SOTA 2015
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Paramita Dasgupta Tewari 13109070 Extensive Air Showers and the Physics of Ultra High Energy 26th June, 2015 (Friday) 4 pm FB-382 The origin of Cosmic Rays(CR) have been intriguing scientists since 1912 when V.Hess carried out his famous balloon flight to measure the ionisation rate in the upper atmosphere. The cosmic rays are energetic particles, which come to us from the outer space and are observed either through satellites or earth based detectors.The spectrum of Cosmic rays can be approximately described by a single power law from energy 10 GeV to the highest energies ever observed ~10^20 eV.During the last decade a significant progress has been made in experimental studies of high energy cosmic rays by means of extensive air shower (EAS) techniques. To a large extent this was due to a new strategy of data analysis, most consequently put forward by the KASCADE experiment.In this talk I will discuss about the particle interactions at ultra high energy using CORSIKA (Cosmic Ray Simulation for Kascade) simulation tools. With different particle interaction models employed, they significantly enhance the accuracy of data analysis, which allows us to obtain impressive results. It is believed that supernovae (SN), supernovae remnants (SNR), pulsars, compact objects in close binary systems and stellar winds are sources to Cosmic rays(CR), but there is still some uncertainty about their origin. Cosmic-rays are energetic particles that have propagated for millions of years in the Interstellar medium (ISM). During this long period of time, the cosmic particles lose or gain energy and even change composition: CR nuclei can decay and become originators of secondary particles and electromagnetic radiation. In the second part of my talk I will discuss The GALPROP code for cosmic-ray transport and diffuse emission production. Some results of our study on neutral and charged pion emissivity at different galactocentric radius and height will be presented at the end of my talk. |
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M M Vinay 13109077 Holographic Quantum Entanglement Entropy 21st May, 2015 (Thursday) 11 am FB-382 Entanglement entropy is used in quantum information as a measure of the quantum entanglement in bipartite quantum states. It was conjectured by Ryu and Takiyanagi few years ago, that the quantum entanglement entropy has a holographic geometrical interpretation through the AdS-CFT correspondence. This correspondence in string theory holographically relates a bulk theory of gravity in Anti de Sitter (AdS) space time with a conformal quantum gauge theory at the conformal boundary in some limit. The Ryu-Takiyanagi conjecture will be reviewed and specifically its implementation in the context of AdS3-CFT2 will be described. |
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Ashish Kumar 13109062 Magnetic Properties of Mn based Pnictogen Compounds 5th May, 2015 (Tuesday) 4.30 pm FB-382 Recently Mn (d5) compounds such as LnMnXO (Ln=lanthanide, X=As,P) and BaMn2X2 (X=As, Bi,Sb) having same structure as transition metal pnictogen family have attracted considerable attention. These compounds are of interest as they have magnetic ordering like cuprates but their crystal structure is like iron pnictides. In this seminar, recent experimental investigations of magnetic properties of BaMn2As2, BaMn2Bi2, and LaMnPO compounds will be reviewed. Recent theoretical investigations of these systems starting with five band interacting electron model propose a rich phase diagram involving a variety of phases such as antiferromagnetic, stripe, flux, paramagnetic, and mixed phases |
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Shubhajyoti Mohapatra 13109075 Magnetic Excitations in Frustrated Quantum Spin Systems 5th May, 2015 (Tuesday) 3.30 pm FB-382 Recent investigations of magnetic excitations and correlations near quantum phase transition (QPT) in antiferromagnetic frustrated spin systems with competing interactions and/or geometric frustration will be discussed, focussing on magnetic field- and pressure-induced QPT in the three dimensional coupled dimer system TlCuCl_3. Magnetic excitations in this system involving triplon excitations are theoretically modeled using the bond-operator technique, and the magnetic field-driven QPT is described through Bose-Einstein condensation in the lowest lying triplon mode. Triangular lattice organic salts will also be discussed, highlighting the QPT arising due to complex interplay between electron correlation, effect of low dimensionality, and magnetic frustration. |
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Sanghamitro Chatterjee 13109884 Plasma Based Low Energy Ion Beams: Extraction, Focusing and Interaction with Matter 22th April, 2015 (Monday) 4 pm FB-382 The application of low energy ion beams in science and technology has recently drawn attention of the scientific community. Low energy ion beams (0 – 5 keV) are interesting because of their unique interaction with matter. Unlike high energy beams, low energy ion beams do not penetrate deep into the substrate, but is confined within a few atomic layers below the surface, thereby bringing about interesting modification of material surface properties. There are various methods of realizing low energy ion beams. A promising and useful technique is low energy extraction of ions from a plasma. In this research, a microwave plasma based multiple ion beamlet system (MIBS) capable of producing both broad beams (diameter ~ 1 cm) and focussed ion beamlets (diameter ~ 30 microns), developed in the laboratory will be utilized. The system provides control over each individual beamlet, to generate desired patterns on a substrate without employing a mask. The characteristics of the plasma and the extracted ion beams in the MIBS have been investigated. At first, we have performed simulations on an inductively coupled plasma to understand the evolution of basic plasma properties like space potential, plasma density, electron energy distribution function, electron temperature, including phase space behavior of electrons and ions. From this study, we conclude that the discharge length (L) and pressure (p) are two important parameters that produce the same effect on discharge dynamics, a phenomenon named as “L-p similarity”. Thereafter, experiments are performed using the MIBS. While investigating the beam characteristics, it is found that the beam current profile is very similar to that of a vacuum diode, having two distinct regimes: (1) space charge limited flow regime and (2) extraction voltage limited flow regime. In the first regime, the profile deviates from the well-known Child-Langmuir law, whereas in the second regime, a Schottky-like behavior is observed, which is attributed to the fact that the collector potential actually perturbs the plasma potential through the plasma electrode apertures. The performance of the switching capable electrode for ion beam patterning has been investigated. Finally, we expect to apply the low energy ion beams for modification of the electrical and field emission properties of Graphene and Carbon Nanotubes respectively. The development of a compact Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) system is proposed, for synthesis of these carbon based materials. A brief review on the structural, electronic, and morphological modification of these materials upon low energy ion implantation will be presented, followed by the plans for future research. |
