the properties of reflected radio pulses from various surface features and study the phase relationship with the HiCal direct events. We find that for some roughness models the reflected pulse shape can be somewhat distorted and may be misidentified as a direct pulse. We studied different HiCal direct pulses as the inputs in our analysis to determine the probability of such misidentification. We see that for HiCal pulses with some special features, the probability of such misidentification is much high. From our analysis, it is clear that the surface wind/ablation crusts, sastrugi, snow-covered crevasses, and similar deviations from smoothness can change the pulse waveform to a great extent. I will explain these results with various HiCal direct pulses in detail. Our proposed roughness model for the Antarctic surface can be applied to future cosmic ray detection experiments in the Antarctic continent and also in Greenland. In the end, I would discuss the Implications of our theoretical framework for the ongoing and future neutrino detection experiments worldwide. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Mr. Pramod Ghising Zakir Hossain 13109071 Study of interface induced phenomena in perovskite oxide thin films 14th July, 2020 (Tuesday) 11:00 AM Online platform Oxide materials have garnered tremendous interest due to their exotic properties which include high temperature superconductivity, colossal magnetoresistance and ferromagnetism among others. With the advent of thin film deposition techniques like pulsed laser deposition, sputtering, molecular beam epitaxy etc., interest in oxide materials have been further rekindled. Of particular interest in oxide thin films is the interface between various overlayers that play host to exciting physics. In this talk, we will explore the interface phenomena in perovskite oxide thin films, fabricated by pulsed laser deposition technique. Perovskite oxides have the generic chemical formula ABO3. Our work focuses on tailoring interfaces of various perovskite oxide films and investigating the emergent phenomena therein. LaTiO3(LTO)/SrTiO3(STO) have been known to exhibit metallic behaviour due to the formation of high density 2D electron gas (2DEG) at the interface. We find that on doping the LTO/STO interface with thin layers of CeTiO3 (CTO), the interface shows interesting properties like Kondo effect and spin orbit interaction (SOI); which can be tuned by the CTO layer thickness. The interfacial phenomena gets more fascinating in La0.7Sr0.3MnO3 (LSMO)/LTO/STO heterostructures, where we demonstrate direct control of its electrical properties using light and gate voltage. With the use of light and gate voltage, we have shown that the electrical properties can be tuned between resistive and conductive states at room temperature. In LSMO, the charge and spin degrees of freedom are strongly correlated. Thus, the ability to tune the electrical states unfolds a strong prospect for the control of magnetic properties of LSMO with light and external electric field. The control of magnetic properties of devices with electric field is much coveted in the field of spintronics. Our investigation in LSMO/LTO/STO unleashes new possibilities for spintronic applications. Furthermore, a unique variant of the hysteresis loop (known as the inverted hysteresis loop) is observed in LSMO/STO thin film, which exhibit negative remanence and coercivity values. The origin of the inverted hysteresis loop (IHL) can be traced to the presence of the LSMO/STO interface. Ferromagnetic resonance studies show competing anisotropies in the LSMO film, while magnetic measurements confirm the presence of positive exchange bias. Based on our experimental results, we unambiguously show that exchange bias leads to the observed IHL. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Prabodh Kumar Pandey Asima Pradhan 10209063 Reconstruction and characterization algorithms for photoacoustic and fluorescence-photoacoustic tomography 30th June 2020 (Tuesday) 3:00 PM Online platform The use of fluorescence in optical-based tomographic imaging helps us to obtain physiological markers of metabolism and pathology before their structural manifestation. Also, the combining of acoustic and optical modalities yields reconstructed images with good contrast(optically based) and resolution(acoustic-based). We set up fluorescence photoacoustic tomographic (FPAT) algorithms for reconstruction of the absorption coefficient of fluorescent markers at the excitation wavelength. We report the first results of FPAT reconstructions from pressure data with diffusion approximation(DA) modeled light propagation in tissue in Jacobian and gradient type algorithm settings. For non-scattering-dominant media, as found in several tissues of interest, where the DA is not valid, we have developed full radiative transport equation modeled Jacobian and gradient-based FPAT algorithms. With the objectives of system design, we have then developed characterization algorithms in the context of universal backprojection based photoacoustic reconstructions, which distinguish between data of different noise levels, as well as provide an appropriate cut-off frequency for photoacoustic reconstructions. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Gyana Ranjan Sahoo Asima Pradhan 13109065 Statistical analysis of fluorescence lifetime and low coherence backscattered images for cervical pre-cancer diagnosis 25th June 2020 (Thursday) 11.30 am Online platform Optical techniques are non-invasive in nature and has shown early promise in diagnosis of cervical cancer. Morphological and biochemical changes occurring in tissue with progression of cancer leads to change in scattering, absorption and fluorescence properties. Such changes can be captured through optical spectroscopic and imaging techniques. Elastic scattering techniques provides information about the morphological changes occurring at cellular and sub-cellular level, whereas, fluorescence techniques provide information about biochemical changes at molecular level. Steady-state fluorescence spectroscopic and imaging techniques have been extensively used to monitor above changes in tissue. However, highly overlapping spectral characteristics of the contributing fluorophores, dependence on fluorophore concentration and illumination intensity limits the efficacy to understand the disease development through these methods. On the other hand, fluorescence lifetime is independent of fluorophore concentration and illumination intensity and is sensitive to its micro-environment. The temporal information, which adds an extra dimension can be used to study complex tissue environments. In this thesis, fluorescence lifetime imaging has been utilized for diagnosis of cervical pre-cancer. Tissue containing both normal and abnormal region was imaged and the result was validated through phantom experiments. Further experiments on normal and abnormal samples were carried out, where both single and double exponential fitting have been exploited to have a better diagnosis. The study indicated that the lifetime is higher in scattering medium. This was followed by the application of principal component analysis (PCA) on spatio-temporal map to study the efficacy of principal components (PC) in capturing changes occurring in fluorophore environment. Preliminary experiments were carried out on two fluorophore phantom followed by tissue experiments and the results were then explained through tissue mimicking phantom experiments. Further, PCA was applied on average fluorescence decay profiles of normal and abnormal samples. It was found that the 2nd PC provides better contrast than lifetime map as it captures both absorption and scattering changes in the medium. In another part, a low coherence backscattered common path interferometric setup was developed, calibrated and imaging was performed on tissue sections. Variation in fractal properties of tissue with progression of cancer was studied by analysing the images through a two-dimensional multifractal analysis. Stronger multifractality was observed with progression of cancer. The calculated multifractal parameters displayed a clear distinction between different grades of cervical pre-cancer. In the last part, support vector machine (SVM) was employed for classification of fluorescence lifetime, PC scores and multifractal parameters obtained in the previous chapters. Different types of kernel functions were first compared for their performance and Gaussian kernel found to have better performance than others and was used for further classification purpose. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Anuj Ram Baitha Prof. Sudeep Bhattacharjee14109875 Production and study of a plasma confined in a dipole magnetic field June 12, 2020 (Friday) 11:00 AMOnline mooKIT platform (link will be provided later and access code to interested participants) Dipole field is one of the most fundamental magnetic field configurations in the universe. There has been a long quest to understand charged particle generation, confinement and underlying complex processes in a plasma confined by a dipole magnetic field. Planetary magnetospheres such as those of Earth and Jupiter are example of such naturally occurring systems. The rather simple magnetic field holds together colossal plasmas in space, and high beta (ratio of plasma to magnetic pressure) plasmas can be sustained. It is of interest to investigate such a magnetic confinement scheme and the resulting plasma behavior, in the laboratory. Unlike other confinement schemes that relies on magnetic curvature and shear, dipole plasma confinement derives stability from plasma compressibility, that utilizes the large ﬂux tube expansion of a dipole ﬁeld. Experiments at MIT, Columbia University and the University of Tokyo, have relied on large devices that employ superconducting electromagnetic coils. In this thesis work, a compact table-top experiment utilizing microwaves to create the plasma has been developed. The system employs a single water cooled cylindrical permanent magnet having surface magnetic field ~ 0.5 T to create the dipole magnetic field. The plasma is generated by electron cyclotron resonance heating. The experiment is unique in terms of low cost, simpler technology, easier plasma accessibility and allows steady-state operation. Voluminous plasmas can be sustained, e.g. a single permanent magnet of volume ~ 17 cm^3 and maximum surface field of ~ 0.5 T, can confine a plasma of volume > 3.4×10^4 cm^3. Visual observations of the plasma indicate alternate bright and relatively less bright regions with resemblance to earth’s radiation belts that trap charged particles. Measurement of plasma parameters such as electron density, electron temperature, and space potentials have been carried out using Langmuir and emissive probes. The plasma was characterized in the polar directions employing special “y-shaped” probes. Optical diagnostics comprising of a photodiode, optical fiber, and a high resolution spectrometer were utilized for measurement of optical emissions from the plasma. Line integrated intensities could be obtained along chords from near the center to the plasma edge, from which the local plasma emissivity could be determined using Abel inversion. Particle balance which results from an inter-relationship between generation and loss was investigated and dependence of production, loss, and plasma retention rates, as a function of wave power and discharge pressure was determined. The developed mathematical model solves the particle balance equation, considering generation through ionization and losses through diffusion and recombination, and incorporates the measured values of plasma parameters and dipole fields in space. The plasma beta determined from experimentally measured data, increases steadily from a small value near the magnet to ~ 7% in the midplane (~ 8 cm from the magnet center), and thereafter the increase is more gradual and almost levels off to ~ 10% at the chamber edge (~ 20 cm). In general, the electron temperature and the plasma potential are higher at the polar cusp regions, and decreases toward the equatorial plane, with the profiles becoming more spherically symmetric away from the magnet. The location of the mid-plane density peak (~ 8 cm) seems to match closely with the region where beta starts to level off, and the space potential starts to decrease exponentially. Particle loss cone analysis in the curved mirror fields, indicates that the percentage of particles lost near the surface of the magnet is much higher ~ 20% than that near the plasma edge ~ 0.07%, this along with the fact that the plasma retention rate is highest a little downstream from the magnet (~ 3 – 7 cm), can explain the density depletion close to the surface of the magnet. An investigation of diffusion induced transport reveals that peaked density profiles, are realized as a natural outcome of the solution of diffusion equation, thereby confirming the phenomenon of inward diffusion as observed in earlier experiments. Two independent diffusion models are developed and the plasma density profiles are determined and compared with those obtained experimentally. Diffusion is found to be predominantly governed by the classical (~1/B^2) scaling law, where B is the magnetic field. The compact device bears promise for basic studies on dipole plasmas, simulation of magnetospheric plasmas, studying dusty plasmas or even for plasma processing, because of the possibility of confining energetic electrons. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Khun Sang Phukon Pankaj Jain 12109065 Study of inspiral dynamics of precessing compact binaries and low-latency searches of gravitational waves 27 May 2020 4 PM Online Platform (https://meetings.iitk.ac.in/b/pan-wmw-nnc) The routine detections of gravitational waves (GWs) from compact binary coalescences (CBCs) by the LIGO and Virgo detectors herald the era of gravitational-wave astronomy. Compact binaries involving black hole pairs, neutron star pairs, and neutron star-black hole pairs are the most promising sources of GWs for the ground-based detectors. In the coming years, the detection rate of compact binaries will increase multifold with the improvement of detectors' sensitivity in the advanced gravitational-wave detector network that comprises of advanced LIGO, advanced Virgo and KAGRA, and with the joining of LIGO-India to this network. In the first part of the talk, I will discuss the dynamics of spinning compact binaries, particularly binary black holes (BBHs) during inspiral. Detailed understanding of the complex spin dynamics of the generic spinning BBHs is crucial to maximizing the scientific outputs of the observations of populations of BBHs. I will discuss spin-precession induced correlations between spin orientations of a population of BBHs in circular orbits at large separations and parameters of the binaries near and after their mergers. I will also address the inspiral dynamics of spinning binaries in elliptical binaries and the effects of eccentricities on the spin morphologies during inspiral. In the second part of this talk, I will focus on algorithmic and hardware optimization related problems to speed up the searches of GWs from compact binaries. Near-real-time implementation of CBC searches in the gravitational-wave detectors' data is crucial for quick follow-up of any potential electromagnetic signal associated with a GW event. I will talk about a random projection based technique that reduces the dimensionality of templates used in the searches of gravitational waves from CBCs and hence the computational cost. Alongside algorithmic ideas, I will discuss the uses of advanced hardware architectures that offer massive parallelism in speeding up CBC searches. I will present speed-up achieved by implementing parts of GW data analysis pipeline on GPUs in comparison to multiple MPI processes running on a CPU cluster. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Dibya Jyoti Sivananda Satyajit Banerjee 11109867 Stepwise disintegration of magnetic domains in a EuB6 single crystal observed by magneto-optical imaging 21 May 2020 (Thursday) 11:00 AM Online Platform The behaviour of magnetism in systems with strong electronic correlations is a fascinating and important area of research with potential for applications in spintronics. In a system like EuB6, the interaction between itinerant electrons with localised ion moments lead to magnetic ordering along with emergence of peculiar features. EuB6 shows colossal magnetoresistance behaviour below a characteristic temperature of Tc1~15.5K and it undergoes paramagnetic to ferromagnetic transition at Tc2~12K. I have used bulk magnetization and high sensitivity magneto-optical imaging technique the explore the para to ferromagnetic (FM) transition in this system. Scaling analysis of bulk magnetization measurements suggests the presence of large critical fluctuations between two different temperature regimes Tc1(H) and Tc2(H). Imaging of the local magnetic field distribution, an additional third temperature window T*(H) is identified in a field - temperature magnetic phase diagram. The imaging of magnetic domains reveals the presence of large magnetized domains below Tc2 (deep in the FM state). With increasing T (> Tc2) the magnetic domains disintegrate into finger-like patterns before fragmenting into disjoint magnetized puddles at Tc1(H) and ultimately disappearing at T*. The results are explained via the formation of magnetic polaronic clusters and their coalescing into larger domains. Another part of the work deals with development of a new technique for sensing of nano-scale vibrations. Sensing of nanoscale to sub-nanoscale vibrations is very important for various ultra-high precision applications like zeptogram mass sensing, chemical sensing, force sensing, etc. Here we use the sensitivity of an STM to measure the time series of the tunneling current to detect sub-nanometer level vibrations. By employing these time series measurements to study the tunneling current fluctuations, we measure the vibration characteristics of a piezo crystal oscillator. The detection of the sub-nanometer vibrations leads us to uncover an unconventional sub-nanometer perpendicular vibrations mode excited on the crystal surface. The direction of these sub-nanometer scale vibrations is perpendicular to the surface of the piezo which is excited in the transverse mode. Near resonance of this unconventional mode, we observe the generation of higher harmonics and suggests the nonlinear modes may yield higher sensitivity. Our hope is that this technique will evolve into a sensitive sensor for a variety of applications, some of which involve sensing extremely weak magnetic phenomena. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Kamalika Nath Satyajit Banerjee 12109872 Study of magnetic response in very high magnetic field and nano-confined structures 15 May 2020, Friday 11 AM Conducted over mooKIT meetings platform The study of magnetism at nanoscale has revealed very unique features in magnetic materials, which are mainly mediated by broken translational symmetry and complex interplay between different competing energy scales. In the first part of the talk, I will present the study of cobalt nanopillars with two different diameters (50 nm and 10 nm), confined in nanoporous alumina (AAO) template. By lowering the temperature below 100 K, we were able to control the magnetic anisotropy of the system from parallel to perpendicular and vice-versa. Along with that, very unusual increase in saturation magnetization and structural distortions in the phases of cobalt were observed below a characteristic temperature. These unique features indicated changes in the magneto-crystalline anisotropy of cobalt in the AAO as a result of the stress developed due to the alumina template at lower temperatures. Our results were supported by micromagnetic simulation studies done using the Landau-Lifshitz-Gilbert equations and give evidence of the interplay between magnetism, structure, and magnetic anisotropy in Co-nanopillars, which can be modified with temperature. In the second part, I will discuss on the generation of completely new magnetic configurations by applying very strong magnetic field through intense laser-matter interaction. By irradiating femtosecond laser pulse on yttrium-iron-garnet films (YIG) and using high sensitivity magneto-optical imaging technique, we observed that complex metastable chiral domain structures are created in YIG after irradiation. Micro-magnetic simulations were done to understand the nature of the excitations, which revealed the propagation of a disturbance in the magnetic configuration of the film, which had the same structure and feature as the domain excitation seen in our experiments. Time permitting, I will also touch upon my work related to fabricating and characterizing CVD graphene based Hall sensors to probe the local magnetization of magnetic samples. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Arif Warsi Laskar Saikat Ghosh 12209063 Quantifying and applying atomic coherence as quantum resource 5th May 2020, Tuesday 4:00 PM Online Platform Superposed atomic states or coherence form the core essence of quantum technologies that promises significant advancements in precision measurements, cryptography and computation. Accordingly, there has been a recent thrust in generating, quantifying and controlling quantum coherence in a wide range of physically realizable systems. In this thesis, we focus on two primary experimental aspects of quantum coherence: its direct quantification in time domain and applying it, towards observation of synchronization in quantum domain. Towards quantification, we start with a detailed study of interplay of classical and quantum dynamics in a thermal ensemble of atoms at room temperature, in a well applied scenario of electromagnetically induced transparency (EIT). We find the generated coherence competes with classical decohering channels (like collision, optical pumping and thermal velocity of atoms) and from the corresponding experimental time traces, we develop a methodology to directly quantify it in time domain. The quantifier effectively captures all aspects of the generated coherence, including “open” and “closed” quantum systems and bench-marking transition from EIT to Autler-Townes regime. Furthermore, we demonstrate a technique to actively compensate for decoherence or decay of quantum coherence, using additional fields, in a scenario akin to quantum zeno effect. In the second half of the thesis, we report experiments on observation of quantum synchronization in the smallest possible physical system: spin-1 atoms. The phenomenon of synchronization in the quantum domain uses coherence as the primary resource. Experimentally, we generate this coherence by storing an optical pulse for a finite time in an ensemble of cold and trapped atoms at a temperature of 100 micro-Kelvin. While stored as two atomic coherences, the excitation evolve in the dark and develop a relative phase difference, in presence of a magnetic field. When the stored excitation is retrieved back as an optical pulse, we observe phase coherent oscillations and corresponding interference fringes. Numerically reconstructed tomography of atomic state, when plotted in phase space, show clear transition from a delocalized limit cycle to a localized synchronized state, only in presence of artificially engineered, anisotropic internal decoherence channels. There is corresponding direct experimental signature in the form of a decrease in visibility of fringes. Furthermore, we observe a suppression or blockade of synchronization due to destructive quantum interference and also, emergence of Arnold tongue-like features with increasing coupling strength. These have been predicted earlier by theorists as typical signatures of quantum synchronization. To the best of our knowledge, this is the first direct observation of quantum synchronization in any physical system. We believe this thesis on experimental quantification of quantum coherence and observation of quantum synchronization in spin-1 atoms would contribute towards development and bench-marking of synchronized large-scale quantum networks, in near future. