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Jhuma Dutta
Y9109069
Photonic and Plasmonic properties of Periodically Patterned Columnar Thin Films
26 November, 2014 (Wednesday)
11 am
FB-382
Columnar 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.
The PP-CTF of various dielectrics like CaF2/ MgF2/SiO2 and metals like silver are shown to be conveniently fabricated by this method. The complete morphology of the PP-CTF is shown to be determined by controlling the angle and flux rate of depositions. 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 plasmonic metals like silver renders it an effectively biaxially anisotropic continuum. PP-CTFs of silver showed strong blazing action and asymmetrically coupled optical radiation to surface-plasmon-polariton(SPP) waves for both p- and s- polarizations propagating only along one direction supported by the CTF/dielectric interfaces.
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 using superstructures 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 two nonporous surfaces such as smooth glass slides and highly reflecting rough aluminum sheets and further enhanced visualization is obtained by treating the deposited CTFs with a fluorescent dye and fluorescence imaging.
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Sourabh Barua
Y7109075
Electrical transport studies of topological insulator
materials.
24th November, 2014
11:30 am
FB-382
Topological insulators are a class of materials which have generated great interest due to their novel properties. The band structure of these materials has surface bands which span the bulk band gaps and the crossing of these bands at the time reversal invariant momentum is protected as long as time reversal symmetry is not broken. This ensures that the materials have a conducting surface despite possessing an insulating bulk. However experimental verification of these surface states in electrical transport studies has been difficult due to the fact that the bulk itself conducts due to defects. In the work done for this thesis we have performed electrical transport studies of the topological insulator materials Bi2Te3 , Sb2Te3 and Bi(2-x)Sb(x)Te(3-y)Se(y). In Bi2Te3, in spite of a metallic bulk, we observe signatures of a topological surface state from the non-zero Berry phase extracted from the Shubnikov-de Haas(SdH) oscillations in the magnetoresistance in addition to a large and linear magnetoresistance. Further angle dependent magnetoresistance studies have revealed interesting asymmetry for positive and negative fields for some angles although the SdH oscillations are more or less symmetric. Sb2Te3 shows initially a negative magnetoresistance which evolves with time to a positive one but the same SdH oscillations persist. There is also asymmetry in the positive and negative fields. In Bi(2-x)Sb(x)Te(3-y)Se(y) we obtained metallic single crystals which showed linear magnetoresistance. We have also made a comparison of the thickness dependence of resistivity reported for various topological insulators in literature with a simple model.
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Anirban Dutta
Y7109062
Temperature dependent scanning tunneling microscopy and spectroscopy (STM/S) studies of Iron-Arsenic based single crystals.
21st October, 2014 (Tuesday)
3 pm
FB-382
Iron-Arsenic (FeAs) based superconductors with high critical temperature have a spin ordered ground state with a spin density wave (SDW) order. Superconductivity emerges in close proximity to the SDW state by suppressing (or weakening) SDW order, thus the SDW order is found to anti-correlate with the superconducting order. My thesis work focuses on the variable temperature scanning tunneling microscopy and spectroscopy (STM/S) studies of AFe2As2 (A: rare earth or alkaline earth element), i.e. 122-series, compounds. We have studied two undoped parent compounds, EuFe2As2 and SrFe2As2, and two doped compounds, CaFe1.96Ni0.04As2 (underdoped) and SrFe1.6Co0.4As2 (optimally doped) with STM/S. The tunnel spectra show homogeneous energy gap in the undoped and underdoped crystals in the SDW state. However, we see spatially inhomogeneous spectra in the SC state in underdoped and optimally doped crystals. We observe an asymmetric suppression in densty of states only at some locations in the SC state for the underdoped crystal, while the optimally doped crystals show a superconducting gap everywhere with spatially varying magnitude. Moreover, for the undoped SrFe2As2 crystals, strength of the SDW gap weakens below 21K, where a small fraction superconducts as found from resistivity and suscptibility measurements. This compound could be more prone to superconducting order and small local perturbations, like strain, may nucleate superconductivity locally in this undoped crystal. Finally we discuss the posiible reasons behind the observed inhomogeneous superconducting state.
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Samit Paul
Y8209868
Micron Focusing, Diagnostics and Interaction of Multi-Element Ion Beams with Matter
22nd September, 2014 (Monday)
12:00 noon
FB-382
A 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, i.e., capable of delivering beams of different ions (H2, 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-2sr-1V^-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 to create the microstructures 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.
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Ms. Vandana Yadav
Y9109082
Polymerization dynamics of microtubules: A boundary layer based approach
4th September, 2014
2:30 pm
FB-382
Microtubules are important components of the cytoskeleton of a cell. These are hollow, tube shaped biopolymers made of alpha,beta-tubulin heterodimers. Experimental studies reveal that microtubules alternate between persistent phases of polymerization and depolymerization. Since microtubules play an important role in various processes such as intracellular organisation, transport, exerting pushing and pulling forces etc., it is crucial to understand the polymerization dynamics of microtubules and its regulations by various other proteins.
In the first part of my seminar, I shall discuss a two-state model of microtubule polymerization with a length-dependent dynamics and obtain the length distributions of microtubules through a discrete formulation.
Next, I shall discuss the boundary layer method and show how it can be used to understand phases and phase transitions of driven many-particle exclusion processes. We develop a 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 be predicted from the stability properties of the fixed points of the boundary layer equation.
In the last part of my seminar, I shall use the boundary layer method to understand the polymerization dynamics of microtubules 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.
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