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Annwesha Dutta 13109061 Degeneracy in genetic code: causes and consequences of codon usage bias 13th April, 2015 (Monday) 4 pm FB-382 The genetic information stored in DNA is a living archive of instructions that a cell uses in sustaining and propagating "life". Nature has designed wonderful machineries for polymerizing such macromolecules (e.g. DNA, RNA, protein) using another biopolymer as the corresponding template. Translation is such a process by which a protein is synthesized from the information contained in a the sequence of nucleotides on a mRNA molecule. Each triplet of nucleotides on the mRNA template constitutes a codon. The genetic code is a set of rules by which a ribosome translates the sequence of codons on the mRNA into the sequence of amino acids, the building blocks of a protein. There are 61 distinct codons corresponding to 20 species of amino acids, which implies that the genetic code is degenerate i.e there are more than one synonymous codon code for the same amino acid. For some time after the discovery of genetic code, it was often believed that synonymous codons were used randomly. With the availability of more and more sequence data, it came to light that synonymous codon usage was non random and that different genomes have different preferred synonyms for any given amino acid. This phenomenon is termed as codon usage bias (CUB). My aim is to explain the evolutionary causes and physiological consequences of CUB using stochastic kinetic models. In this talk I'll begin by reviewing some proposed models for explaining the consequences of CUB. Then I'll briefly mention a theoretical model that I have developed very recently, capturing codon usage bias. |
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Soumendu Ghosh 13109076 Polymerase traffic on nucleic acid tracks: effects of stochasticity, queueing and interference 13th April, 2015 (Monday) 4 pm FB-382 Nucleic acid strands, namely DNA and messenger RNA (mRNA) serve as templates for polymerization of "macromolecules of life". For example, a RNA polymerase (RNAP) synthesizes a mRNA strand which is complementary to a single-stranded DNA template. While polymerizing a RNA, the RNAP walks step-by-step on the ssDNA strand in a specific direction consuming input chemical energy. Thus, a RNAP can also be regarded as a molecular motor. A large number of such motors move simultaneously along the same track or on adjacent tracks; the collective movement of such motors on the template strand (track) is often referred to as traffic because of the superficial similarities with vehicular traffic. In this talk I'll first present an overview of the rules that govern these types of molecular motor traffic. Then I'll briefly present a theoretical model for RNAP traffic by incorporating the steric interactions between RNAPs and the mechano-chemical cycle of individual RNAPs during the elongation stage, including the possibility of RNAP backtracking. The results of this model explain the physical origin of the switch-like regulation of the two interfering genes in both co-directional and contra-directional traffic of the two groups of RNAPs |
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Himanshu Gupta 13209861 Rayleigh-Benard Convection Problem in Intracluster Medium 24th March, 2015 (Tuesday) 3 pm FB-382 Galaxy clusters are the largest known gravitationally bound structures which consist of very hot (10-100 Mega-Kelvin) plasma, called intracluster medium (ICM). This plasma is mainly composed of Hydrogen & Helium. In ICM, both of the species are fully ionized. ICM is a weakly magnetized (~1 micro-Gauss), weakly-collisional and high-beta plasma in which the anisotropic transport of heat and momentum, and diffusion of ions is predominantly along the direction of the magnetic field. It is well known that ICM can become convectively unstable via either magneto-thermal instability or heat-flux-buoyancy-driven instability. This talk concerns with aforementioned two instabilities in ICM. I plan to review them and discuss the outcomes of the linear stability analysis performed on the equations governing ICM modeled as a Rayleigh-Benard set-up. |
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Ashutosh Kumar Singh 13109879 Wave packet dynamics (collapse, revival etc.) and ultrafast carrier dynamics in various two dimensional systems. 24th March, 2015 (Tuesday) 11 am FB-382 This talk is divided into two parts. In the first part we will discuss about spontaneous collapse and consequent quantum revival of injected wave-packet in two dimensional systems with non-equidistant energy levels. Initially when a well localized wave packet is injected into a 2D system with non-equidistant Landau energy levels, it undergoes cyclotron motion and evolves quasi-classically for a number of cycles, with its probability density spreading around the quasi-classical trajectory. Non-equidistant nature of the discrete energy spectrum then leads to destructive quantum interference and consequently the collapse of the wave-packet. The collapsed wave-packet regain their initial waveform and oscillate again with the quasi-classical periodicity on a much longer time scale known as revival time. In the second part we will talk about the carrier dynamics in Graphene under exposure to light beam which theoretically investigated either in the field theoretic language or is based on solving a set of extended optical Bloch equations obtained from time dependent Dirac/Schrodinger equation. |
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Pramod Ghising 13109071 MFM Studies on Ferromagnets and superconductors 27 February 2015 (Monday) 12:00 Noon FB-382 |
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Rajesh Tripathi 13109074 Low temperature properties of rare earth based ‘122’ phosphide 27 February 2015 (Monday) 11:00 am FB-382 |
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Jitendra Kumar Pradhan 13109066 Infrared sensors based on Metamaterial Perfect Absorbers 23 Feb. 2015 (Monday) 4.00 pm FB-382 |