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Ankit Kumar Satyajit Banerjee 12209062 Exploring low magnetic field instabilities in the static and driven vortex state of K-doped BaFe2As2 superconductor 29 April 2020, Wednesday 4:00 PM Online Platform Melting of vortex state in superconductors is like an ice to water transformation. Depending on the competition between different energy scales associated with this system, different ordered and disordered phases of the vortex matter arise. Results on magneto-optical imaging of the low field vortex melting done for the first time in Ba0.6K0.4Fe2As2 single crystal will be presented. Scaling of the magnetization (M)-temperature (T) data at different fields shows the low dimensionality of the melting process. Results on angular dependence of bulk magnetization using SQUID magnetometer will be discussed to show the presence of extended defect planes extending through the sample thickness. These defects play a crucial role in governing the properties of the low field melting transition. Results on imaging the self field generated by electric currents, to visualize the distribution of current density around the low field solid and liquid phases, will be presented. Results on development of inhomogeneous current distribution near the solid to liquid phase transformation will be shown. We will show that current shifts the melting phase boundary and the entire vortex phase diagram is modified. We show that for transport current I > 50 mA Joule heating lowers the melting phase boundary. For I < 50 mA we show there is a new dynamic drive induced destabilization of the vortex lattice which lowers the melting. In the final part of the presentation, a parallel aspect in the thesis work will be presented, namely exploring charge conduction and the role of water in transport properties of high resistance natural fibers. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Rajesh Tripathi Zakir Hossain 13109074 Griffiths Phase and Quantum Criticality in Heavy Fermion System Ce(Cu1-xCox)2Ge2 2nd March, 2020 (Monday) 4:00 PM (Tea at 3:45 PM) FB 382 (Amal Raychaudhuri Seminar Room) After the discovery of superconductivity in CeCu2Si2 by Steglich et al. strong progress have been made towards understanding the low-temperature properties of heavy fermion (HF) metals. The system CeCu2Ge2 is an example of HF metal. It exhibits interesting physical properties close to quantum critical point (QCP) like non-Fermi-liquid (NFL) behavior and unconventional superconductivity. External parameters, such as doping or application of pressure or magnetic field, can easily tune this system from magnetically ordered to paramagnetic states. At the point where magnetism is suppressed to T = 0 is known as QCP. In this context, the substitution of Co at Cu site makes the compound Ce(Cu1-xCox)2Ge2 interesting to look for QCP and some novel phases around it. This work is the results of numerous experiments and observations on Ce(Cu1-xCox)2Ge2. The macroscopic measurements like thermal transport, magnetic susceptibility, and specific heat are supplemented by microscopic measurements such as Muon Spin Relaxation/Rotation (μSR) and neutron investigations. The long-range anti-ferromagnetic order established in CeCu2Ge2 at T_N = 4.1 K, can be suppressed by Co-doping, and at critical composition x_c = 0.6 (T_N → 0 K) a QCP has been observed. The magnetic susceptibility and specific heat data reveal the signatures of quantum Griffiths phase near the AFM QCP, accompanied by non-Fermi-liquid behavior inferred from the power-law dependence of heat capacity and susceptibility i.e., C(T)/T and χ(T) ∝ T^(-1+α) down to 0.4 K. The QCP is also manifested in the power law divergence of exponential depolarization, i.e. λ ∝ T^(-1+α) down to 0.1 K. The relaxation rate of x = 0.6, obeys the time-field scaling relation Gz(t,H) = Gz(t,H^γ), which is considered to be a characteristic feature of quantum critical magnetic fluctuations. Furthermore, for x = 0.6, the exponents of M ∼ H^η and magnetization-field-temperature scaling are consistent with the ZF-µSR data. These results show that around the quantum phase transition (at x = 0.6), the Griffiths phase crucially controls the low-temperature spin dynamics and is responsible for NFL behavior. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Saikat Sur V. Subrahmanyam(Thesis Supervisor) and Harshawardhan Wanare(Thesis Co-Supervisor) 14109272 Effect of local dynamical processes on spin chain dynamics 06.02.2020 (Thursday) 11:30 AM FB-382 (Physics Seminar Room)Quantum Information and communication aspect of quantum spin chains has been investigated over the last few years. From quantum information theory point of view, a quantum spin chain is a many-qubit system that can undergo various multiparty operations, both global and local along with background unitary evolution. A local operation that interrupts the background evolution can occur from a local quantum decohering process or a local coherent operation. Any local quantum dynamical process occurring on a many qubit state can change the distribution of correlations and entanglement structure. Various subparts of a multi-qubit system undergoing QDPs can be thought of as a simple model for decoherence in many body systems. We will focus on the propagation of the signal due to a locally intervening QDP in spin chains. The possibility of detecting the occurrence of the QDP at farther sites from the dynamical evolution of the state after the epoch time of the QDP will be discussed for various model Hamiltonians. The local QDP can interfere with the quantum state propagation in the system and change the quantum state transfer fidelity. The effect of QDP on state transfer fidelity will be highlighted for integrable and nonintegrable models. We will discuss various measures of bipartite quantum correlations and quantum information. We will also make a connection between the dynamics of bipartite quantum correlations and quantum information scrambling by computing tripartite mutual information (TMI) in different spin models. It has been observed that the sharing of bipartite correlations implies a negative value of the TMI. We show that the bipartite correlations can be increased depending on the location and the time of the QDP occurrence and local QDPs can cause scrambling even when the background dynamics in non scrambling in nature. Multiple incoherent QDPs intervening the dynamics at regular intervals can be thought of as if the system is interacting with an external decohering environment, while the action of multiple coherent QDPs though does not cause decoherence, can generate nonintegrability in the system. We will discuss the behaviour of the Loschmidt Echo, a measure of revival of a quantum state, to show a contrast between integrable and nonintegrable behaviour. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Pulastya Parekh (Physics, IITK)Arjun Bagchi16109277 Tensionless Strings: A perspective from the worldsheet January 13, 2020, Monday2:30 PMFB-382I will be speaking about the construction of the tensionless limit of closed bosonic string theory in the covariant formulation in the light of Bondi-Metnzer-Sachs (BMS) symmetry that rises as the residual gauge symmetry on the worldsheet. I will discuss how the classical analysis of the fundamental tensionless theory is viewed as a contraction of worldsheet coordinates. In the quantum regime, I will show that there are multiple ways to impose constraints to restrict the physical Hilbert space, which in turn lead to three distinct choices of tensionless vacua: the Oscillator, Induced and Flipped vacua. I will analyse these vacua in detail, commenting on various aspects like the central charges and the spectrum around each of them. I will make connections to higher spins, Hagedorn physics and ambitwistors for the three representations. I will also show how a Bose-Einstein like condensation occurs on the worldsheet while considering the vacuum in the induced representation. The analysis can also be extended to the closed superstring where the symmetry algebra is the Super BMS (SBMS). This is realised in two distinct ways for the tensionless string: the Homogenous and the Inhomogenous SBMS. I will also brieﬂy discuss some features of the quantum theory of tensionless superstrings, especially the map between the supersymmetric vacua, for both homogeneous and inhomogeneous cases. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Mr. Ravindra Kumar VermaProf. Pankaj Jain and Prof. Shashi Dugad (Thesis Supervisers)13109883 Search for a Charged Higgs Boson at 13 TeV in the CMS experiment at the LHC, CERN 27th Dec. 2019, Friday12:00 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)A search is conducted for a low-mass charged Higgs boson produced in a top quark decay and subsequently decaying into a charm and an antistrange quark. The data sample was recorded in proton-proton collisions at center of mass energy 13 TeV by the CMS experiment at the LHC and correspond to an integrated luminosity of 35.9 /fb. The signal search is conducted in the process of top-quark pair production, where one top quark decays into a bottom quark and a charged Higgs boson, and the other to a bottom quark and a W-boson. With the W boson decaying to a charged lepton (electron or muon) and a neutrino, the final state comprises an isolated lepton, missing transverse momentum, and at least four jets, of which two are tagged as b-jets. To enhance the search sensitivity, one of the jets originating from the charged Higgs boson is required to satisfy a charm tagging requirement. No significant excess beyond standard model predictions is found in the dijet invariant mass distribution. An upper limit is set on the branching fraction of the top quark decay to the charged Higgs boson and bottom quark for a Higgs mass between 80 and 160 GeV. In this talk, first I will briefly discuss the theoretical details of the standard model of particle physics and its extension in which the charged Higgs boson is produced. Then, a short description of the LHC and CMS experiment will be presented. Finally, I will discuss the detailed analysis for the search of light charged Higgs boson in the c and sbar channel at center of mass energy of 13 TeV from the collision data recorded in 2016 by the CMS experiment. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Mr. Sougata MardanyaProf. Amit Agarwal (Thesis Supervisor)14109882 First principle investigation of topological phases and properties of crystalline materials16th Dec. 2019, Monday11:00 AMFB382 (Prof. A. K. Raychaudhuri seminar hall)Following the discovery of topological insulator, materials with topologically protected surface state have been attracting significant attention due to their potential application in the field of electronics, spintronics and quantum computation. The topological protection of these surface states makes them robust against the local perturbation and ensure a backscattering free dissipation-less flow of current at the boundary. More recently, the focus has shifted from the gapped insulating state to the gapless semi-metallic state and even metallic topological materials. This was primarily motivated by the theoretical prediction, as well as experimental observation of Weyl and Dirac semimetals in crystalline materials. In addition to the topologically protected surface states, the key feature of these topological semimetal is linearly dispersing band touching points at discrete values of the crystal momenta in the Brillouin zone (BZ). The low energy excitations around these band crossings can be treated as quasi-particle, which are condensed matter equivalent of relativistic and massless Fermions. Additionally, due to the existence of non-trivial bulk and surface electronic states, the topological semimetals exhibit intriguing transport properties, such as large magneto-resistance, Chiral anomaly, and non-local transport signatures. In this thesis, I explore the connection between various topological semi-metallic phase appearing in crystalline solids. In general, the non-trivial band crossings in topological semimetals are dictated by crystalline symmetries of the system and the composition of the constituent atoms. The present thesis explores different topological phases in two- and three- dimensional crystalline materials using first principle calculation and low energy models. Additionally, I also explore the connection between different topological phases in the same family of materials, by systematically relaxing the crystal symmetries. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Anupam Ghosh Prof. Sagar Chakraborty (Thesis Supervisor)14109876 Comprehending Occasional Uncoupling Induced Chaotic 11th Dec. 2019, Wednesday04:30 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)The word `synchronization’ implies the evolution of two or more interacting systems in unison. In our real-life, flashing of fireflies, metabolic processes in cells, etc., are some examples where we observe this phenomenon. Synchronization in coupled chaotic systems got popular after 1990;although, in dynamical systems, this phenomenon was first discovered in the seventeenth century by C. Huygens. Also, one of the most counter-intuitive ideas came in 1993—-the occasional uncoupling-—where the participating systems are coupled occasionally, instead of continuously,and synchronization is observed. Interestingly, it is also observed that the occasional uncoupling leads to robust synchronization, unlike the continuous coupling. There are various occasional uncoupling schemes reported in the literature, viz., the stochastic on-off coupling, the on-off coupling, the transient uncoupling, etc. In many of the cases, the corresponding scheme’s parameter(s) for a given coupled chaotic systems is (are) chosen purely on the basis of trial and error. In this talk, we investigate and understand why and how the aforesaid occasional uncoupling schemes work. Initially, we understand how the transient uncoupling works for coupled chaotic oscillators from local linear stability analysis. The real part of the eigenvalues of the local Jacobian of the coupled oscillators are the indicators of the local expanding directions. If we suppress the local expansions(due to the inherent chaotic nature of the oscillator) using an accurately calculated coupling term,the synchronized state can be obtained. We use the aforementioned local analysis to explain the reasons behind the effectiveness of the transient uncoupling to obtain synchrony at higher coupling strength unlike the continuous coupling. Furthermore, there is stochastic occasional uncoupling scheme, where synchronized state can be established even after coupling the interacting systems randomly. In the process of our attempt to understand the mechanisms behind the success of occasional uncoupling schemes, we devise a hybrid between the transient uncoupling and the stochastic on-off coupling, and aptly name it the transient stochastic uncoupling—yet another stochastic occasional uncoupling method.Furthermore, through the transient stochastic uncoupling, we establish that one most probably needs to know the global statistics (viz., autocorrelation function of the corresponding chaotic time series)in order to understand the occasional uncoupling scheme. In addition, we investigate synchronization in coupled Hamiltonian systems. The absence of attractor in Hamiltonian systems leads to a different kind of synchronization: measure synchronization, when two Hamiltonian systems are coupled appropriately. In measure synchronization, two orbits—one from each of the identical coupled systems—have identical invariant measure on the portion of phase space that they share. Furthermore, we extend the concept of occasional uncoupling to the coupled Hamiltonian systems and employ the on-off coupling to overcome measure desynchronized state. Until our work, the applicability of the occasional uncoupling has been restricted in the domain of dissipative chaotic systems. In addition, the fast switching of the coupling term, we observe that the on-off coupling is equivalent to the continuous coupling with a proper scaling of the coupling strength parameter. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Rajan Kumar Singh Prof. Saikat Ghosh (Thesis Supervisor)12209067 Hybrid Nano-Mechanical Resonators 19th Nov. 2019, Tuesday09:30 AMFB382 (Prof. A. K. Raychaudhuri seminar hall)Mechanical resonators have played an important role in the field of sensing and metrology, from the classic 18th-century experiment by Cavendish estimating gravitational constant to present day NEMS (nano-electro-mechanical-systems) resonator based accelerometers, gyroscopes and sensors that are sensitive to masses at the level of a single molecule or even a proton. More recently, with the advent of quantum technologies along with advancement in fabrication techniques, engineered high-quality factor (Q) NEMS resonators with Silicon Nitride are being actively pursued as quantum limited force sensors at room-temperature. Additionally, such NEMS resonators offer on-chip integrability with existing optical, microwave and mechanical systems towards classical and quantum information processors. In this seminar, we will introduce a hybrid platform consisting of a high-Q Silicon Nitride (SiNx) resonator coupled to atomically thin, gate tunable, freely suspended graphene membranes. We find the platform to be extremely rich in dynamics and offer efficient applications in several scenarios. In one of the works, using the platform, we demonstrate a motion amplifier with record detection sensitivities. In particular, we demonstrate an amplification of 38 dB of a driven SiNx mechanical mode, when detected on graphene. Furthermore, with additional parametric driving we demonstrate thermomechanical squeezing of noise and record a measurement sensitivity of 3.8 femto-meter, integrated over a second. To further improve performance of the amplifier, we use resolved sideband cooling of the amplifier mode and its amplification due to intra-modal coupling, to effectively cool down the mode to 200 K. In another work, we observe the back action force of graphene to induce a giant mechanical nonlinearity on the SiNx resonator modes. In particular, the induced nonlinearity of SiNx hybrid mode is tunable with graphene's resonant frequency. We also observe a rich interplay between nonlinear damping and Duffing nonlinearity of the hybrid modes, leading to formation of a novel frequency comb beyond a parametric threshold. We analyze the comb using a theoretical model and numerical simulations. Our numerical simulations are in excellent agreement with observations. From the analysis, we estimate a giant eight order of magnitude enhancement in effective nonlinearity of SiNx resonator, due to its strong coupling with graphene. We will end the seminar with a discussion on possible routes of integrating our proposed hybrid nano-mechanical resonator as an essential element with the existing toolbox of opto- and electro mechanics towards storing and processing quantum information. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Pavan Kumar Prof. Asima Pradhan (Thesis Supervisor)11209067 In vivo and Ex vivo Detection of Head and Neck Cancer with Different Diagnostic Media and Spectroscopic Techniques 23rd Oct. 2019, Wednesday04:00 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)Light based techniques which are non-invasive by nature are being used to detect and diagnose various cancers. Fluorescence, Raman and diffuse reflectance spectroscopy and imaging are most widely used techniques for in vivo detection of head and neck cancer. Conventional technique with histopathological examination is the gold standard but it is an invasive process. In this thesis, in vivo and ex vivo studies for head-neck cancer detection using different spectroscopic techniques and diagnostic mediums (tissue and saliva) have been performed for a comparative assessment between the diagnostic media and among the different spectroscopic techniques. Fluorescence, Stokes shift and diffuse reflectance have been utilized for ex vivo oral precancer detection. Statistical tools such as principle component analysis (PCA), Mahalanobis distance analysis and receiver operating characteristic (ROC) analysis, applied on the spectra have classified normal, dysplastic and oral squamous carcinoma (OSCC) groups. Mahalanobis based classification on Stokes shift spectroscopy has shown higher sensitivity and specificity than the other two spectroscopic techniques. In the next chapter, human saliva is tested as a diagnostic medium for oral precancer detection using fluorescence and Stokes shift spectroscopy. Here, statistical tools PCA, linear discriminant analysis (LDA) are used for classifying normal, oral submucosal fibrosis (OSMF) and OSCC samples. Again Stokes shift spectroscopy has shown a better discrimination than fluorescence. Further, a comparative study between tissue and saliva using spectroscopic techniques and statistical tools (PCA, LDA, Mahalanobis and ROC) has revealed that human saliva is a reliable as human oral tissue. It may be thus an alternate diagnostic medium for oral cancer detection. Since collection of saliva is easy and non-invasive for patients and volunteers of any age group, this makes it an unique bio-fluid for diagnostic purpose. The work is further extended to in vivo clinical studies using an in-house fabricated fluorescence based spectroscopic device. With this device, measurements are performed on OSCC patients, dysplastic patients and normal volunteers and results indicate that porphyrin may be a key diagnostic marker. This device is further upgraded to an imaging system which helps to expose a larger area of oral cavity and thus avoids multiple scans. Preliminary results from imaging show promise. Further in the thesis, human saliva has been studied as a diagnostic medium for the inaccessible cancer of the throat. Samples are collected from SCC, dysplastic and normal groups. It has been found that SCC and dysplastic groups are differentiated from normal with higher values of sensitivity and specificity. It thus shows that saliva can be an excellent diagnostic medium for throat cancer detection. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Utso BhattacharyaProf. Amit Dutta (Thesis Supervisor)14109885 Aspects of non-equilibrium dynamics of closed quantum systems: information, phase transitions and topology 23rd Oct. 2019, Wednesday04:00 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)Recently, there has been a burst of activity in the field of topological semi-metals and insulators, which are new phases of quantum matter. One possible way of attaining the new phases is to periodically drive a parameter of a quantum many-body system. Instead of periodic driving, one may also choose to induce non-equilibrium dynamics by suddenly changing or quenching the same parameter. Therefore, the key idea here is to use out of equilibrium dynamics to probe exotic quantum phase transitions, which may or may not have an equilibrium counterpart. The goal of this talk is therefore to look at some aspects of non-equilibrium dynamics brought about by quenches and time-periodic drives, focussing on the study of non-interacting topological and interacting quantum light-matter systems. We use quantum information theoretic measures to not only probe non-equilibrium criticality such as “dynamical quantum phase transitions” but also to look for signs of classical chaoticity in the systems mentioned above. Furthermore, in such systems, in addition to the emergence of new non-equilibrium quantum phase transitions and novel topological behaviour, we also question the effect of out of equilibrium dynamics on the preservation of existing equilibrium topological behavior. Thus, this interconnection between information, phase transitions and topology is the central theme of this thesis. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Bhavya MishraProf. Amit Dutta (Programme Coordinator) and Prof. Debashish Chowdhury (Thesis Supervisor)13109063 Molecular motor traffic on Nucleic Acid Strands: Traffic Congestion in Decoding and Recoding of Genetic Message 25th July 2019, Thursday03:00 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)Life is sustained in a state far from thermodynamic equilibrium where a non-vanishing flux of energy and matter exist. In spite of the stochastic nature of all key processes, there is a surprisingly high level of coordination among the molecular machines that drive intracellular processes. Even when the system attains a non-equilibrium steady state, the directed movement of motors are noisy. Study of movement of molecular motors along a nucleic acid strand under the condition of heavy traffic congestion (where multiple motors walk along the track simultaneously) is of great interest. Moreover, in addition to the random errors caused by the stochasticity of the various steps of gene expression, systems are also designed for inducing some "programmed" errors. Programmed translational errors are an integral part of what are known as "non-canonical" translation. Translation of genetic message encoded chemically in the sequence of mRNA template is carried out by a molecular machine called ribosome. Each ribosome treats the template mRNA as a track and walks along it, in a step by step manner, albeit stochastically, in an environment that is far from thermodynamic equilibrium. In this thesis we develop mathematical models for non-canonical translation processes. To capture the essence of the simultaneous directed movement of multiple ribosomes along the same mRNA, we model the system as a single (or multi-) species totally asymmetric simple exclusion process (TASEP) of hard rods under the open boundary conditions. We have developed a TASEP-based model and studied the phenomenon of programmed ribosomal frameshift (PFA). We apply the Master equations technique under mean-field approximation (MFA) to study the efficiency of frameshift under the traffic congestion, density profiles of ribosomes along the mRNA track and rate of protein synthesis for frame-shifted and non-frame shifted ribosomes. The usefulness of the methodology is also tested by Monte-Carlo (MC) simulations. Developing another TASEP-based model, we have investigated the consequences of phenomenon of internal ribosome entry site (IRES). In this study, we develop a new methodology of modelling, based on non-homogeneous TASEP, by dividing the whole track into multiple identical homogeneous segments. We also study the fluctuations in the IRES model. We apply the technique of matrix product ansatz (MPA) to get the exact expressions of average current, the fluctuations around the mean value, and some other measurable quantities. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Girish KulkarniProf. Anand Kumar Jha14109264 Novel tools for characterizing photon correlations in parametric down-conversion 16th July 2019, Tuesday03:30 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)Over the last two decades, numerous experimental studies have demonstrated that quantum systems are capable of performing information processing tasks – such as teleportation, superdense coding, secure communication, and efficient integer factorization – which are widely considered to be impossible with classical systems. These enhanced capabilities are believed to arise at least partly from the nonlocal correlations present in entangled quantum systems. While the intrinsic correlations that underlie interference effects are a general feature of both quantum and classical systems, the nonlocal correlations of entangled quantum systems have no known classical counterpart. In addition to being a vital resource for quantum applications, these nonlocal correlations also pose fundamental questions regarding the consistency and completeness of the quantum description of physical reality. Currently, the most important experimental source of entangled states is parametric down-conversion – a second-order nonlinear optical process in which a single photon, referred to as pump, is annihilated in its interaction with a nonlinear medium to create a pair of entangled photons, referred to as signal and idler. This talk will draw on concepts from optical coherence theory and quantum information theory, and present some tools and techniques for characterizing the correlations of the signal-idler photons and their relationship to the intrinsic correlations of the pump photon in various degrees of freedom. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Soumendu GhoshProf. Satyajit Banerjee (Programme Coordinator) and Prof. Debashish Chowdhury (Thesis Supervisor)13109076 Polymerase traffic on DNA tracks: stochasticity, interference and conflict resolution26th July 2019, Friday11:00 AMFB382 (Prof. A. K. Raychaudhuri seminar hall)Each cell, the basic unit of life, can be thought of as a micro factory where the molecular machines work together in a coordinated way. Machines take chemical energy as input and do mechanical work. Transcription is a process by which a single stranded DNA is copied into a RNA by a molecular machine, called RNA Polymerase (RNAP). Each RNAP synthesizes a copy of the RNA in a step-by-step manner where, in each step, it elongates the RNA by one monomeric subunit and moves ahead by one step on the template DNA that also serves as the track for its motor-like motion. The transcription process is stochastic. Moreover, none of the sites on the template DNA can be covered simultaneously by more than one RNAP motor. The traffic-like collective movement of RNAP on DNA has been modelled as an exclusion process. In this thesis, I have studied the phenomenon of transcriptional interference (TI) theoretically by introducing exclusion models of two distinguishable species of hard rods with their distinct sites of entry and exit under open boundary conditions. In the first model, both of the species of rods move in the same direction whereas in the other two models, they move in the opposite directions. We have also introduced a novel exclusion model to calculate mean time needed for successful search for a specific target site on a DNA strand by transcription factors in the presence of active crowders. Finally, I'll also describe another two-species exclusion model that we have introduced to describe the key features of the conflict between the RNAP motor traffic, engaged in the transcription of a segment of DNA, concomitant with the progress of two DNA replication forks on the same DNA segment. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Bharat Lal MeenaProf. Asima Pradhan10109064 Development and Testing of a Portable Device for In-vivo Detection of Cervical Pre-cancer Based on Intrinsic Fluorescence18th June 2019, Tuesday11:00 AMFB382 (Prof. A. K. Raychaudhuri seminar hall)Optical detection methods such as fluorescence spectroscopy, elastic scattering, and imaging have the potential for early diagnosis and are able to monitor cellular and chemical changes with disease progression. Fluorescence spectroscopy is one of the relatively sensitive methods to probe subtle biochemical changes. Since the fluorescence from tissue is significantly modified by absorption and scattering effects at both excitation and emission wavelengths, important diagnostic information of biochemical changes with disease progression are hidden. We have earlier developed an algorithm based on measured polarized fluorescence and polarized elastic scattering to extract intrinsic fluorescence, validated and tested it on tissue-mimicking phantoms and biological tissue samples with a commercial spectrofluorometer. The intrinsic fluorescence is free from the distortion effects and hence provides more precise information about biochemical changes compared to the co-, cross-, and unpolarized spectrum. Based on this concept a portable device has been designed, fabricated and validated with tissue mimicking phantoms and tested on human tissue samples. A 405 nm diode laser and white light sources have been used to collect the polarized fluorescence and polarized elastic scattering data respectively. Flavin adenine dinucleotide (FAD) is the dominating fluorophore at this wavelength with fluorescence peak around 510 nm. The promising results of this study on biopsy and total hysterectomy samples have shown that the device has the potential to use it for in-vivo detection of cervical pre-cancer. Principal component analysis (PCA) has been applied to reduce the dimension of the data without loosing original information. To discriminate the different grades of the samples linear discriminant analysis (LDA) and Mahalanobis distance (MD)have been applied. Preliminary in-vivo testing of the device on patients was conducted in the GSVM medical college as well as AIIMS Bhubaneswar in the presence of medical experts. The comparison of spectroscopic data and histopathology of each sample was done and used as data for further analysis. These ex-vivo and preliminary in-vivo results indicate the efficacy of the handheld device for screening cervical pre-cancer using intrinsic fluorescence. With disease progression the contribution from different fluorophores changes and this information can be used as biomarker for discrimination of different grades of the samples. In this study Nelder-Mead method has been utilized to fit the spectral profile with a Gaussian to decouple the different fluorescence bands of contributing fluorophores (FAD and porphyrin). The change in concentration of FAD during disease progression manifests in the change of the ration of total area to FWHM of its Gaussian profile. The results show that FAD may be an effective biomarker in discriminating different grades of samples. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Sourav BiswasProf. Jayita Nayak and Prof. Anjan K Gupta12209070 Phase Dynamic Regime in micro-SQUIDs & Its Optimization for Probing Nanomagnetism13th June 2019, Friday04:00 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)Superconducting QUantum interference devices (SQUID) are the most sensitive magnetic flux detectors to date. The miniaturized micro- or nano-SQUIDs, with weak links (WLs) as Josephson junctions, can lead to magnetic moment resolution down to below an electron's magnetic moment. Thus, m-SQUIDs are of recent research interest for probing nanomagnetism at low temperatures. A SQUID operates as a flux-to-voltage transducer in its phase dynamic regime. This regime is the most suitable as it offers good flux resolution, fast response and with conventional electronics. However, the Joule heating effects substantially affect this regime leading to even its non-existence at low temperatures unless the devices are carefully optimized. This thesis deals with the optimization of m-SQUIDs by systematically understanding the phase-dynamics in a WL with incorporation of the heating effects. A dynamic thermal model (DTM) reveals a new phase dynamic regime defined by two different types of retrapping currents. More importantly, a single dimensionless parameter \beta is introduced that determines the supercurrent amplitude in the dissipative phase-dynamic state and hence the voltage modulation magnitude. We understand and verify the DTM in the transport results from Nb-based m-SQUIDs, fabricated using electron beam lithography technique. Magnetic flux dependent voltage is observed even in the hysteretic regime of optimally designed m-SQUIDs. The experimental observations are quantitatively explained using the DTM. In the next part, guided by the DTM, we demonstrate a new handle of eliminating thermal hysteresis very efficiently down to 1.3 K in WLs with an inductive shunt. An inductive shunt with well-chosen shunt parameters introduces an unusual nonlinear dynamics that destabilizes an otherwise stable fixed point through a Hopf bifurcation in the dynamic-dissipative branch of the WL. This leads to a nonhysteretic behavior with much-improved voltage oscillations in intrinsically hysteretic m-SQUIDs, as observed in adequately (inductively) shunted devices. Above a threshold shunt-inductance value, the relaxation oscillations in SQUID voltage appear consistent with the model. Some preliminary results on low-temperature nanoparticle Magnetometry using thus optimized m-SQUIDs, but intrinsically hysteretic, are discussed towards the future research prospects. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Pritam Kumar RoyProf. Krishnacharya13109882 Surface and Interfacial Phenomena on Soft Elastomeric Surfaces3rd May 2019, Friday10:00 AMFB382 (Prof. A. K. Raychaudhuri seminar hall)In our daily life, nature shows various kind of instabilities in form of different patterns, for example sand dunes, sea waves, cloud pattern, breaking of water into droplets, wrinkling of skin to name a few. In earlier times, instabilities were often perceived as a failure mechanism, but recently patterns formed by various instabilities have found their way in many practical applications e.g. photonic crystals, micro/nano-fabrication, controlling of wetting behavior, tuning of surface adhesion etc. In my presentation, I will discuss about various interfacial phenomenon on one dimensional wrinkles formed with soft elastic substrates. The most important characteristic of wrinkle is their mechanically tunable topography. Wetting behavior on such one dimensional wrinkles is drastically different due to underlying topography than that on a flat surface. Since the topography of wrinkles can be manipulated mechanically, so can be their wetting behavior. Micron sized wrinkles are fabricated on elastic polymer surfaces with nanoscale roughness generated by replicating through a nano-master pattern. Due to hierarchical roughness of same type material, wrinkle surfaces show stable and robust superhydrophobic behavior. Also their surface wettability con be tuned reversibly, between superhydrophobic and hydrophobic states, applying external mechanical strain. Subsequently, I also demonstrate how to unite superhydrophobic, superoleophobic and lubricating fluid infused slippery behavior on copper oxide nanostructure based surfaces. Following Nepenthes pitcher plants, slippery surfaces are fabricated by coating a suitable lubricating fluid on rough surfaces. Slippery surfaces, when fabricated on wrinkles, provide freedom to tune the slippery behavior as a function of mechanical stress. Towards the end, I will talk about some fundamental aspects of lubricating fluid coated slippery surfaces such as Neumann’s angle at the three phase contact point, kink formation due to deformation at the three phase contact point, variation of lubricating film thickness between underneath the drop and outside the drop and skin formation due to cloaking of drops. I will discuss in details the origin and investigation of these characteristics and comparison with the state of the art results. I will also demonstrate some technological applications of superhydrophobic, superoleophobic and slippery surfaces. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Saikat ChakrabortyProf. Kaushik Bhattacharya14109881 Bouncing Cosmology in f(R) Theories of Gravity26th April 2019, Friday2:45 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)Although the currently dominant paradigm for the early universe, the inflationary paradigm, has been successful in resolving many conceptual problems of big-bang cosmology, it is by no means free from flaws. Problems such as super-Planckian excursion of the inflaton in the field space for large field models, assumption of special initial conditions and existence of an initial singularity plagues the inflationary paradigm. This has fuelled the search for alternatives to inflation, among which a popular one is the bouncing scenario. Bouncing cosmologies do not have a singularity in it, and also solves almost all the problems that inflation does. But the bouncing scenario also has it's own share of problems, It is difficult to implement a bouncing scenario in the framework of general relativity with ordinary matter satisfying the familiar energy conditions; typically one needs an ultra-stiff and a null energy condition violating matter component to model a successful bounce. Furthermore, considering general relativity as the theory of gravity in that energy scale is also questionable. Here lies the motivation to consider the bouncing scenario in modified gravity theories, the simplest being the f(R) gravity. In my talk I will address the attempts to implement a bouncing scenario in simple polynomial f(R) gravity theories, and it's advantages and disadvantages. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Saikat SurV. Subrahmanyam and H. Wanare14109272 Effects of local dynamical processes on spin chain dynamics 3 April, 2019, Wednesday10:00 AMFB-382 Quantum Information and communication aspect of quantum spin chains has been investigated over the last few years. From quantum information theory point of view, a quantum spin chain is a many-qubit system that can undergo various multiparty operations, both global and local along with background unitary evolution. A local operation that interrupts the background evolution can occur from a local quantum decohering process or local coherent operation. Any local quantum dynamical process occurring on a many qubit state can change the distribution of correlations and entanglement structure. The various sub parts of a multiqubit system undergoing non unitary operations can be thought of as a simple model for decoherence in many body systems. We will focus on the propagation of the signal of the intervening QDP in the chain. The possibility of detecting the occurrence of the QDP at farther sites from the dynamical evolution of the state after the epoch time of the QDP will be discussed for various models.The local QDP can interfere with the quantum state propagation in the system and change the quantum state transfer fidelity. The effect of QDP on state transfer fidelity will be highlighted for integrable and nonintegrable models. The Loschmidt Echo is used as a measure of revival of a quantum state when imperfect time reversal procedures take place during the evolution. Multiple incoherent QDPs intervening the dynamics at regular intervals can be thought of as if the system is interacting with an external decohering environment, while the action of multiple number of coherent QDPs though does not cause decoherence can generate nonintegrability in the system. We will discuss various measures of two party correlations and some multiparty correlations in spin systems. Depending on the location and time of QDP how does these correlation measures between two parties change will be addressed. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Raghwendra KumarProf. S. Anantha Ramakrishna12109067Metamaterial and Plasmonic Microstructures for Controlling Infra-red Properties22nd Feb 2019, Friday10:30 AMFB382 (Prof. A. K. Raychaudhuri seminar hall)Micro/nano-fabrication techniques have enabled new possibilities with plasmonic structured materials and metamaterials as they make possible highly reproducible structures with complex geometries and functionalities. Applications include solar cells, photo-detectors, band selective thermal emitters, chemical and bio-sensors, and many more. Here we discuss the design, fabrication, and characterization of plasmonic and metamaterial structures that show the electromagnetic response at infra-red frequencies. Structured conducting surfaces can support surface electromagnetic modes even at low frequencies similar to surface plasmon polaritons at optical frequencies that exist at the interface between plasmonic and dielectric materials. Some novel applications that utilize such “spoof” surface plasmons at infra-red frequencies are developed in this work, for enhancing the coupling to photo-detectors and for developing surfaces with band-selective infra-red emittance. To enhance the transmittance through a subwavelength hole array in a thin gold film, dielectric micro-domes were embedded into the hole. A simpler metamaterial absorber consisting of a photoresist/polymer disk array covered by a double-layer of (ZnS/Au) has been developed to show band selective absorption or emission. Fano-like resonance has been observed in a metamaterial absorber consisting of an array of photoresist disks on silicon substrate covered by a tri-layer of (Au/ZnS/Au) due to the interference of bright mode (cavity mode) and dark mode (Wood’s anomalies and guided mode resonance). Soft-lithography techniques have been used to fabricate the metamaterial absorber over large areas of about 100 cm2. To fabricate more complex micro-structures, a two-photon absorption based laser writing system with sub-micrometer resolution has been developed using an inexpensive sub-nanosecond laser for two/three-dimensional structuring in photosensitive materials. Speaker:Supervisors:Roll No.:Title:Date:Time:Venue:Abstract: Ritu GuptaProf. Zakir Hossain and K.P. Rajeev12109068Physical Properties of Charge Density Wave Superconductor LaPt2Si222nd Feb 2019, Friday10:30 AMFB382 (Prof. A. K. Raychaudhuri seminar hall)Charge density wave (CDW) is a periodic modulation in electronic charge density which is found in some low dimensional materials due to strong Fermi surface nesting. However, three dimensional materials with layered structures also sometimes show CDWs due to their quasi-low dimensional Fermi surfaces. CDW state opens up a gap (or a partial gap) at the Fermi surface which leads to a metal to insulator (or semimetal) transition in contrast to a superconducting transition which manifests itself with an infinite electrical conductivity. So, systems which exhibit both CDW and superconductivity (SC) are of great interest. LaPt2Si2 shows a first order structural transition from tetragonal to orthorhombic accompanied by a CDW transition at 112 K along with a superconducting transition at around 1.5 K. After a brief introduction of the CDW, I shall present the structural details of LaPt2Si2 which crystallizes in CaBe2Ge2 type structure. Absence of mirror symmetry in this material makes it suitable candidates for non-centrosymmetric (NC) superconductivity. We present temperature dependent electrical resistivity, magnetization, thermoelectric power, thermal conductivity, thermal expansion measurements etc. Competing nature of CDW and SC has been presented through application of negative chemical pressure in LaPt2(Si1-xGex)2. We have also measured anisotropy in various physical properties on the single crystals grown using Czochralski pulling method. Unconventional nature of the superconductivity has been confirmed in temperature dependent upper critical field and specific heat measurements. BCS fitting with various models including nodal gap (d-wave), isotropic gap (s-wave) and multigap (s+s or s+d) to the specific heat below TC will be discussed. Further analysis of the superconducting gap has been done through muon spin relaxation/rotation (MUSR) technique. Through zero field MUSR spectra, it has been shown that time reversal symmetry is preserved in the system. Magnetic penetration depth, which has been calculated through transverse field MUSR spectra, further provides the symmetry of the superconducting gap. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Anil Kumar SinghProf. Anjan Kumar Gupta12109062Role of Interface States on Electronic Properties of Graphene3rd Jan 2019, Thursday4:00 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)Inhomogeneity due to disorder in graphene prevents us from precisely accessing its Dirac point. In graphene field-effect-transistors with SiO2 gate, the disorder and residual doping are mainly attributed to the defects and/or trap states at graphene-SiO2 interface. Potential landscape created by charged defects at graphene-SiO2 interface leads to electron and hole puddles in graphene. Such inhomogeneity is undesired as it limits the carrier mobility. Moreover, due to charge transfer between graphene and adsorbed species on SiO2 surface, the graphene field-effect-transistors show significant hysteresis. The hysteresis, on the other hand, offers applications in data storage. Thus understanding these interface charge traps, particularly for finding ways to control and probe the charge stored in them, is important. Here I shall discuss the role of interface states on electronic properties of graphene on SiO2 substrate using STM/S and transport measurements. We measure the local tunnel spectra and conductance maps as a function of gate voltage. The local tunnel spectra show certain features that evolve with the back gate voltage. This evolution is understood using the effect of tip-gating and interface states. We find that a broad energy dependent interface states' density, Dit(E) that leads to an effect similar to a reduction in the Fermi velocity while a narrow Dit(E) leads to the pinning of the Fermi energy close to the Dirac point. The spatial evolution of electronic inhomogeneity with back-gate voltage shows a systematic reversal of contrast in some places in the STS maps and sharp changes in cross-correlations between topographic and conductance maps, as the Fermi level approaches the Dirac point. These are attributed to the change in charge-state of interface defects. The spatial correlations in the conductance maps are described by two different length scales whose growth during approach to Dirac point shows a qualitative agreement with the linear screening theory of graphene. Further, these interface states are utilized for reversibly controlling the doping in graphene by manipulating the charge state of the interface defects. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Vinay M MProf. Gautam Sengupta14109883Holographic Entanglement Negativity in AdS3/CFT227. 12. 2018 ( Thursday)11:00 AMFB382 (Prof. A. K. Raychaudhuri seminar hall)In the past decade, the study of holographic entanglement entropy through the Ryu-Takayanagi cnjecture in the context of the AdS-CFT correspondence has revealed a novel connection between space-time geometry and quantum entanglement in dual conformal field theories. It is well known however in quantum information theory that the entanglement entropy which serves as a unique entanglement measure for bipartite pure states, ceases to be valid for mixed quantum states. In this context, entanglement negativity is a significant computable quantum information theoretic measure that characterizes the upper bound on the distillable entanglement for bipartite mixed states.In this thesis, we propose a holographic entanglement negativity conjecture for bipartite pure and mixed states of a (1+1)-dimensional conformal field theories (CFT) through the AdS3/CFT2 correspondence. Application of our conjecture to specific examples of pure vacuum state and finite temperature mixed states exactly reproduces the corresponding replica technique results in the large central charge limit of the CFT. Subsequently, we establish the consistency of our conjecture through the large central charge analysis of the entanglement negativity for a relevant mixed state configuration. We further extend our holographic entanglement negativity conjecture to time-dependent bipartite states of (1+1) dimensional CFTs dual to non-static bulk AdS3 configurations. Once again our results exactly reproduce the corresponding replica technique results for suitable examples in the large central charge limit. We briefly allude to a possible higher dimensional extension of our conjecture in a generic AdS(d+1)/CFTd scenario. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Tripurari SrivastavaProf. Joydeep Chakrabortty14109883Phenomenological and Theoretical aspects of BSM scalar in the light of low energy constraint20.11.2018 (Thursday)12:00 PMFB382 (Prof. A. K. Raychaudhuri seminar hall)In 2012 the last piece of the most successful theory of elementary particles, the "Standard Model", was discovered in the form of a neutral massive scalar, the "Higgs", at the Large Hadron Collider (LHC). Though this completes the story of the SM, many observations hint that there must be a theory Beyond the SM (BSM). Most of the proposed BSM frameworks contain an extended scalar sector to break extended gauge symmetries and(or) generate masses for the exotic fermions. Thus, it is very important to analyse the impact of these "BSM" scalars. We have performed a model independent analysis of a doubly charged scalar coupled with charged leptons and gauge bosons. Restricting ourselves to the regime of conserved charged-parity (CP), we assume only a few nonzero Yukawa couplings between the doubly charged scalar and the charged leptons. Our choices allow the doubly charged scalar to impinge on low-energy processes like anomalous magnetic moment of muon and a few possible charged lepton flavour violating (CLFV) processes. These same Yukawa couplings are instrumental in producing same-sign dilepton signatures at the LHC. Our findings would be useful to test the phenomenological significance of a doubly charged scalar by using complementary information from muon (g − 2), CLFV and collider experiments.Having discussed that, I will move on to a specific scenario of BSM. The left-right symmetric model is considered to be an excellent candidate for BSM physics. This model contains doubly charged scalars accompanied by many other exotic particles. I will discuss the theoretical constraints on the extended scalar sector, namely perturbativity, unitarity and vacuum stability. As these exotic particles are heavy, there is a possibility that they appear as off-shell propagators in different low energy processes. In the presence of off-shell propagators, the decay width calculation is a little involved. I will talk about the proposal which can ease this computation. Though this prescription was proposed initially for off-shell scalars, it has been later modified such that the effect of off-shell particles having any quantum numbers can be encompassed. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Ashutosh Kumar SinghProf. Amit Agarwal13109879Nonlinear and anisotropic optical conductivity in Dirac materials15.11.2018 (Thursday)10:00 AM(Refreshment @ 09:45 AM)FB382 (Prof. A. K. Raychaudhuri seminar hall)Optical response, in general, can be quantified in terms of optical susceptibility or optical conductivity which reveal the system’s behavior to an external field in terms of Polarization or Current density respectively. Such description of response function is best provided in terms of density matrix, which is used to describe the carrier dynamics via Equation of Motion (EOM) approach. The EOM yields a set of coupled differential equations popularly known as Optical Bloch Equations (OBEs). Most of the studies in Nonlinear optics have been carried out either by direct integration of OBEs or they involve perturbative treatments in powers of the electric field. These kinds of study are very useful for ultrafast spectroscopy when the timescale over which a pulse lasts is approximate of the order of few femtoseconds. However, if a Continuous Wave (CW) is used instead of ultrafast pulses and different kind of decay channels are replaced by phenomenological damping in OBEs, the time evolution of the system attains quasi-equilibrium steady state owing to the balance between continuous excitation of carriers and relaxation due to damping. This approach leads to rather elegant and intuitive results which can easily be manifested in various kinds of optical transport measurements. The present thesis explores the nonlinear optical response in Dirac materials in the presence of a CW light. We develop an analytical framework to obtain the steady state density matrix of a two band system in general. Using the steady state solution of the photo-excited density matrix, we study in detail about the nonlinear optical conductivity, polarization rotation, anisotropic photo-conductivity and wave-mixing effects in graphene. Interestingly the well known Kubo formula for optical conductivity, appears as a limiting case (linear response and Infinite coherence time) of our more general non-linear formulation. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Himanshu GuptaProf. Sagar Chakraborty13209861Fluid Instabilities in Inhomogeneous Intracluster Medium02.11.2018 (Friday)11:30 AM(Refreshment @ 11.15 AM)FB382 (Prof. A. K. Raychaudhuri seminar hall)Galaxy clusters are the largest known gravitationally bound objects in the Universe. The convective instabilities in the intracluster medium (ICM), a weakly collisional dilute plasma permeating the galaxy cluster, is of main interest in the thesis. It consists mostly of fully ionized hydrogen ions, helium ions, and the free electrons. The magnetic field in the ICM is too weak to be of any dynamical significance. However, it is capable of making the heat conduction as well as the particle diffusion strongly anisotropic. This anisotropy helps the system in becoming convectively unstable, arguably leading to a turbulent state and thus to a more "conducting" state. The ICM facilitates the heat-flux buoyancy driven instability (HBI) & the magnetothermal instability (MTI) depending on the sign of the temperature gradient along with specific configurations of the gravity and the very weak magnetic field. The HBI dominates in the core of a galaxy cluster while the MTI in the outskirts. We have applied the stability analysis techniques, usually employed for studying the Rayleigh–Bénard convection, to analyze the MTI and the HBI. Our formalism has made it easier to incorporate the explicit boundary conditions used in the numerical experiments of the ICM. We find how the marginal state depends on the plasma-beta factor, which mode becomes unstable first, etc. The process of diffusion may lead to helium sedimentation in the cluster core. Hence, the composition of the ICM can vary radially from the cluster center. In order to account for such inhomogeneity and its possible effect on the MTI and the HBI, we have introduced a concentration gradient and magnetic field-mediated anisotropic diffusion in our model. Our studies show that a suitable concentration profile of helium ions is capable of stabilizing the aforementioned instabilities. Subsequently, we have also found that many cool-core galaxy clusters may be in the supercritical regime and hence there is a possibility of the existence of helium fingers (defined in line with the salt fingers found in the oceans) in the core of the ICM conducive to the HBI. A part of the thesis work is focused on the outskirts of the cluster where the MTI is supposedly in action. We show that the outcomes of the stability analysis as done on the plasma in the sheared state due to, say, the ubiquitous differential rotation. We have analyzed how the stability of such a sheared state depends on the instabilities generated due to the concentration gradient of the plasma. We also show that a suitable concentration gradient can counteract both the Kelvin-Helmholtz instability and the magnetorotational instability as found in the sheared ICM. References: [1] S. A. Balbus, Astrophys. J., 562 (2):909, 2001. [2] E. Quataert, Astrophys. J., 673:758, 2008. [3] M. E. Pessah and S. Chakraborty, Astrophys. J., 764:13, 2013. [4] H. Gupta, S. K. Rathor, M. E. Pessah, and S. Chakraborty, Phys. Lett. A, 380 (31-32):2407, 2016. [5] S. Sadhukhan, H. Gupta, and S. Chakraborty, Mon. Notices Royal Astron. Soc., 469 (3):2595, 2017. [6] H. Gupta, A. Chaudhuri, S. Sadhukhan, and S. Chakraborty, Mon. Notices Royal Astron. Soc., 474 (1):636, 2018. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Shamik GhoshProf. Pankaj Jain12109875Cosmology beyond the Cosmological Principle25.10.2018 (Thursday)11:00 AM(Refreshment @ 11:45 AM)FB382 (Prof. A. K. Raychaudhuri seminar hall)The Cosmological Principle implies that the distribution of matter and radiation in the Universe is spatially isotropic and homogeneous. The Cosmological Principle is an assumption and is not a manifestation of symmetries of the underlying theory. For our (3+1) dimensional spacetime, this assumption allows us to treat the 3-space as a maximally symmetric subspace which gives us the Friedmann metric. Thus it is a foundation of the very successful LCDM model of cosmology, which is built on the Friedmann metric. However over the last decade gathering evidence suggest that on the largest length scales there is a departure from statistical isotropy.One key observation has been the hemispherical power asymmetry (HPA) in the Cosmic Microwave Background (CMB) temperature. The HPA implies a difference in power of the CMB in the two hemispheres about (l=232 deg, b=–14 deg). I will discuss some theoretical expectations for an identical HPA which might be observable in the CMB polarization signal. I will talk of estimators we introduced to test this signal in both the harmonic space and the pixel space. I will also discuss in brief the effects of sky masking and mitigating estimator bias. I will present our current limits on the HPA in CMB polarization from PLANCK 2015 polarization data. I will also present forecasts for the detection of this signal from future CMB experiments like CORE.The dipole in the NVSS radio galaxy catalog is another observation that violates the assumptions of statistical isotropy. Due to the local motion of our frame of observation, we do expect to observe a dipole in both the CMB and the radio sky. But the velocity from the radio continuum survey is found to be much larger than the measurements from CMB. This can arise if there is an intrinsic dipole in the large scale structures. I will discuss the theoretical possibility of such an intrinsic dipole arising from a large wavelength mode perturbation. I will also discuss the measurement of variation of the log N - log S slope. Finally I will cover some updates to these work and future course of action. Speaker:Supervisor:Roll No.:Title: Date:Time:Venue:Abstract: Sanghamitro ChatterjeeProf. Sudeep Bhattacharjee13109884Studies on surface wettability of atomically heterogeneous systems created by microwave plasmagenerated low energy inert gaseous ion beams21.08.2018 (Wednesday)12:00 AM(Refreshment @ 11:45 AM)FB382 (Prof. A. K. Raychaudhuri seminar hall)Ion beams are increasingly becoming one of the most important tools, with the rapid development of many basic, applied and inter-disciplinary research in physics and material science. Researchers use ion beams having energies ranging from a few eV to some MeV for studying the basic and applied aspects of ion-matter interactions. Low energy (0 - 5 keV) ion beams (LEIB) have attracted special attention due to their nature of interactions with matter, where, a few sub-surface atomic layers can be tailored. Thus, materials irradiated by LEIB exhibit interesting surface properties that were lacking in the unirradiated condition.Surface wettability, which is the spreading and gathering of a liquid drop on a solid surface, is an important surface property that is determined by the interplay between the cohesive and adhesive intermolecular interaction between the solid and liquid surfaces in contact. Thus, wetting is related to the surface free energy of a solid and indirectly dictates its adhesiveness. Thereby, it contains useful information about the surface topography, heterogeneity, and composition. The phenomena of wetting is ubiquitous in nature; starting from non-wetting lotus leaf to the lubricating fluid present in the cornea of human eyes, and thereby has drawn significant attraction of scientists. It is also important in many industrial processes such as lubrication, self-cleaning, and microfluidics. Hence, controlling wettability of a solid surface is an important requirement. Conventionally, wettability is controlled by engineering of surface topography, chemical texture or chemical coating, which widely falls under the so-called Wenzel and Cassie-Baxter regimes. Textured surfaces are expected to produce significant hysteresis brought about by the defect induced pinning.The present research demonstrates a distinct way of controlling and studying wettability. Here, metallic (Au, Cu, Al) thin films (~ 200 nm) are irradiated by low energy (0.5 keV) noble gas (Ar, Kr, Ne) ion beams, obtained from a microwave induced plasma. It has been found that the irradiated metallic films become hydrophobic and the large scale tunability in the wetting behaviour cannot be explained by the nanometer scale roughness generated due to irradiation. However, the implanted noble gas ions stay within the near surface atomic layers and are responsible for an appreciable modification in the dispersive intermolecular interaction in the near surface region, leading to a reduced surface free energy which explains the observed transition. A few degrees of hysteresis is also observed, which is believed to have stemmed from retention of a thin liquid layer behind the receding three-phase contact line, and that the nanoroughness or the atomic scale heterogeneity do not induce appreciable pinning.Since, the present investigation demonstrates utilization of a microwave plasma ion source for studies on surface wetting, it represents a cross-disciplinary area of research- where the physics of plasmas, ion beams and surface wettability have interplaying roles. Hence, the systematic study follows the following sequences: first, a computational work has been performed on a simple 1-D wave induced plasma where it has been found that the effect of varying length and neutral gas pressure have self-similar effects on various discharge parameters, namely, the electron energy probability function, electron temperature and plasma density, which was further validated by experiments on the existing microwave plasma setup. Thereafter, successful extraction of beams have been demonstrated experimentally with the available set up and different regimes of the current-voltage characteristic curve was analysed, followed by measurements of beam flux at different plasma conditions. It has been observed that the low voltage (≤ -50 V) regime of the beam characteristic is due to space charge limited flow, which deviates from the well-known Child-Langmuir law. The high voltage (-50V to -3kV) regime is Schottky-like, which stems from penetration of extraction electric field through the plasma electrode apertures. These are original researches carried out and also aimed at optimization of the system. These studies were followed by the investigations of wetting phenomena as described above. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Ms. SuchitaProf. R.Vijaya12109876Continuous wave, broadband, erbium-doped fiber laser and study of its temporal coherence characteristics08.08.2018 (Wednesday)08:30 AMFB382 (Prof. A. K. Raychaudhuri seminar hall)A spectrally broadened light output can be generated from an erbium doped fiber ring laser (EDFRL) by introducing a specialty fiber inside the cavity, and thus enabling the third order nonlinear optical process of four wave mixing (FWM). The intra-cavity FWM will generate new sidebands which will undergo further gain and generate more mixing products during the multiple roundtrips, leading to spectral broadening. During this process, the gain and the new sidebands play an important role in modifying the phase correlations in the spectral output of the laser. The temporal coherence length of broadband lasers is important in deciding their suitability for applications ranging from coherent communications to incoherent imaging, and has to be quantitatively estimated through reliable experiments.In this thesis, a broadband EDFRL is constructed based on multi-pump-induced FWM in the wavelength range of 1560-1600 nm. The required multiple pumps for the FWM process are generated within the ring cavity itself by a suitable length of erbium-doped fiber and specialty fiber. The prevalent FWM theory is improved to interpret the two-pump and four-pump induced side band generation in the first and second orders, after including the effect of phase mismatch at the wavelengths used in the experiments. The temporal coherence of this broadband EDFRL is studied and compared with the results from different optical sources by analyzing the interference features from a fiber-based Mach-Zehnder interferometer (MZI). The fixed path length MZI is able to extract the temporal coherence characteristics of the source from the wavelength-dependent visibility results. A standard narrow line-width commercial laser is used as a reference source in these studies. The broadband source is filtered down to a multi-spectral source with several narrow spectral lines, and made comparable in linewidth to that of the standard laser source for clarity in interpretation of results.The work also involves simulations by including all the likely phase contributions. The combined results from experiments and simulations clearly imply that the process of generation of the light in the broadband source is influencing its temporal coherence. The FWM process and the ring cavity have a combined effect on the temporal coherence property of the nonlinearity-assisted broadband EDFRL. The FWM sidebands are also individually tested for their temporal coherence property with and without the cavity effect, which helps to explain the loss in visibility due to the intra-cavity noise arising from the amplified spontaneous emission which is not entirely annulled in broadband lasers. In addition, the phase correlation within the spectrum of the broadband source is also demonstrated by providing two optical inputs to the MZI. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Mr. Ummer K.VProf. R.Vijaya10709070Role of Band-edge modes and in-plane modes in two- and three-dimensional photonic crystals 02.08.2018 (Thursday)11:00 AM (Tea will be served at 10.45 AM)FB382 (Prof. A. K. Raychaudhuri seminar hall)Periodic dielectric structures have been studied in recent years for many interesting optical properties such as negative refraction, superprism, and super-collimation effect. The periodicity in their dielectric constant will discretize the electromagnetic modes within the structure into allowed propagating modes and the modes with no propagating component – constituting the photonic stopband. Depending on the extent of overlap of the photonic stopband with the emission spectrum of an embedded emitter in a photonic crystal (PhC), the suppression in spontaneous emission within the stopband and enhancement in emission at the band-edges can be expected. The reduced group velocity at the band-edges provides increased time duration for light-matter interaction.In this thesis, the quantitative enhancement in emission and absorption of Rhodamine-B dye due to the band-edge effect in 3-D ordered pseudo gap polymer-based photonic crystals has been studied. The motivation is to identify the ideal parameters for designing stand-alone miniature low-threshold lasers through a combined effect of absorption and emission enhancement from band-edges. The combined effect has resulted in a 70% enhancement in emission intensity over the intrinsic emission of the dye within a PhC.Silicon-based PhC platforms are desirable since they help in large-scale on-chip integration of optical components for miniaturization of optical devices. In this work, the 2-D photonic crystal slab with a hexagonal lattice of air holes on SOI wafer has been fabricated by electron beam lithography (EBL), and its Fano resonances are characterized by a simple out-of-plane experimental set-up. The quality factors of these Fano resonances are associated with the lifetime of the leaky modes of the slab which are present in the band structure above the light line for the cladding. A higher quality factor at any wavelength will be helpful in building applications such as Si-based micro-devices and narrowband optical filters.In addition to the applications based on the photonic stopband, the allowed Bloch modes of photonic crystals possess unusual dispersion characteristics which result in various optical phenomena such as negative refraction, self-collimation, and super-prism effects. In our work, an all-angle (±90º) self-collimation effect with a bandwidth of 15% has been obtained from a two-dimensional rectangular PhC. All-angle (±90º) negative refraction from a two-dimensional honeycomb lattice of air holes in Si shows a wavelength resolution of 0.2λ when imaged with a point source. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Mr. Alekha Chandra NayakProf. Pankaj Jain11209062Phenomenological study of very special relativity July 3, 2018 (Tuesday)4 PM (Tea @ 3.45 p.m.)FB382 (Prof. A. K. Raychaudhuri seminar hall)The special theory of relativity has been experimentally tested to unprecedented degree of accuracy. The Standard model of particle physics respects Lorentz invariance and successfully describes all particle interactions. However, it breaks several discrete symmetries such as P, T, CP or CT, which were once thought to be preserved in nature. With the violation of these discrete symmetries, it is possible that only a subgroup of the Lorentz group may be preserved in nature. The resulting framework along with translational symmetry is known as very special relativity (VSR). It turns out that this is both necessary and sufficient to explain the null result of Michelson-Morley experiment and its consequences. VSR has a preferred direction which manifestly breaks Lorentz invariance. Remarkably it provides an alternative approach for neutrino masses which does not require addition of a new fields in the Standard Model and it also conserves lepton number. We study collider, astrophysical and cosmological implications of models based on VSR. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Tanmay MajiProf. Dipankar Chakrabarti12209874Transverse structure of proton and azimuthal spin asymmetries in the SIDIS processesJune 5, 2018 (Tuesday)4:00pm(tea @ 3:45pm)FB382 (Prof. A. K. Raychaudhuri seminar hall)In last two decades, there have been a lot of investigations on the three dimensional structure of nucleons. Understanding of the spin and the orbital angular momentum (OAM) structure in partonic level is a challenging task in this field. Three dimensional structures are encoded into the distribution functions e.g., generalized parton distribution functions(GPDs), transverse momentum dependent parton distribution(TMDs), generalized TMDs (GTMDs) etc. Wigner distributions are higher dimensional distributions which are not accessible in experiments but at different limits they reduce to different parton distributions and thus play important role in understanding the 3D structure of nucleons. Since these distributions are non-perturbative in nature and difficult to be calculated in full QCD, people study these objects in QCD inspired models. We present a model prediction to the non-perturbative transverse structures of proton. To investigate these distribution functions of nucleons, we construct the Light-front quark-diquark model (LFQDM) where the wave functions are adopted from soft-wall AdS/QCD. The model is consistent with the quark counting rules and the parameters of the model are determined by fitting the experimental data of Dirac form factor and Pauli form factor. The TMDs provide a three-dimensional structure with the probability of finding a quark with longitudinal momentum fraction $x$ and transverse momentum $\bf{p}_\perp$ inside a proton. We also calculate the five-dimensional Wigner distributions and present the quark orbital angular momentum(OAM), spin-OAM correlations inside a proton.In this model, We also present the single spin asymmetries (SSAs) for SIDIS e.g., Collins asymmetries, as well as the double spin asymmetries (DSAs) which involve the T-even TMDs at the leading twist. The model results of asymmetries are compared with the COMPASS and HERMES data for $\pi^+$ and $\pi^-$ channels. Fragmentation functions are taken as a phenomenological input. We propose a parameter evolution approach for TMD evolution and compare with the QCD evolution of unpolarised TMDs. The parameter evolution approach for TMD evolution gives quite satisfactory result for azimuthal asymmetries as the asymmetries are defined by the ratio of TMDs. Our model has predicted sizeable Collins asymmetry for the upcoming EIC data. We then extend our investigation for T-odd TMDs and azimuthal spin asymmetries generated by them in SIDIS process. The spin asymmetries associated with T-odd TMDs namely, Sivers and Boer-Mulders asymmetries require gluon exchange in the final state. To get the nonzero T-odd TMDs, the wave functions are modified by including a spin dependent complex phase which reproduces the final state interaction (FSI) effect. We also predict the Sivers asymmetry and Boer-Mulders asymmetries in the SIDIS process for $\pi^+$ and $\pi^-$ channels and compared with the HERMES, COMPASS measurements. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Anmol ThakurProf. Amit Agarwal12109063Density and Current Responses in Dirac Materials18th May, 201811:00 am (Tea: 10.45 am onwards)FB382 (Prof. A. K. Raychaudhuri seminar hall) Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Sunil KumarProf. Y. N. Mohapatra Y9109079 Organic Semiconductor Homojunction Diodes: Mechanisms Controlling Current and Capacitance Characteristics15th April 201803:30 PM SCDT (Seminar Room) Electronic devices based on organic materials, particularly, diodes and transistors have significant technological potential due to their advantages in processibility, flexibility and cost-effectiveness. The role of organic/organic & metal/organic interfaces is crucial in controlling the device characteristics. However the underlying physical mechanisms taking place at these interfaces are not well understood, and need special attention. A detailed understanding of mechanisms at the interface will help design structures and applications in the field of organic electronics. My thesis aims to study in detail the undoped homojunction diode, which is a sandwich structure consisting of doped and undoped layer of the same organic semiconductor. Our principal focus is to demonstrate the effect of the thickness of the undoped layer on the device characteristics. The configuration of the specifically designed device under test is ITO|p-MTDATA|i-MTDATA|Al. Intrinsic layer thickness is varied from 10 nm to 100 nm so as to demonstrate the control over both forward and reverse current density-voltage (J-V) and capacitance-voltage (C-V) characteristics. The specially designed device structures for this purpose have been fabricated using a state-of-the-art automated multi-chamber Cluster tool. The prototype materials m-MTDATA and F4-TCNQ are used as matrix and dopant, respectively and the doping is achieved by co-evaporation. The devices with thin intrinsic layer showed Zener type behavior exhibiting high current density beyond a certain reverse threshold voltage. In order to delineate underlying mechanisms, the J-V characteristics were studied by varying the temperature from 200K to 300K. The forward bias characteristics are controlled by the high-low junction to result in nearly temperature independent ideal exponential J-V regime prior to the space charge limited regime even for samples with intrinsic layer thickness as low as 10 nm. However, the reverse characteristics are controlled by Fowler-Nordheim (F-N) tunneling at the cathode/intrinsic layer interface. The barrier controlling F-N tunneling is found to be temperature independent but sensitive to the intrinsic layer thickness. This is interpreted in the framework that the thickness of the intrinsic layer modulates the Fermi level responsible for barrier height. The capacitance spectroscopy of m-MTDATA based homojunction and intrinsic diodes has been analyzed to investigate the physical mechanisms controlling the charge processes. The capacitance exhibits voltage dependence in reverse bias which is a signature of the defect states acting as traps. In order to investigate the defect states, temperature dependent capacitance-frequency (C-f) characteristics have been analyzed. The defect states are estimated to have Gaussian distribution and the associated parameters have been calculated. In the low thickness homojunction we are able to probe the parameters associated with localized level in the HOMO states, demonstrating consistency with Gaussian disorder model of energetic in organic semiconductors. In homojunction diode, having low intrinsic layer thickness (10nm), the capacitance in deep reverse bias starts decreasing nearly exponentially and goes below the geometrical capacitance (Cg) after a critical electric field inside the device. This decrease in the capacitance is interpreted on the basis of F-N tunneling of carriers. The small signal capacitance in such cases will have negative contribution which is directly related to delay time introduced by charge transport. Using F-N tunneling, the characteristics have been modeled in deep reverse bias, and the estimated barrier height matches with the values calculated from J-V characteristics. The technique also allows the determination of mobility, which in turn yields the disorder parameters through its temperature and field dependence. The study reported in the thesis thus gives a coherent account of mechanisms underlying current and capacitance characteristics of homojunction organic diodes. We thus demonstrate determination of useful intrinsic transport and disorder parameters from such characteristics. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Pratim RoyProf. Tapobrata Sarkar 11109067 Information Geometry: An AdS/CFT Perspective26th April 20183:00 PM FB382 (Prof. A. K. Raychaudhuri seminar hall) In the context of statistical physics, information geometry addresses the question - how do we distinguish two infintesimally separated states in a physical system? This is done by defining a Fisher information metric on the parameter space of the system (classical or quantum). This metric in turn allows the introduction of the notion of a line element which quantifies the distance between two infinitesimally separated states. Recently, there has also been a proposal to calculate a counterpart of this information metric from the celebrated AdS/CFT correspondence, which relates classical gravity to strongly coupled field theories. In the first part of the present thesis, to illustrate the utility of the information geometric approach to physical systems, we investigate phase transitions in magnetic systems, as well as a toy model of a liquid system. With this serving as the motivation, we undertake to investigate information geometry in the context of AdS/CFT. As the first investigation in an AdS/CFT context, we first investigate the phase transition of rotating and non-rotating black holes in the extended phase space paradigm. Briefly, this proposal modifies the well-known first law of black hole thermodynamics by considering the cosmological constant as a variable parameter. Next, we introduce the notion of holographic complexity, which is a quantity related to the Fisher information. We explicitly calculate complexity for field theories dual to Schwarzschild and Reissner-Nordstrom black holes in a near horizon expansion. We have also numerically computed the complexity for a Reissner-Nordstrom black hole with a spherical horizon. Lastly, we investigate how the complexity behaves along the renormalization group flow by holographic methods. In systems where there is a phase transition, we track the behaviour of the complexity across it and comment on how the complexity behaves near the phase transition. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Alestin MawrieProf. Tarun Kanti Ghosh 12109061 Electrical, optical and thermoelectric transport properties of spin-orbit-coupled fermionic systemsFeb 06, 20184:30 PM FB382 (Prof. A. K. Raychaudhuri seminar hall) The fundamental role of spin-orbit interaction (SOI) in condensed matter systems is one of the interesting topics in modern days research. The strong SOI in group III-V semiconductors makes them a suitable candidate for the spintronic application. The SOI associated with holes in p-doped III-V semiconductors is comparatively stronger than that of the electrons in n-doped III-V semiconductors. The hole in bulk p-doped III-V semiconductors is fully described by the 6 X 6 Luttinger Hamiltonian which gives a quadratic momentum dependence of SOI. On the other hand, two-dimensional heavy-hole gas (2DHG) formed at the p-type GaAs/AlGaAs heterostructure is associated with two types of SOIs, namely Rashba SOI (RSOI) and Dresselhaus SOI (DSOI), which are both cubic in momentum. Using the Kubo formula and the Green's function technique, we present a theoretical study to understand the influence of higher-order SOIs on electrical and thermoelectric transport coefficients as well as on optical properties. We also discuss magneto-transport properties of 2DHG with k-cubic RSOI in presence of a quantizing magnetic field. References: [1] R. Winkler, Spin-orbit Coupling Effects in Two-Dimensional Electron and Hole Systems (Springer, Berlin 2003) [2] A. Mawrie, T. Biswas and T. K. Ghosh, Journal of Physics: Condensed Matter 26, 405301 (2014). [arXiv: 1405.4533] [3] A. Mawrie and T. K. Ghosh, Journal of Applied Physics 119, 044303 (2016). [arXiv: 1511:04917] [4] A. Mawrie and T. K. Ghosh, Journal of Physics: Condensed Matter 28, 425302 (2016). [arXiv: 1601:00591] [5] A. Mawrie, P. Halder, B. Ghosh and T. K. Ghosh, Journal of Applied Physics 120, 124309 (2016). [arXiv: 1605:05162] [6] A. Mawrie, S. Verma and T. K. Ghosh, Journal of Physics: Condensed Matter 29, 465303 (2017). [arXiv:1705.02483] Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Jishnu Goswami Prof. Dipankar Chakrabarti 11109065 Numerical Studies of Borici Creutz fermions on lattice21st December, 20173:30pm (tea@ 3:15pm)FB382Lattice field theory is a well known technique to study any field theory non perturbatively from the first principle. However the discretization of the fermion action suffers from the notorious fermion doubling problem. There are many prescriptions to overcome this issue but in the price of some basic symmetries of the lattice action. There is a no-go theorem which tells that fermionic lattice action with locality, hermiticity and chiral symmetry must have doublers. In general, chiral symmetry is sacrificed to get rid of the doublers. Nevertheless the search for chiral symmetric lattice action which is computationally not very demanding is still going on. Recently motivated by the fact that electrons on a graphene lattice are described by a massless Dirac-like equation, Creutz and Borici proposed a four dimensional euclidean lattice action describing two flavors of fermions on an orthogonal lattice. This action minimize the doubling problem but in this case hypercubic symmetry is reduced to cubic symmetry. There is not much evidence in the literature to prove whether this fermion is suitable or not for numerical studies. So, we initiated a systematic numerical investigation of the Borici-Creutz action. We first studied the chiral symmetry breaking and mass spectrum in Gross Neveu model with this fermion formulation using HMC(hybrid monte carlo method) in two dimensions. Encouraged by the two dimensional results we extend our work to four dimensions, where we use mixed action approach to study this fermion. We use Borici Creutz valence quarks and asqtad sea quarks. As Borici Creutz action breaks hypercubic symmetry, we need to add counter terms which can be generated through radiative corrections. In our study we fix the counter terms non perturbatively. We show that our results for partial quenching, pion chiral log and delta mix etc. are consistent with the chiral perturbation theory predictions. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Dipak RoutProf. R.Vijaya 11209064 Localized surface plasmon resonance effect on the optical properties of metal-dielectric photonic crystals06.11.2017 (Monday)11:30 AM (Tea @ 11.15 AM) FB382Many exotic phenomena are observed during electromagnetic wave propagation through periodic dielectric media with feature sizes comparable to the length scale of the wavelength of light. Photonic crystals are one such class of periodic structures which effectively block a set of wavelengths in transmission leading to band gap effect. The optical properties of these dielectric crystals can be modified further by introducing metal nanoparticles into them. Metal-dielectric photonic crystals allow better control of light-matter interaction as they offer a higher refractive index contrast to the incident radiation and support plasmonic resonances. They are useful for applications ranging from integrated photonics and solar energy conversion to chemical and bio-molecular sensing. This work explores the possibilities of tailoring the optical properties of metal-dielectric hybrid structures by a synergistic combination of the photonic stopband and the plasmon resonance of metal nanoparticles. By a proper choice of the size of dye-doped polymeric colloids and the gold nanoparticles, we have studied the effect of plasmonic resonance on the stopband characteristics of photonic crystals. In addition, the fluorescence intensity of the dye molecules in the crystals is measured using a standard laser-induced photoluminescence experiment, and the fluorescence decay time of the excited state is measured using a time-correlated single photon counting system. We have demonstrated an enhanced emission from gold-infiltrated photonic crystals as a result of increased plasmonic absorption and a resonant energy transfer between the dye molecule and the gold nanoparticles. A reduction in the radiative decay time and a complete suppression of the non-radiative decay route is achieved in the metal-dielectric photonic crystal. Suppression of non-radiative decay route is useful to enhance the fluorescence efficiency of the dye and to reduce the threshold for lasing. In addition, a considerable increment in the Raman signal intensity is achieved when the gold nanoparticles are surface-coated over the dielectric photonic crystals due to the combined effect of the stop band and the localized surface plasmon resonance of the gold nanoparticles. The plasmonic-photonic interaction in enhancing the dye emission is further studied in complex designs such as heterostructures made from dye doped 3-D colloidal photonic crystals embedding a planar SiO2 defect doped with gold nanoparticles. The reason is the physical intuition to be gained about spontaneous emission control in non-local designs. Significant spectral narrowing and reduction in the fluorescence lifetime is achieved in such structures. The role of defect mode and localized plasmons in modifying the emission characteristics from the heterostructure is addressed through this work. Thus the thesis establishes the possibilities of localized surface plasmon modes in modifying the optical properties of metal-dielectric photonic crystals towards amplification of emission and low-threshold lasing applications. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Debarchan DasProf. Zakir Hossain 11109866 Heavy fermion behavior and magnetic ordering in Ce(Cr,Ti)Ge3 with hexagonal perovskite structure16-08-2017 (Wednesday)4:30 p.m. (Tea at 4:15 p.m.) FB382Strongly correlated electron systems have attracted considerable attention of condensed matter physics research community as they exhibit wide variety of fascinating phenomena which include heavy fermion behavior, Kondo effect, valance fluctuation, quantum criticality, superconductivity etc[1,2]. However, among the vast number of existing intermetallic compounds, only a small fraction exhibits orders of magnitude enhancement of the conduction electron effective mass at low temperature, known as heavy fermion system and sometimes they even enter in the superconducting state alongside this enhanced conduction electron effective mass, known as heavy fermion superconductors (HFSC). Unlike the conventional s-wave superconductor where superconducting pairing is mediated by phonons, in HFSC superconducting pairing is believed to be mediated by magnetic interactions. Thus, Heavy fermion and HFSC systems have continued to dominate the field of condensed matter research for the past several decades. In our effort to search for new rare earth based intermetallic compounds we have synthesized polycrystalline samples of CeCrGe3 and performed a comprehensive study of the low temperature properties by means of x-ray absorption spectroscopy (XAS), magnetic susceptibility, isothermal magnetization, electrical resistivity, specific heat and thermoelectric power measurements. The results corroborate that the compound undergoes a ferromagnetic transition below Tc ~ 70K. The Kondo lattice type of the resistivity and the large value of Sommerfeld coefficient (γ = 130mJ/mol/K2) convince us that CeCrGe3 is a moderate heavy fermion system[3]. The observation of HF/ Kondo lattice behavior even in the ferromagnetically ordered state is quite rare. In order to gain further insight into the magnetic ordering in CeCrGe3, we have performed muon spin relaxation (μSR) experiments which reveal the presence of a bulk magnetically ordered state[4]. In order to unveil the magnetic structure of this system, we have carried out powder neutron diffraction study on CeCrGe3. Neutron diffraction investigation reveals that both Ce and Cr orders ferromagnetically at ~70 K [5]. Interestingly, the observation of this high value of ordering temperature for the Ce moments is very unusual. In addition, we have tried to tune the magnetic ordering in CeCrGe3 by Ti doping at Cr site. The results on CeCr1-xTixGe3 suggest a possible existence of a bi-critical point in the system[4]. Superconductivity was not found down to 2K. [1] G. R. Stewart, Rev. Mod. Phys. 56, 755 (1984). [2] G. R. Stewart; Rev. Mod. Phys. 73, 797 (2001). [3] D. Das, T. Gruner, H. Pfau, U.B. Paramanik, U. Burkhardt, C. Geibel and Z. Hossain, J. Phys.: Condens. Matter 26, 106001 (2014). [4] D. Das, A. Bhattacharyya, V. K. Anand, A. D. Hillier, J. W. Taylor, T.Gruner, C. Geibel, D. T. Adroja and Z. Hossain, J. Phys.: Condens. Matter 27, 016004 (2015). [5] D. Das, S. Nandi, I. da Silva, D. T. Adroja and Z. Hossain, Phys. Rev. B 94, 174415 (2016). Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Sayandip GhoshProf. Avinash Singh 11109869 Unified Description of Electronic Structure and Anti Ferromagnetism in a Three-Orbital Model for Iron Pnictides 18/07/2017 (Tuesday)11:00 AM (Tea @ 10:45 AM) FB382Iron pnictides, the new class of high T_c superconductors after cuprates, exhibit a rich temperature-doping phase diagram in which the parent compounds undergo a tetragonal-to-orthorhombic structural transition and a magnetic phase transition to the stripe antiferromagnetic (AFM) state. With doping, both are suppressed and superconductivity arises. The electronic structure calculated using first-principle techniques and probed by angle resolved photoemission spectroscopy experiments reveal multiple bands crossing the Fermi level resulting in two nearly circular hole pockets and two elliptical electron pockets. Magnetic excitations in the magnetic state examined by inelastic neutron scattering measurements yield well defined spin wave excitations with a maximum at the ferromagnetic zone boundary. Moreover, existence of ferro-orbital ordering between the xz and yz Fe 3d orbitals is also observed. Understanding these electronic and magnetic properties within a single theoretical framework has been an outstanding challenge. The single-band Hubbard model is our starting point in which the stripe AFM state is stabilized at intermediate hole doping. More realistic two-orbital models yield correct Fermi surface topology, but I will show that the spin wave dispersion does not agree with experimental findings. Motivated by this, I will present and investigate a three-orbital itinerant-electron model and show that essential features of electronic structure and magnetism can be well understood. Existence of the weakly gapped magnetic state will be shown to arise from a composite effect of orbital hybridization and magnetic order. I will also demonstrate how the effective spin couplings are generated from the particle-hole propagator, which explains the microscopic origin of the ferromagnetic spin coupling along b direction required to understand the observed spin wave dispersion. Finally, the genesis and stability of the newly discovered double-Q spin-charge-ordered state will be discussed. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Nimisha RaghuvanshiProf. Avinash Singh Y9209042 Magnetic excitations in multi orbital models for iron pnictide.16/05/2017 (Friday)11:00 AM (Tea @ 10:45 AM) FB382Study of magnetic excitations provides detailed understanding of effective magnetic interactions in correlated electron systems. Spin wave excitations in iron pnictides have been intensively investigated since the discovery of magnetism in these iron based superconductors. Angle resolved photo emission spectroscopy and inelastic neutron scattering experiments have revealed various features of electronic Fermi surface and magnetic excitations in this (pi, 0, pi) ordered antiferromagnet. These observations have highlighted the limitations of various existing theoretical models and approaches. In this talk, I will discuss calculation of magnetic excitations in different multi-orbital models for iron pnictides aimed at understanding the major characteristic features of magnetic excitation spectra within a single theoretical framework. We propose that the characteristic maximum spin wave energy at the ferromagnetic zone boundary is the result of strong ferromagnetic spin couplings generated due to the exchange of the (virtual) particle-hole pair. Conditions on the microscopic Hamiltonian parameters of the multi-orbital model which favor the stabilization of the spin density wave state and correct sign of the orbital order within the constraints on the Fermi surface structure are identified. The role of nesting in the stabilization of the spin density wave ordered state is examined. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Anup Kumar SinghProf. Rajeev Gupta Y8109062 Anomalous resistivity behaviors of thin films and environmental effects in devices of amorphous Indium Gallium Zinc Oxide.17/05/2017 (Wednesday) Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Biplab BagProf. Satyajit Banerjee 10109873 A study of interplay between magnetic and superconducting order in BaFe2-xCoxAs2, and driven vortex state properties in 2H-NbS2 superconductor.17/05/2017 (Wednesday)3.30 PML-6 In iron pnictide superconductors, we investigate the competition and interplay between magnetic fluctuations and superconductivity. Using bulk and local magnetization techniques, we do a detailed doping dependence study to observe the interplay between superconductivity and magnetic correlations with doping in BaFe2-xCoxAs2 single crystals (underdoped, optimally doped and overdoped). Our study reveals an inhomogeneous superconducting state below the superconducting transition temperature (Tc). Above Tc, we observe the presence of positive magnetic response in our samples. Using high sensitivity magneto-optical imaging we find that the positive magnetic response in our samples appears due to presence of a short range magnetic correlation in the sample. On application of magnetic field we find an unusual suppression in the magnetic response along with an enhancement in the diamagnetic shielding (superconducting) response in our samples. We show the presence of superconducting fluctuation response above the bulk superconducting transition temperature. We have found that the strength of the short range magnetic order, and superconducting fluctuations are the strongest for the optimally doped sample (which possess the highest Tc) and weakest for the over doped sample. We believe our results provide some evidence of interplay between magnetic and superconducting order in iron pnictide superconductors and magnetic fluctuations may play a role in mediating superconductivity in these compounds. A part of the thesis work is also devoted to exploring a new jamming transition in the driven vortex matter in 2H-Nbs2 superconductors. Using four probe transport characteristics, we reveal two different depinning threshold regime; (i) conventional depinning from the static pinned vortex state and (ii) unjamming transition from the drive-induced jammed state. We show the unjamming transition has unusual negative velocity fluctuations. We identify the unique characteristics of the unjamming transition which are found to be distinct from conventional depinning. We analyze this transition in terms of IV scaling relations and Cohen-Gallavotti non-equilibrium fluctuation relation (GC-NEFR). Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Anshuman DeyProf. Tapobrata Sarkar 11109063 Holographic Renyi entropy, entanglement entropy and thermalization in higher derivative gravity theories. 21st April, 201711 a.m. FB382 The present thesis seeks to understand three important aspects of quantum field theory : thermalization, entanglement entropy and R ́enyi entropy, in the context of the gauge/gravity duality with higher derivative corrections. All these as- pects are of great interest in the condensed matter and statistical physics community, and it is important to understand the predictions of the AdS/CFT correspondence in these contexts. In the first part of the thesis, we consider a gravity model for holographic thermalization, generalizing the Einstein-Maxwell action by including a specific class of four derivative terms which involves a coupling of the Maxwell field to the bulk Weyl tensor. The key interest here is to understand how the coupling constant of the four derivative term affects the thermalization in the boundary field theory. Here we also investigate the effect of the chemical potential in the boundary theory on thermalization, and find subtle interplay between the coupling constant and this chemical potential. In the second part of the thesis, considering a similar gravity model, we set out to explore the rich phase structure of the dual field theory in a fixed charge ensemble, and for this purpose we construct appropriate black hole solutions perturbatively, and analyze black hole thermodynamics and compute the entanglement entropy of the dual CFT. Finally, the third part of the thesis is devoted to the study of another important quan- tity in quantum information theory, namely the R ́enyi entropy, from the holographic viewpoint, in the context of modified gravity. We investigate this by considering four different gravity models, charged and uncharged, and carry out extensive investigations of some aspects of the dual conformal theories. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Seema DeviProf. Asima Pradhan Y9109081 Application of correction techniques and statistical analysis on polarized fluorescence and synchronous fluorescence spectroscopy and study of polarized fluorescence imaging for characterization of cervical precancerous tissue.27/03/2017 (Monday) FB382 Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Upkar Kumar VermaProf. Y. N. Mohapatra Y8209064 Trapping and recombination in P3HT:PCBM bulk heterojunction solar cells; influence of device structure using photocapacitance, impedence and photovoltage decay techniques.28/12/2016 (Wednesday) SCDT seminar room Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Jitesh BarmanProf. Krishnacharya 111009868 Electric Field Controlled Interfacial Phenomenon at Liquid-Solid/Liquid Interfaces.27/12/2016 (Tuesday)11.00 AM FB382 Wetting of liquids on solid/liquid surfaces is one of the most important interfacial phenomenon from fundamental as well as application point of view. Wettability of a surface can be controlled by manipulating the surface free energy of solids either passively by coating or actively using external stimulus e.g. temperature, radiation, electric filed etc. Here we report external electric field induced manipulation of liquids on solid and liquid surfaces. Electrowetting on dielectrics (EWOD) has been established as an effective tool to reversibly manipulate the surface wettability. Firstly I will talk about the application of EWOD in open-microfluidics. Surface grooves with triangular cross-section will be used as open-microfluidics. Static and dynamics of liquid transport in the grooves, actuated via electrowetting, will be discussed along with appropriate theoretical model. Subsequently, EWOD on dielectric lubricating fluid coated solid surfaces, to reduce hysteresis, will be presented. Nepenthes pitcher plants have motivated researchers to explore lubricating fluid coated slippery surfaces. I will demonstrate how using external electric field, electrically tunable slippery surfaces can be fabricated with electric field dependent slip velocity. Multiple aqueous droplets on such lubricating fluid coated surfaces show spontaneous coalescence or pseudo non coalescence depending on the lubricating fluid film thickness. External electric field can also be used to control the coalescence or non-coalescence on such surfaces. Towards the end, I will talk about specially engineered superoleophobic surfaces which repel low surface tension liquid (oils and hydrocarbons). Ultraviolet radiation induced reversible wettability change, from superoleophobic to oleophilic, will also be discussed. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Pankaj ChaturvediProf. Gautam Sengupta 10109874 AdS/CFT correspondence, holographic superconductors and quantum phase transitions.25/11/2016 (Friday)12.30 PMFB382 We will describe three specific examples of the application of the AdS/CFT Correspondence ( Gauge Theory-Gravity Duality) to the construction of holographic superconductors and the study of holographic quantum phase transitions. The first example will deal with a (1+1)-dimensional holographic superconductor from a boundary quantum field theory dual to a bulk (2+1)-dimensional charged rotating BTZ black hole. Subsequently we will describe a (2+1)-dimensional p-wave holographic superconductor from a boundary field theory dual to a (3+1)-dimensional bulk charged AdS-Born-Infeld black hole. Finally we will characterize a holographic quantum phase transition in a (2+1)-dimensional boundary field theory dual to a system of two mutually interacting massive real scalar fields in a (3+1)-dimensional bulk AdS soliton background. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Tanay NagProf. Amit Dutta 11109870 Quenched and periodically driven closed quantum systems: entanglement, decoherence and dynamical localization.04/11/2016 (Friday)10 AM FB382 Dynamics across a quantum critical point is necessarily non-adiabatic due to a diverging time scale in the vicinity of the quantum critical point (QCP) where the system becomes infinitely sluggish. The defect density in the final state is given by the celebrated Kibble-Zurek (KZ) law where the defect density is expressed in terms of quenching rate and the critical exponent associated with the quantum phase transition (QPT). We probe the temporal behavior of decoherence factor (DF) by considering a central spin model where an external qubit is globally connected to the environmental spin chain. We show that the DF shows power law behavior in the vicinity of a QCP while on the far limit DF exhibits a Gaussian fall for integrable environment. We successfully predict the scaling behavior of the decay constant associated with DF in both the limits using KZ scaling argument. In parallel, we probe the periodically driven system in the light of Floquet theory. We have studied the dynamics of a one-dimensional lattice model of hard core bosons (HCB) which has an initial current carrying ground state in the superfluid phase. The system is subjected to periodic delta-function kicks in the staggered on-site potential. It is found that the current vanishes (and the work done saturates) in the in the limit of an infinite number of kicks and large frequency. This phenomena can be attributed to a dynamic localization of the hard core bosons. We also have shown that the particles propagates in a light-cone-like fashion in real space. This study has been generalized to a situation when the staggered potential varying sinusoidally in time to estimate the behavior of maximum group velocity as a function of frequency and amplitude. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Gyanendra KumarProf. R. Vijaya Y9109068 Nonlinear dynamical studies on Erbium doped fibre laser subjected to cavity loss modulation.26/09/2016 (Monday)10.00 AM FB382 Highly-stable erbium-doped fibre amplifiers and lasers are useful in traditional fibre-optic communication due to their wavelength range of operation lying in the low-loss window of glass fibres. Fibre lasers operating in the chaotic domain are equally useful, but for widely disparate applications ranging from secure communication systems to chaos-based LiDARs and random number generators. An understanding of the conditions for switch-over from linear to nonlinear dynamical regime of fibre lasers, their tendency for chaotic operation, and possibilities of controlling their dynamics are essential for these applications. We study the fundamental as well as sub- and super-harmonic resonance characteristics of the erbium-doped fibre laser in the regime of its relaxation oscillation frequency, the evolution of its chaotic dynamics, and the conditions leading to its bi-stable and multi-stable operation using the cavity-loss modulation technique through experiments. At appropriate modulation parameters, the laser exhibits different periodic states (period-1, period-2, period-4, period-8 etc) eventually leading to deterministic chaos through the period-doubling route. This work shows that we can repetitively drive the non-autonomous class-B laser into and out of linear, nonlinear (of different extents), and chaotic dynamics with a very simple approach as required for applications. The nonlinear oscillator model of the laser provides a good agreement between the theory and experiments. Convenient user-defined parameters such as the pumping ratio and the biasing voltage of modulation enable a fine control on the laser dynamics. The alternate technique of pump modulation is also analyzed for the sake of completeness, and the advantage of loss modulation is established. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Chandan MandalProf. Dipankar Chakrabarti11109864Nucleon structure in light front quark models in AdS/QCD.14/09/2016(wednesday) 3.30 PM FB382The investigation of the structure of nucleon has a long history which goes back to early fifties but the structures are still not completely resolved. In parallel to the efforts of theoretical modelling, different experiments are still going on to understand the insights into nucleon structure. In the recent time, light-front AdS/QCD has emerged as one of the most promising theoretical framework to investigate the non-perturbative QCD/hadronic structure. The thesis mainly focuses on the application of AdS/QCD models to investigate several nucleon properties such as electromagnetic and gravitational form-factors, generalized partion distributions (GPDs), transverse charge and magnetization densities in transverse plane etc. In the second part of the work, we investigate various quark-diquark model where the nucleon wavefunctions are constructed from the soft-wall AdS/QCD correspondence and compare with the soft-wall AdS/QCD models. The results obtained are compared with experimental data wherever available or other phenomenological models. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Anirban BaguiProf. S. Sundar Kumar Iyer Y7109861 Effect of electric-field annealing during solvent drying step of active layer in organic solar cell devices. 05/08/2016 (Wednesday)12.00 PM SCDT seminar room The performance of organic electronic devices can often be improved by improving the nano-morphology of organic active layers in those devices. In this thesis, the role of thermal annealing of polymer layers during solvent drying in the presence of a constant electric-field to improve the the nano-morphology of the films thus formed was studied. Comparative studies of morphological and electro-optical properties of the P3HT:PC61BM based blend film and bulk hetero-structure solar cell devices were carried out to understand the role of 'electric-field annealing'. The external quantum efficiency (EQE) and power conversion efficiency (PCE) of the solar cells made showed significant enhancement due to better charge transport in the case of electric-field annealed devices. In order to study this process step on the hole mobility in the P3HT layer, devices with holes as the primary charge carriers were fabricated. The hole mobility in P3HT was found to increase monotonically for annealing field strengths up to 2000 V/cm. The current density voltage (J V) data corresponding to the space charge limited currents (SCLC) at various low temperatures for P3HT based hole-only devices were fitted with the empirical model and the Gaussian disorder model (GDM) to interpret the data. X-ray diffraction (XRD) measurements confirmed increase in crystallinity and crystallite size of the films for electric-field annealed samples. The observed changes in the charge transport properties due to electric-field annealing of the films at the time of their fabrication have been explained based on the above measurements and analysis. Solar cells in inverted structure have also been made with appropriate modification in device structure to get better performing devices by electric-field annealing. Finally, the effect of electric field assisted treatments was studied on devices fabricated with PTB7 (a new promising polymer for organic solar cells) as the device active layer. The experimental results presented in this thesis confirm that application of electric-field during annealing of organic films during their formation is a useful processing step to fabricate higher mobility polymer films for building improved organic electronic devices. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Gangadhar BeheraProf. S. Anantha Ramakrishna Y9109065 Nanostructured plasmonic thin films for enhanced optical properties.20/04/2016 (Wednesday)4.00 PM FB382 The unique optical properties of nano-structured plasmonic films have attracted great attention due to their potential applications in solar cells, photo-detectors, sensors, nano-imaging devices, thermal emitters and many more. Surface plasmon polaritons (SPPs) are surface electromagnetic waves that exist at the interface between a metal and a dielectric material. In this work, the significant role of the surface plasmons in novel optical phenomena like enhanced transmission or enhanced absorption in nano-structured plasmonic thin films is investigated in detail. The extra-ordinary transmission of light through an array of holes in thin plasmonic films for use as conducting transparent electrodes with enhanced functionality due to large local fields is proposed. By experiments and simulations, these structured metallic electrodes are also shown to enhanced absorption for solar cell applications. Complementary layers of ladder-like plasmonic structures fabricated by laser interference lithography (LIL) are investigated for enhanced optical properties in the visible-IR bands. Possible applications as polarization dependent sensors at IR frequencies is also discussed. Enhanced absorption from a trilayer plasmonic system consisting of structured hole arrays in gold film separated from the bottom gold layer by dielectric spacer is reported. The fabrication by LIL and optical characterization of these samples are presented. A new approach to design dual band perfect absorber in the visible to the NIR region with top metallic patches on a SiO2 coated Si substrate is reported. A physical model for these absorbers is presented. A combined structuring of metallic disc and grating arrays on glass substrates that give rise to triple-band perfect absorption at visible frequencies is also reported. The physics behind these enhanced phenomena are discussed. Further, a system consists of metal-dielectric multilayers grating structures that show polarization dependent and independent unit absorption at visible to near-infrared frequencies are also discussed. The results have good potentialfor realizing low-cost large area nanostructured plasmonic thin film devices for different device applications. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Reeta PantProf. KrishnacharyaY9209066 Investigation of Tunable Wetting and Slippery Behaviour on Hydrophilic Surfaces.19/04/2016 (Tuesday)11.00 AM FB382 Interaction of a liquid with a solid surface is one of the most fundamental surface/interfacial phenomenon which primarily governs the wetting behaviour of the liquid-solid system. The surface wettability can be controlled by manipulating the surface free energy of the solid and/or liquid-solid interfacial energy with passive or active methods. Reversible switching of surface wettability using polystyrene/titania nanocomposite based responsive surfaces upon ultraviolet (UV) exposure and annealing will be discussed. As prepared nanocomposite coating shows hydrophilic behavior which become superhydrophoboc upon annealing at 180 degree for 90 mins. This happens due to increase in surface roughness during annealing. Subsequently upon UV exposure, the superhydrophobicity slowly decrease and the samples become superhydropholic with almost 0 degree water contact angle. The UV exposed superhydrophilic samples recover their superhydrophobicity upon annealing again at 180 degree. Detailed investigation of static and dynamic wetting transition will also be presented. Subsequently superhydrophobic and lubricating fluid infused slippery surfaces will be discussed. Based on energy minimization, stability criteria for lubricating fluid infused slippery surfaces will be established. Initially we observed that slipping water droplets sink into lubricating silicone oil layer due to strong polar interaction between hydrophilic substrate and water molecules. This sinking can be prevented by using hydrophobic surfaces. Annealing silicone oil coated substrates makes the surface hydrophobic. Optimization of annealing parameters will be discussed in detail. Characterization of the stable slippery surfaces using contact angle hysteresis and slip velocity measurements will be presented. Subsequently, I will discuss the effect of substrate roughness on the stability of slippery surfaces. It was found that small nanoscale roughness enhances the stability of the lubricating fluid as well total slippery behavior. Fabrication of stable slippery surfaces based on nanostructured formed by nanoparticles enable us to get rid of the hydrophobic surface requirement. Hydrophilic silica and titania nanoparticles were used to cast nanostructured film which after coating with silicone oil show stable slippery behavior. Towards the end, I will discuss about the effect of surface wettability on the lubricating film thickness underneath a test drop. Electrical measurements were used to measure the thickness of the lubricating film underneath a test drop. Speaker:Supervisor:Roll No.:Title:Date:Time:Venue:Abstract: Dheeraj PratapProf. S. Anantha Ramakrishna Y9109063 Anisotropic and Hyperbolic Metamaterials in the Cylindrical Geometry.25/03/2016 (Tuesday)11.00 AM FB382 Metamaterials are artificially designed composite materials that are structured on subwavelength lengthscales. They are often inherently optically anisotropic, having direction dependent properties, due to their structure. Acid anodization of an aluminium micro wire was used to fabricate a nanoporous alumina microtube with non-branching nanopores aligned along the radial direction. This is essentially a metamaterial fiber with anisotropy in the cylindrical geometry. These nanoporous alumina micro tubes show enhanced optical and thermal properties, of which a study of the optical properties is presented here. Not only are these nanoporous alumina structures anisotropic, but they are radially inhomogeneous as well due to the radius dependent size of the nanopores. These metamaterials were homogenized to obtain the electromagnetic effective medium parameters (dielectric permitivity tensor)using techniques of transformation optics and Maxwell-Garnett homogenisation theory. Nanowires of silver were deposited electrochemically within the nanopores of the cylindrical nanoporous alumina to yield a metamaterial with hyperbolic dispersion. These metamaterial fibers exhibit novel optical modes that are described by Bessel functions of fractional and imaginary orders solely due to the anisotropy. Particularly for the high transverse momentum modes (whispering gallery modes), these modes are confined increasing near the centre of the waveguide in sheer contrast to conventional fibers where they localise to the edge. The inhomogeneous nature of the metamaterial fibers also cause extremely strong confinement of the modes near the center. The nanoporous alumina microtubes coated with active dye molecules of Rhodamine-6G show highly enhanced fluorescence, which is highly quenched in the presence of silver nanowires within the nanopores. Other studies, such as the possibility of manipulating the structure of the nanopores and a study of fluorescence from molecules deposited on nanoporous alumina, in the flat geometry were also carried out. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Abhishek KumarMahendra K. Verma11109062Energy spectra and fluxes of buoyancy-driven turbulent flows23/01/2017 (MON)3:00 PM (Tea at 2:45 PM) FB 382 (Physics seminar room)Buoyancy-driven turbulent flows are often encountered in geophysics, astrophysics, atmospheric and solar physics, and engineering. An important unsolved problem in this field is how to quantify the small-scale quantities, e.g., spectra and fluxes of kinetic energy (KE) and potential energy (PE) of these flows. Using ideas of energy flux in direct numerical simulation (DNS) and shell model simulation, we show that the KE spectrum for stably stratified flows has −11/5 spectral exponent, while Rayleigh-Bénard convection (RBC) has Kolmogorov’s −5/3 spectral exponent. For RBC, we show the applicability of Taylor’s hypothesis in the presence of steady large-scale circulation in a cubical geometry. Our simulations indicate that a cubical geometry is better suited for spectral studies than a cylindrical one due to the steady nature of large-scale circulation in a cube. We also show a modified Bolgiano-Obukhov spectrum for the velocity field, and the presence of internal gravity waves at large-scale in two-dimensional stably stratified turbulence. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Ambrish PandeyMahendra K. Verma10109872Scaling of large-scale quantities in Rayleigh-Bénard convection06-Oct-201611:30 AM (Tea at 11:15 AM) FB 382 (Physics seminar room)Thermal convection plays an active role in many natural systems, e.g., in stars, Earth's outer core and mantle, atmosphere, etc. Convection has many applications in industrial processes as well. An idealized model of thermal convection is the Rayleigh-Bénard convection (RBC) in which a fluid, confined between two horizontal plates, is heated from below and cooled from top. Dynamics of the RBC is governed primarily by two nondimensional parameters – the Prandtl number (Pr) and the Rayleigh number (Ra). The response parameters are the transport of heat and momentum, which are quantified as the Nusselt (Nu) and the Reynolds numbers (Re) respectively. We study the scaling of Nu(Ra,Pr) and Pe(Ra,Pr) using direct numerical simulations and derive a formula to analytically compute the Péclet number for a given Ra and Pr that match well with simulation and experimental data. We observe that Nu and Pe follow different power laws in the viscous and the turbulent regimes. We also study the scaling of kinetic energy and entropy spectra for very large Prandtl numbers, and observe that the kinetic energy spectrum does not follow the Kolmogorov scaling, instead it scales with wavenumber (k) as k-13/3. The entropy spectrum exhibits dual branches in the inertial range. Moreover for very large Prandtl numbers, we observe that Nu and Pe, as well as the energy and entropy spectra scale similarly in three- and two-dimensions Speaker:Supervisor:Roll no:Title: Date:Time:Venue:Abstract: Mr. Ravi Pratap Singh R K Thareja10109068Time resolved investigations of laser ablated single and colliding carbon plasma/plumes.May 20, 2016 (Friday)11:00 AM onwardsFB 382 (Physics Department Seminar Room)--- Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Shraddha SharmaAmit DuttaY9209067Fidelity and Loschmidt echo: quenches, non-analyticities and emergent thermodynamics2nd May (Monday), 201610 AMFB 382 (Physics seminar room)Our work focuses on relating quantum information theoretic measures such as the ground state fidelity and the Loschmidt echo (LE) to quantum phase transitions (QPTs) and dynamics. Fidelity is the modulus of the static overlap of the two ground states of a system at different parameter values; whereas, the LE is the modulus of the overlap of two states evolved from the same initial state with two different Hamiltonians. Therefore, one can map a connection between the two; both the tools effectively detects the quantum critical point (QCP) and follows universal scaling behavior close to it. We shall discuss the marginal behavior of fidelity and LE and explain how logarithmic scaling behavior enters in the scaling of fidelity in such situations. Furthermore, we shall relate LE to dynamical fidelity (obtained for a periodically driven quantum Hamiltonian), dynamical phase transitions. Exploring the finite temperature LE, we shall further study the emerging thermodynamics and analyze the behavior of the average irreversible work and the irreversible entropy in a many body quantum system following a sudden quench. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Mr. Dushyant Kumar Prof. R.C. Budhani and Prof. Z. HossainY9109064Spin and Electronic Charge Diffusion in Superconducting NbN and Disordered SrTiO3 Under the Influence of Magnetic Field and LightApril 28, 2016 (Thursday)10:00 AM (Tea at 09:45 AM)FB 382 (Physics Seminar Room)Spin-based electronics generally called “spintronics” encodes and processes information in the quantum-mechanical spin of the electrons leading to reduced power consumption with faster operation. However, the generation of pure spin currents, its transport and detection remain a challenge. A comprehensive understanding of these phenomena requires study of spin diffusion in metals, semiconductors, and superconductors. It has been of fundamental interest to search for the sources of pure spin currents and the materials which can allow long spin diffusion lengths. In this thesis work we have addressed both these issues. We used the full Heusler alloy Co2MnSi (CMS) as a source of polarized spins. This Heusler compound is a half-metallic ferromagnet. Such a band structure suggests 100% spin polarization, thus making the ordered cubic phase of Co2MnSi a good candidate for spintronic devices. As for the material for efficient transport of spin current, we have concentrated on SrTiO3, a cubic band insulator well known for its useful dielectric and optoelectronic properties. The high mobility (∼2 x 103 cm2/Vs at 15 K) electron gas was induced in the few nanometer thickness of STO by irradiating its surface using Ar+-ions. The process converts ~4 nm thick surface layers of STO into a metal while over the remaining thickness of a 0.5 mm wafer remains insulating. The unperturbed base material has been used as a gate dielectric in a back gated geometry. For device perspective of the surface electron gas of reduced STO, it is desirable to study its response to photon and electrostatic fields. In this connection, we have studied the photoconductivity (PC) and photoluminescence (PL) of ion irradiated STO and tuned them using electrostatic gate field. Speaker:Supervisor:Roll no:Title: Date:Time:Venue:Abstract: Nikhil Kumar Prof. Anjan GuptaY9109074Controlling thermal hysteresis in superconducting weak links and micron size superconducting quantum interference device.April 27, 201611:00 AM onwardsFB 382 (Physics Department Seminar Room)--- Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Gangadhar Behera S. Anantha Ramakrishna Y9109065Nanostructured plasmonic thin films for enhanced optical properties20 April 2016 (Wednesday)4.00 p.m. (Tea will be served at 3.45 p.m.) FB 382 (Physics department conference room)TThe unique optical properties of nano-structured plasmonic films have attracted great attention due to their potential applications in solar cells, photo-detectors, sensors, nano-imaging devices, thermal emitters and many more. Surface plasmon polaritons (SPPs) are surface electromagnetic waves that exist at the interface between a metal and a dielectric material. In this work, the significant role of the surface plasmons in novel optical phenomena like enhanced transmission or enhanced absorption in nano-structured plasmonic thin films is investigated in detail. The extra-ordinary transmission of light through an array of holes in thin plasmonic films for use as conducting transparent electrodes with enhanced functionality due to large local fields is proposed. By experiments and simulations, these structured metallic electrodes are also shown to enhanced absorption for solar cell applications. Complementary layers of ladder-like plasmonic structures fabricated by laser interference lithography (LIL) are investigated for enhanced optical properties in the visible-IR bands. Possible applications as polarization dependent sensors at IR frequencies is also discussed. Enhanced absorption from a trilayer plasmonic system consisting of structured hole arrays in gold film separated from the bottom gold layer by dielectric spacer is reported. The fabrication by LIL and optical characterization of these samples are presented. A new approach to design dual band perfect absorber in the visible to the NIR region with top metallic patches on a SiO2 coated Si substrate is reported. A physical model for these absorbers is presented. A combined structuring of metallic disc and grating arrays on glass substrates that give rise to triple-band perfect absorption at visible frequencies is also reported. The physics behind these enhanced phenomena are discussed. Further, a system consists of metal-dielectric multilayers grating structures that show polarization dependent and independent unit absorption at visible to near-infrared frequencies are also discussed. The results have good potential for realizing low-cost large area nanostructured plasmonic thin film devices for different device applications. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Ms. Reeta Pant Krishnacharya Y9209066Investigation of Tunable Wetting and Slippery Behavior on Hydrophilic Surfaces519 April 2016 (Tuesday)11:00 AM (Tea will be served at 10:45 AM) FB382 (Physics Seminar Room)Interaction of a liquid with a solid surface is one of the most fundamental surface/interfacial phenomenon which primarily governs the wetting behavior of the liquid-solid system. The surface wettability can be controlled by manipulating the surface free energy of the solid and/or liquid-solid interfacial energy with passive or active methods. Reversible switching of surface wettability using polystyrene/titania nanocomposite based responsive surfaces upon ultraviolet (UV) exposure and annealing will be discussed. As prepared nanocomposite coating shows hydrophilic behavior which become superhydrophobic upon annealing at 180°C for 90 mins. This happens due to increase in surface roughness during annealing. Subsequently upon UV exposure, the superhydrophobicity slowly decreases and the samples become superhydropholic with almost 0° water contact angle. The UV exposed superhydrophilic samples recover their superhydrophobicity upon annealing again at 180°C. Detailed investigation of static and dynamic wetting transition will also be presented. Subsequently superhydrophobic and lubricating fluid infused slippery surfaces will be discussed. Based on energy minimization, stability criteria for lubricating fluid infused slippery surfaces will be established. Initially we observed that slipping water droplets sink into lubricating silicone oil layer due to strong polar interaction between hydrophilic substrate and water molecules. This sinking can be prevented by using hydrophobic surfaces. Annealing silicone oil coated substrates makes the surface hydrophobic. Optimization of annealing parameters will be discussed in detail. Characterization of the stable slippery surfaces using contact angle hysteresis and slip velocity measurements will be presented. Subsequently, I will discuss the effect of substrate roughness on the stability of slippery surfaces. It was found that small nanoscale roughness enhances the stability of the lubricating fluid as well total slippery behavior. Fabrication of stable slippery surfaces based on nanostructured formed by nanoparticles enable us to get rid of the hydrophobic surface requirement. Hydrophilic silica and titania nanoparticles were used to cast nanostructured film which after coating with silicone oil show stable slippery behavior. Towards the end, I will discuss about the effect of surface wettability on the lubricating film thickness underneath a test drop. Electrical measurements were used to measure the thickness of the lubricating film underneath a test drop. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Rohit KumarProf. Mahendra K. Verma10109875Energy transfers in dynamos with small and large magnetic Prandtl numbers03.11.20152:30 PMFB 382 (Physics Seminar Room)The presence of magnetic field in celestial bodies is explained by dynamo mechanism in which a conducting fluid in motion generates a self-sustained magnetic field. In dynamo, energy is transferred form velocity field to magnetic field and consequently the growth of magnetic energy takes place. The magnetic Prandtl number (Pm), the ratio of kinematic viscosity and magnetic diffusivity, is an important non-dimensional parameter for dynamos. We study energy transfers in dynamos with small- and large-Pm by performing direct numerical simulations (DNS) in a 3D periodic box on 10243 grid, using a pseudo-spectral solver Tarang. Energy fluxes and shell-to-shell energy transfers show that for dynamo with large-Pm, the growth of magnetic energy takes place due to nonlocal energy transfers from large-scale velocity field to small-scale magnetic field. For dynamo with small-Pm on the other hand, the magnetic energy grows due to local energy transfers from large-scale velocity field to large-scale magnetic field. We also use a shell model of dynamo to understand the energy transfers for extreme values of Pm, which are otherwise inaccessible to DNS due to the computational constraints. The energy fluxes in our shell model simulation are in qualitative agreement with the DNS results. We also construct a low-dimensional model to study the dynamo transition for very small to very large Pm and observe that the critical magnetic Reynolds number for dynamo transition, Rmc, saturates to constant values in the two limiting cases of Pm. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Mr. Shubhankar DasR.C. Budhani and Zakir HossainY9109078Magnetoresistance and Magnetothermopower studies of delta-doped 2-dimensional electron gas at the interface of LaTiO3/SrTiO31st April (Friday), 20168:45 AMFB 382 (Physics Seminar Room)Studies of the 2-dimensional electron gas (2DEG) at the interface of LaAlO3 (LAO) or LaTiO3 (LTO) and TiO2 terminated SrTiO3 (STO) has attracted much attention in recent years due to its interesting properties like metal-insulator transition, magnetism, superconductivity and strong spin-orbit interaction effects in the electronics transport. We have used a new approach to bring about modification in the electronic properties of this 2DEG. This involves delta (δ) doping at the interface of LTO/STO by an iso-structural antiferromagnetic perovskite (TN = 298 K) LaCrO3 (LCO). This δ-layer dramatically alters the properties of the 2DEG. The changes includes increase in room temperature sheet resistance (R□), drop in the sheet carrier density (n□) almost exponentially with the layer thickness, and emergence of new features in the temperature dependence of R□ at T ≤ 50 K. Our spectroscopic measurement along with density functional theorem (DFT) calculations show that the Cr-ions at the interface act like a trap for electrons which are transferred from the LTO to STO surface. Extensive measurements of out-of-plane and in-plane magnetoresistance (MR) have been carried out on all the samples to address issue such as weak antilocalization and Kondo scattering. We have also observed a gradual crossover from positive out-of-plane MR to negative in-plane MR when magnetic field is titled with respect to the film surface.The MR measurements are augmented by the measurement of thermopower (S) which increases dramatically with δ-layer thickness at ambient temperature. The linear temperature dependence of S in the temperature range 100 to 300 K is indicative of diffusion thermopower. We also observed a large enhancement in thermopower in the temperature range where a minimum in R□ is observed. This enhancement is attributed to Kondo scattering. The thermopower is suppressed in the presence of a magnetic field and the suppression is isotropic with respect to the field direction.We will also present a tunable Rashba S-O interaction in these interfaces by δ-doping with another iso-structural ferromagnetic perovskite LaCoO3 (LCoO). In LCoO-doped sample, the inelastic scattering time varies as 1/T and the S-O scattering time remains constant in temperature, which suggests that the spin relaxation follows the D’yakonov-Perel mechanism. The δ-doping also results in 3 orders of magnitude decrease in τso whereas the inelastic scattering time increases very slowly with doping. A detailed analysis of anisotropic MR when the field is applied in the plane of the sample displays the effect of mixing of spin-up d-states and spin-down d-state at Fermi level due to spin-orbit interaction. Reference: 1. Shubhankar Das et al., Phys. Rev. B 90, 081107(R) (2014). 2. Shubhankar Das et al., Phys. Rev. B 90, 075133 (2014). 3. Shubhankar Das et al., Under review in Phys. Rev. B. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: GopalPankaj JainY9109066Cosmological and Colliders Implications of Conformal Symmetry21-3-2016 (Monday) 4.00 p.m. (Tea @ 3.45 p.m.)FB 382 (Physics Conference room) Symmetry has always played an important role in understanding Nature. At present most of our theoretical knowledge comes from the symmetry principles. In this talk I will discuss the implications of scale or conformal symmetry. In particular, I will propose a possible solution to the fine tuning problem of cosmological constant within the framework of softly broken conformally symmetric model. Cosmological constant is potentially a source of Dark energy, which constitutes about 70% of the energy density of the Universe. A major problem with the cosmological constant is that it gets very large quantum contributions from the matter sector at each order in perturbation theory. So we need to cancel these large contributions in order to maintain the small value of observed dark energy density, leading to an acute fine tuning problem. In our proposed solution we have shown that the matter sector will not contribute to the cosmological constant and hence we have solved the fine tuning problem. Here we have not considered the quantum gravity effects since quantum theory of gravity is not well formulated. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Dheeraj PratapProf. S. Anantha RamakrishnaY9109063Anisotropic and Hyperbolic Metamaterials in the Cylindrical Geometry15 Mar. 2016 (Tuesday)11 a.m. (Tea will be served at 10:45) L6, Lecture hall complexMetamaterials are artificially designed composite materials that are structured on subwavelength lengthscales. They are often inherently optically anisotropic, having direction dependent properties, due to their structure. Acid anodization of an aluminium micro wire was used to fabricate a nanoporous alumina microtube with non-branching nanopores aligned along the radial direction. This is essentially a metamaterial fiber with anisotropy in the cylindrical geometry. These nanoporous alumina micro tubes show enhanced optical and thermal properties, of which a study of the optical properties is presented here. Not only are these nanoporous alumina structures anisotropic, but they are radially inhomogeneous as well due to the radius dependent size of the nanopores. These metamaterials were homogenized to obtain the electromagnetic effective medium parameters (dielectric permitivity tensor)using techniques of transformation optics and Maxwell-Garnett homogenisation theory. Nanowires of silver were deposited electrochemically within the nanopores of the cylindrical nanoporous alumina to yield a metamaterial with hyperbolic dispersion. These metamaterial fibers exhibit novel optical modes that are described by Bessel functions of fractional and imaginary orders solely due to the anisotropy. Particularly for the high transverse momentum modes (whispering gallery modes), these modes are confined increasing near the centre of the waveguide in sheer contrast to conventional fibers where they localise to the edge. The inhomogeneous nature of the metamaterial fibers also cause extremely strong confinement of the modes near the center. The nanoporous alumina microtubes coated with active dye molecules of Rhodamine-6G show highly enhanced fluorescence, which is highly quenched in the presence of silver nanowires within the nanopores. Other studies, such as the possibility of manipulating the structure of the nanopores and a study of fluorescence from molecules deposited on nanoporous alumina, in the flat geometry were also carried out. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Jhuma DuttaProf. S. Anantha Ramakrishna and Prof. Harshwardhan WanareY9109069Photonic and Plasmonic properties of Periodically Patterned Columnar Thin Films19 Feb. 2016 (Friday)11 a.m. (Tea will be served at 10:45) L8, Lecture hall complexColumnar thin films are known to be deposited by making incident a collimated vapor flux at large oblique angles to a substrate. Using a substrate that is periodically patterned at micro/nanoscales, new kinds of periodically patterned columnar thin films (PP-CTFs) have been fabricated. The extremely anisotropic optical properties offered by the nanocolumnar structure with larger scale structures for photonic bandgap or diffractive effects provide for new physical phenomenon. These structures were also found to be extremely reconfigurable giving rise to enormous flexibility in applications.PP-CTFs are shown to have enhanced photonic and plasmonic properties. Dielectric PP-CTFs are shown to function as blazed diffraction gratings with large asymmetric diffraction efficiencies. A CTF made of plasmonicmetals like silver renders it an effectively biaxially anisotropic continuum. PP-CTFs of silver showed strong blazing action and unidirectionally coupled optical radiation to surface-plasmon-polariton(SPP) waves for both p- and s-polarizations.The blazing effect of the gratings of dielectric CTFs as a result of the spatially linear phase shifts caused by prismatic air cavities was understood using Kirchhoff-Fresnel diffraction theory. Homogenization of the metallic CTFs using the Bruggeman formalism revealed them to display hyperbolic dispersion, and the dispersion of SPP waves on both 1D and 2D gratings of such anisotropic hyperbolic media was found to be adequately described thereby. Detailed electromagnetic simulations of the grating structures reveal large changes in the photonic properties with the slant angle such as diffraction efficiencies and the electromagnetic near fields. Furthermore, these slanted nanocolumnar structure can be uniformly reconfigured by ion beam irradiation method and gives rise to reconfigurable blazed gratings, thereby maximizing the diffraction efficiencies for different wavelength bands by changing the blazing (angle) condition. A novel application of CTFs to fingerprint visualization was developed. Visualization of latent fingerprints is enhanced by deposition of columnar thin films at large oblique angle of CaF2 and SiO2 on fingerprint marks on nonporous surfaces and further enhanced visualization is obtained by treating the deposited CTFs with a fluorescent dye and fluorescence imaging. Speaker:Supervisor:Roll no:Title: Date:Time:Venue:Abstract: Jagtap J Manihrao Prof. V. Subhramanyam and Prof. Asima PradhanY9109074Mueller matrix spectroscopy imaging and image analysists techniques for human cervical pre-cancer discrimination.17.12.201511:00 AM onwardsFB 382 (Physics Department Seminar Room)--- Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Vandana YadavProf. V. SubrahmanyamY9109082Steady state properties of nonequilibrium growth and transport processes: A boundary layer based approach14 December, 20152:30 PMFB 382Microtubules are important components of the cytoskeleton of a cell. These are hollow,tube shaped biopolymers made of alpha, beta-tubulin heterodimers. Experimental studies reveal an interesting growth dynamics of microtubule which play an important role in various processes such as intracellular organization, transport, exerting pushing and pulling forces etc., it is crucial to understand the growth dynamics of microtubules and its regulations by various regulatory proteins. It has been predicted experimentally that motor proteins belonging to Kinesin-5 and Kinesin-8 family can regulate the dynamics of microtubule in a length dependent manner. Here I discuss a two-state model of microtubule polymerization with a length-dependent dynamics and obtain the length distributions of microtubules through a discrete formulation. These investigations suggest that the length dependent regulation leads to a much faster decay of distributions as compared to an exponential decay. In the latter part, I shall discuss about the boundary layer technique and show how it can be used to understand phases and phase transitions of driven many-particle exclusion processes. We develop the fixed point based boundary layer method and apply this to two distinct driven exclusion models to show how the shape of the particle distribution profile, the location of the boundary layer etc can bepredicted from the stability properties of the fixed points of the boundary layer equation. Further, I shall discuss the applicability of boundary layer method in order to understand the polymerization dynamics in the presence of diffusing tubulins. Since various rates associated with the polymerization dynamics are expected to be influenced by the number of available tubulins, the growth dynamics and the diffusive dynamics of tubulins are coupled in a nonlinear way. The boundary layer analysis emerges as a powerful method that allows us to obtain analytical solutions for the length distributions in a systematic way. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Mr. Sourabh BaruaProf. K P RajeevY7109075Electrical Transport Studies of Topological Insulator Materials Bi2Te3, Sb2Te3 and Bi2-xSbxTe3-ySeyMonday, 30 Nov 201510 AMFB 382Topological insulators have generated a lot of interest because of the revolutionizing properties predicted for these materials. These materials are supposed to have a conducting surface and an insulating bulk and the surface carriers have spin locked to their momentum. The surface conduction is furthermore topologically protected from disorders as the surface bands traversing the bulk band gap cross at the time reversal invariant momenta and these crossings are guaranteed unless time reversal symmetry is broken by application of a magnetic field or magnetic impurities. However, experimental verification of the surface conduction from these topological surface states in electrical transport studies has been difficult because of the bulk conduction in these materials due to crystalline defects. The focus of this thesis is the electrical transport studies of single crystals of topological insulator materials: Bi2Te3, Sb2Te3 and Bi2-xSbxTe3-ySey. In case of Bi2Te3, even though the sample was metallic, we were able to obtain signatures of the topological surface states from the Shubnikov-de Haas (SdH) oscillations in the magnetoresistance, such as a non-zero Berry phase, magnitude of surface conduction similar to that expected for topological insulators and also large and linear magnetoresistance. We also performed angle dependent magnetoresistance studies which have revealed that the SdH oscillations do not show the angle dependence expected for a two dimensional Fermi surface of the topological surface states. Further, the angle dependence of the frequency of the oscillations does not adhere to the behavior expected for the conventional three dimensional bulk Fermi surface in this material. We also observe asymmetry in the magnetoresistance in positive and negative fields, which has been attributed to anisotropic bulk transport earlier, but we show that such asymmetry can arise due to the mixing of Hall signal with longitudinal resistance in this material. In Sb2Te3, we observed prominent SdH oscillations although the Berry phase extracted was inconclusive in determining whether these were from the surface states. In Bi2-xSbxTe3-ySey, the single crystals obtained were metallic and these showed linear magnetoresistance. We also made a simple model of the dependence of resistivity on thickness for a topological insulator and compared it to data for various topological insulators published in the literature. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Anirban BaguiProf. S. Sundar Kumar IyerY7109861Effect of electric-field annealing during solvent drying step ofactive layer in organic solar cell devices5th August, 201512 noonSCDT seminar roomThe performance of organic electronic devices can often be improved by improving the nano-morphology of organic active layers in those devices. In this thesis, the role of thermal annealing of polymer layers during solvent drying in the presence of a constant electric-field to improve the the nano-morphology of the films thus formed was studied.Comparative studies of morphological and electro-optical properties of the P3HT:PC61BM based blend film and bulk hetero-structure solar cell devices were carried out to understand the role of 'electric-field annealing'. The external quantum efficiency (EQE) and power conversion efficiency (PCE) of the solar cells made showed significant enhancement due to better charge transport in the case of electric-field annealed devices.In order to study this process step on the hole mobility in the P3HT layer, devices with holes as the primary charge carriers were fabricated. The hole mobility in P3HT was found to increase monotonically for annealing field strengths up to 2000 V/cm. The current density – voltage (J–V) data corresponding to the space charge limited currents (SCLC) at various low temperatures for P3HT based hole-only devices were fitted with the empirical model and the Gaussian disorder model (GDM) to interpret the data. X-ray diffraction (XRD) measurements confirmed increase in crystallinity and crystallite size of the films for electric-field annealed samples. The observed changes in the charge transport properties due to electric-field annealing of the films at the time of their fabrication have been explained based on the above measurements and analysis.Solar cells in inverted structure have also been made with appropriate modification in device structure to get better performing devices by electric-field annealing. Finally, the effect of electric–field assisted treatments was studied on devices fabricated with PTB7 (a new promising polymer for organic solar cells) as the device active layer. The experimental results presented in this thesis confirm that application of electric-field during annealing of organic films during their formation is a useful processing step to fabricate higher mobility polymer films for building improved organic electronic devices. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Pranati Kumari RathProf. Pankaj JainY9109077Large Scale Anisotropy in Cosmic Microwave Background Radiation and its Relationship with Primordial Power Spectrum27th July, 20154 pmFB-382The Cosmological principle says that the universe at large distance scale is assumed to be statistically homogeneous and isotropic. The Cosmic Microwave Background Radiation (CMBR) is considered to be one of the best experimental evidence supporting this principle. However, there exists considerable evidence in cosmological data which suggests violation of this principle.In this talk, I will revisit two of these anomalies observed in the CMBR temperature data provided by WMAP and recently by PLANCK team. These includes the alignment of the quadrupole (l=2) and octopole (l=3) and the hemispherical power asymmetry. To explain the low-l alignment, I will discuss some anisotropic inflationary models within the framework of the Big Bang cosmology. In these models, the anisotropy decays very quickly during the inflationary phase of expansion. The resulting direction dependent power spectrum in this anisotropic background leads to violation of isotropy and hence explain the alignment of the low l CMBR modes. I will also discuss an inhomogeneous power spectrum model in order to explain the hemispherical power asymmetry, observed in the CMBR data. The hemispherical asymmetry can be parametrized in terms of the dipole modulation model. Alternatively, I will show that, an anisotropic dipolar imaginary primordial power spectrum, which is possible within the framework of noncommutative space-times, also provides a good description of the observed dipole modulation in CMBR data. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Shail PandeyProf. Sudeep BhattacharjeeY7209063Pulse modulated microwave plasmas : self-excited instabilityand plasma states3rd July, 20154.30 pmL-5Waves and instabilities are inherent to plasma and have been a widely studied subject owing to its importance in electron acceleration, wave absorption, and plasma heating. While most of the earlier studies discuss these in a preformed steady-state plasma, limited studies exist on excitation of waves and instabilities in pulsed plasmas. In the pulsed regime, large amplitude electromagnetic pulses can interact with a self-generated growing plasma, leading to the formation of instabilities. The present thesis carries out an experimental investigation of the plasma state created by pulsed electromagnetic waves in the microwave regime and excitation of instabilities due to counter streaming charged particles in the ionization front.Pulsed microwaves of 2.45 GHz are launched in a low pressure argon (0.2 – 2.5 mTorr) gas to generate a temporally growing ionization front which subsequently develops into a full fledged plasma. Depending upon the peak power and time duration of the pulses, plasmas are characterized by an afterglow state (AGP) or an interpulse state (IPP). It is found that : (i) the AGP state characterized by a temporally decaying plasma with a low value of electron temperature ~ 1 eV, occurs at larger pulse duration > 50 microseconds and (ii) the IPP state, where the plasma grows beyond the pulse duration, often forming a quasi-steady state in the power off phase, is realized at shorter pulse duration. The distinction between the two is further investigated by measurements of the electron energy probability function (EEPF).The IPP state is governed by plasma growth within the pulse, where strong interaction with the injected microwaves can be realized leading to formation of instabilities, which is investigated next. The measured particle current profile shows two phases of plasma development: (i) self-excited wave (SEW) in the ionization front, followed by (ii) rapid plasma growth. The growth rate of the SEW comes out to be ~ (1.0 – 4.5) × 10^7 s^-1, depending upon the pressure, which is further confirmed with optical emission spectroscopy measurements. Time – Frequency Analysis (TFA) indicates the presence of two frequency bands (~ 4 MHz) centered around 3.8 MHz and 13.0 MHz.The profile of electron thermal velocity v_th(t) and relative charged particle drift velocities V_D(t) in the SEW, obtained from the measured EEPF, indicate V_D > v_th – a condition necessary for excitation of Buneman instability (BI). The calculated growth rate and oscillation frequency of BI further confirms its existence in the SEW. Finally, the energy transfer from the instability leading to the heating of the plasma during the pulse is investigated. Speaker:Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Subhash MahapatraProf. Tapobrata Sarkar10109069Applications of the Gauge/Gravity duality : Superconductivity,Optics and Entanglement Thermodynamics.1st June, 201510 amL-6A remarkable connection, in the context of string theory, has emerged in last few years between gravity and the strongly coupled field theories. This connection, which is generally called as the gauge/gravity duality or the AdS/CFT correspondence, maps a quantum theory of gravity in (D+1) dimension Anti de sitter (AdS) spacetime to D dimensional quantum field theory - living at the boundary of AdS spacetime. An important feature of this duality is that in certain approximation, the strongly coupled limit of field theory corresponds to the weakly coupled limit of the gravity theory. This strong-weak nature of the correspondence has provided a unique approach to address some questions in strongly coupled condensed matter systems which otherwise would be intractable.In this thesis, we discuss important applications of the gauge/gravity duality, on three main directions. It includes the phenomenon of superconductivity, optics and entanglement thermodynamics.In this thesis work, we have studied and developed various generalized models for holographic superconductors and have computed response functions of these generalized superconductors using the gauge/gravity duality. We have been able to show that the occurrence of negative refractive index is a generic feature of holographic superconductors for specified ranges of frequencies. For these superconductors, we also showed that the holographic entanglement entropy is a good measure to probe different phases of the system. We further found a indication of the first law of thermodynamics like relation in the context of entanglement entropy for holographic superconductors. Speaker: Supervisor:Roll no:Title:Date:Time:Venue:Abstract: Anukul Prasad ParhiProf. S. Sundar Kumar IyerY5209061Annealing in the presence and absence of electric field of copper phthalocyanine based thin films and their applications in organic solar cells18th May, 20156 pmSamtel seminar roomThe ability to tailor nano morphology by influencing the growth and crystallization process of organic thin films is desired for various applications. Annealing is known to help in modifying thin film textures and properties and is easier to implement during device fabrication. In this thesis, films of copper phthalocyanine (CuPc) and their blends have been subjected to thermal annealing in the presence and absence of electric field. The films were then characterized by various methods including x-ray diffraction (XRD), scanning electron microscope (SEM) and by taking the film’s absorption spectrum. These were analysed to explain the effect of the annealing of the films.Initially, pristine CuPc films were annealed at different temperatures, electric fields and annealing ambiance - in vacuum as well as in nitrogen. It was observed that there was enhanced re-crystallisation by annealing at higher temperatures, in higher electric fields and in vacuum.Blends of CuPc with buckminsterfullerene (C60) films of different volume ratios (3:1, 1:1 and 1:3) were then annealed at different temperatures both in the presence and absence electric field. While the blending inhibited re-crystallisation (when compared to pristine films of CuPc), above certain temperatures, the two constitutes, CuPc and C60, were phase-segregated. The phenomenon of abrupt phase segregation was observed in 1:1 and 1:3 blends under electric field. Analyses of the absorption spectra carried out confirm blending and phase-segregation of the two constitutes in the film during annealing. Using density functional theory (DFT), the poloarisability of the constituent molecules were calculated and the trends in the intermolecular attraction and segregation were explained.Bi-layer organic solar cells were fabricated with CuPc and C60 films. The effect of annealed CuPc films and CuPc-C60 bi-layer films on the solar cell performance and their current-voltage characteristics were analysed. It was observed that annealing of the bi-layer stack resulted in improved solar cell performance. However, the application of electric field during annealing of the bilayers does not significantly add to the improvement in performance of the solar cells observed due to thermal annealing. Speaker:Roll no:Title:Date:Time:Venue: Supervisor:Abstract: Bahadur SinghY9209061First-principles investigations of topological phases in materials11th May, 201511 amFB-382Prof. Rajendra PrasadA new paradigm for classifying condensed matter systems by topology of bulk band structure has spurred the discovery of several new states of matter having exotic properties. In particular, topological insulators are such new states of matter that are distinct from conventional insulators. These materials support spin-polarized conducting states at the boundaries/surfaces while remaining insulating in the bulk. The surface states often exhibit Dirac-cone energy dispersion with helical or chiral spin-texture and are protected by time reversal symmetry. The topological protection ensures the backscattering free transport at the boundaries of topological insulators. The recent developments in the field show that these nontrivial states not only offer potential applications in quantum computing and spintronics, but also provide platforms for realizing novel quantum phenomena such as Weyl semimetals, and Majorana fermions in a condensed matter system.The thesis presents an analysis of topological surface state properties of several selected materials and predict new materials or thin films of materials that realize the quantum spin Hall state, Dirac semimetal, Weyl semimetal, or Rasbha effect, using the ab-initio density functional theory framework and k.p theory. Through systematic analysis of bulk and surface electronic structures, it is shown that thallium based ternary III-V-VI2 series of compounds, TlMQ2 (where M= Bi or Sb and Q= S, Se, or Te), contain both topological and normal insulators. Therefore, it is possible to study the topological phase transition (TPT) in these compounds by tuning the lattice parameters as well as spin-orbit coupling (SOC). The electronic structure and spin-texture analysis of topological surface states show that the surface states form an unusual planer metal that is essentially half of an ordinary two dimensional (2D) conductor and carry nontrivial $\pi$ Berry phase. Furthermore, a possible TPT in Ge(BixSb1-x)2Te4 thin films is discussed as a function of layer thickness and Bi concentration x. The systematic examination of band topologies suggests that thin films of Ge(BixSb1-x)2Te4 are viable candidates for 2D topological insulators, which would undergo a 2D-TPT as a function of x. Finally, it is shown that the inversion asymmetric compound, Sb2Se2Te, harbors both a novel nontrivial band topology and giant Rasbha-type spin splitting in its native form driven by strong SOC. The Rasbha splitting in the bulk bands of Sb2Se2Te is the largest that has been found to date and attributed to large polarization field (electric field) and small band gap in the system. Speaker:Roll no:Title:Date:Time:Venue: Supervisor:Abstract: Samit PaulY8209868Micron Focusing, Diagnostics and Interaction of Multi-ElementIon Beams with Matter7th May, 201511 amL6Prof. Sudeep BhattacharjeeA focused ion beam (FIB) system is an inevitable tool for research and applications in nano-science and technology. However, the availability of only gallium (Ga) ions as projectile, in liquid metal ion source (LMIS) based FIB systems limits its applicability. Moreover, metallic Ga has contamination issues and provides less milling yield. In order to ameliorate the above problems, an emerging area in this field is development of focused ion beams of a variety of elements utilizing plasmas. A compact, high current density (~ 1 A/cm^2), microwave plasma based multi-element focused ion beam (MEFIB) system has been developed in the laboratory, that is capable of delivering beams of different ions (H_2 , Ar, Kr, Ne) of energies up to 18 keV and spot size of ~ 25 µm, using a 1 mm plasma electrode (PE) aperture. The beam has an emmittance of ~ 0.1 mm-mrad and brightness of ~ 10^5 Am^-2 sr^-1 V^-1, thereby making it an excellent candidate for FIB.In this thesis our target has been to reduce the beam to submicron spot size and make it operate at higher energies (30 keV) comparable to that of conventional FIBs. In parallel, we developed a new diagnostics (spider probe) for measuring micron size beams and have investigated the time dependent physics of the interaction with matter, particularly, the milling capability to make micro-pores and microstructures on metallic foils and thin films.In order to achieve the above, first the plasma has been optimized to maximize the extracted beam current density maintaining tolerable ion energy spread at the meniscus (~5 eV). The utilization of two Einzel lenses in conjunction with a PE and a beam limiting (BL) aperture, is found to have better control over the beam energy and current density. Controlled sculpting of micron scale pores in aluminum foil with Ar and Kr ion beams is demonstrated. The temporal evolution of the micro pores caused by beam interaction on metallic foils has been studied and smallest pore diameter of ~ 3 µm has been successfully created. This shows that with proper control of the irradiation time, submicron pores can be fabricated. Further experiments, using smaller PE apertures of 45 µm and 31 µm, created by MEFIB, have been used to obtain a focused beam size ~ 1 µm. A time-dependent exposure method to determine the exact location of the focal point of the beam has been invented that takes care of the over irradiation issue. Extensive use of Lab-View software is made for manipulating the beams to create several microstructures such as circular and rectangular trenches, gratings, symbols and alphabetical letters on 50 nm Cu thin films.In order to reduce the beam spot size further, guiding and transmission of extracted Ar ion beams with the help of straight and tapered micro-glass-capillary have been investigated. The beam current through the capillary is found to have a threshold extraction voltage and observed to exhibit hysteresis, with a unique self-focusing capability. The temporal and dimensional dependence of the hysteresis have been studied. The guiding capability of the tapered capillary is found to be more effective where beam size reduction is desired without compromising total beam current unlike electrostatic beam limiters. For further understanding the phenomena, Particle-In-Cell simulations that solves Poisson's equation and equation of motion self consistently are carried out, and the experimental results are found to have a reasonable agreement with simulations. Speaker:Roll no:Title:Date:Time:Venue:Abstract: Prabwal Jyoti PhukonY8209867Studies on the Physics of Brames from the bulk to the boundary27th October 2014 (Monday)3 pmFB-382Branes play an important role in our understanding of physics at different length scales: from black holes to condensed matter systems. They arise as a class of dynamical, extended objects in string theory and other closely related theories like M-theory and are specified by the number of spatial dimensions they possess (a brane in p spatial dimensions is called a p-brane). In this thesis, we are particularly interested in the D-p-branes of string theory and M-p-branes of M-theory. We primarily focus on exploring three important directions related to brane configurations: its bulk in the low energy limit,i.e supergravity theory, bulk to boundary correspondence and a class of conjectured world-volume theories on branes. The thesis addresses three illustrative issues in the context of each of these three directions in three portions. To start with, we undertake an analysis of branes with reference to asymptotically AdS black holes and their thermodynamics. Here, we seek to understand logarithmic corrections to the Bekenstein-Hawking entropy for these black holes due to thermal fluctuations in the grand canonical ensemble. Then, in the second part of the thesis, we carry out a bulk to boundary theory study of branes in the backdrop of gauge-gravity duality, focussing on R-charged black holes. We study the electromagnetic response functions of strongly coupled media whose dual gravitational descriptions are given by these R-charged black holes. In dimensions 4, 5 and 7, these correspond to rotating configurations of M2, D3 and M5-branes, respectively. The third part of the thesis work attempts to understand the moduli space of a class of conjectured worldvolume theories on M2-branes. These are known as quiver gauge theories. As a specific example, we investigate the (2+1) dimensional N = 2 Chern-Simons (CS) quiver gauge theories which appear as worldvolume theories on a stack of M2-branes probing a special class of Calabi-Yau (CY) four folds, namely complex cones over smooth Fano 3-folds. In this portion, we put special emphasis on examining the Higgsing and unHiggsing of these theories via a "brane tiling" method. We conclude by highlighting some novel results in black hole thermodynamics, holographic optic and Chern-Simons quiver gauge theories that have been obtained in course of this thesis work. Speaker:Roll no:Title: Date:Time:Venue: Hemanadhan M.Y8209866Study of excited-state energy density functionalsconstructed by splitting k-space for homogeneous electron gas19th September 2014 (Friday)9:30 amFB-382 Speaker: Roll no: Title: Date: Time: Venue:Abstract: Prabhakar TiwariY9109075Observations of Large Scale Anisotropy and Cosmological Models8th September 2014 (Monday)12.00 pmFB-382The observable Universe is simply huge! ∼10^{26} meters in every direction, ~14 billion years old and contains ∼10 ^{80} hydrogen atoms. The modern cosmology is the science of the entire Universe. We, here on a small planet, can only assume that the Universe is knowable and physics is followed everywhere in the same manner. Furthermore it is reasonable to assume that the observable Universe is statistically same for all observers, located anywhere in the Universe. Today, we demand this uniformity as “Cosmological Principle” which assumes homogeneity and isotropy at large distance scales. This thesis is a critical examination of the cosmological principle. In the thesis we review the present observations of large scale anisotropy and discuss possible theoretical models to explain these observations. Speaker: Roll no: Title: Date: Time: Venue:Abstract: Mr. Shyam Lal GuptaY6209062Dynamics of Laser Ablated ZnO Colliding Plasma Plumes19th August, 2014 (Tuesday)03:00 pmFB-382Irradiation of the target material with a laser pulse having fluence higher than its ablation threshold fluence results in formation of its plasma. UV laser irradiation of target surface results in smaller/comparable thermal diffusion length compared to absorption length resulting in congruent ablation unlike the chunk ablation due to bulk target heating with longer wavelength irradiation. The properties of the plasma plume viz. temperature, electron number density, and ion velocities etc. affect the properties and quality of pulsed laser deposited thin films. The properties of the plasma plume can significantly be varied by using various configurations of colliding plasma plumes. The plasma plumes evolve with time due to the pressure difference with respect to the ambient and move due to the thermal kinetic energy gained by the plasma species from the ablating laser pulse. The velocity components of seed plasma plumes in the direction of the interaction contribute to processes occurring in the interaction zone, where as the forward plume front velocity components result in the movement of the interaction zone. The properties of interaction zone (interpenetration or stagnation) are tuned by varying the inter-plume distance and the ion-ion mean free path. The ion â€“ ion mean free path depends on relative collision velocity of two seed plasmas, the ionization state, and the ion density of the plasma plumes. In the present work we report the dynamical behavior of collinearly colliding ZnO plumes and formation of interaction zone / stagnation layer. The plasma plume dynamics is studied using fast imaging and optical emission spectroscopy. Two seed plasmas initially evolve independently and then start interacting at ~15 ns along their lateral dimensions. The interaction of seed plasma plumes results in a stagnation layer having abundance of the higher ionic states, high electron number density and temperature with slower decaying rate as compared to the single plume that lasts for longer time delays as compared to single plume. At the later stages of plasma evolution the vapor phase is populated by nanoparticles /nanoclusters of ZnO. The presence of nanoparticles/clusters of ZnO in the vapor phase for longer time delays in colliding plumes is confirmed using dynamic laser light scattering (Rayleigh) and photoluminescence (PL) techniques. The configuration is used for deposition of ZnO thin films on glass substrates using 1064 nm irradiation. The deposited films are characterized using X-ray diffraction, AFM, optical transmission in the UV-visible range and photoluminescence measurements. IR ablated colliding plasma plumes results in deposition of a-plane ZnO thin films with better optical properties as compared to c-axis oriented thin films obtained using conventional UV (355 nm) single plume deposition. The observed differences in the quality and properties of thin films are attributed to the flux of mono-energetic plasma species with almost uniform kinetic energy and higher thermal velocity reaching the substrate from interaction/stagnation zone of colliding plasma plumes.