List of Ph.D. Theses

 
 

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Abhishek Kumar
Mahendra K. Verma
11109062
Energy spectra and fluxes of buoyancy-driven turbulent flows
23/01/2017 (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.


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Ambrish Pandey
Mahendra K. Verma
10109872
Scaling of large-scale quantities in Rayleigh-Bénard convection
06-Oct-2016
11:30 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


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Mr. Ravi Pratap Singh
R K Thareja
10109068
Time resolved investigations of laser ablated single and colliding carbon plasma/plumes.
May 20, 2016 (Friday)
11:00 AM onwards
FB 382 (Physics Department Seminar Room)
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Shraddha Sharma
Amit Dutta
Y9209067
Fidelity and Loschmidt echo: quenches, non-analyticities and emergent thermodynamics
2nd May (Monday), 2016
10 AM
FB 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.


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Mr. Dushyant Kumar
Prof. R.C. Budhani and Prof. Z. Hossain
Y9109064
Spin and Electronic Charge Diffusion in Superconducting NbN and Disordered SrTiO3 Under the Influence of Magnetic Field and Light
April 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.


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Nikhil Kumar
Prof. Anjan Gupta
Y9109074
Controlling thermal hysteresis in superconducting weak links and micron size superconducting quantum interference device.
April 27, 2016
11:00 AM onwards
FB 382 (Physics Department Seminar Room)
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Gangadhar Behera
S. Anantha Ramakrishna
Y9109065
Nanostructured plasmonic thin films for enhanced optical properties
20 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.


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Ms. Reeta Pant
Krishnacharya
Y9209066
Investigation of Tunable Wetting and Slippery Behavior on Hydrophilic Surfaces
519 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.


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Rohit Kumar
Prof. Mahendra K. Verma
10109875
Energy transfers in dynamos with small and large magnetic Prandtl numbers
03.11.2015
2:30 PM
FB 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.


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Mr. Shubhankar Das
R.C. Budhani and Zakir Hossain
Y9109078
Magnetoresistance and Magnetothermopower studies of delta-doped 2-dimensional electron gas at the interface of LaTiO3/SrTiO3
1st April (Friday), 2016
8:45 AM
FB 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.


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Gopal
Pankaj Jain
Y9109066
Cosmological and Colliders Implications of Conformal Symmetry
21-3-2016 (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.


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Dheeraj Pratap
Prof. S. Anantha Ramakrishna
Y9109063
Anisotropic and Hyperbolic Metamaterials in the Cylindrical Geometry
15 Mar. 2016 (Tuesday)
11 a.m. (Tea will be served at 10:45)
L6, Lecture hall complex
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.


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Jhuma Dutta
Prof. S. Anantha Ramakrishna and Prof. Harshwardhan Wanare
Y9109069
Photonic and Plasmonic properties of Periodically Patterned Columnar Thin Films
19 Feb. 2016 (Friday)
11 a.m. (Tea will be served at 10:45)
L8, Lecture hall complex
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.

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.


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Jagtap J Manihrao
Prof. V. Subhramanyam and Prof. Asima Pradhan
Y9109074
Mueller matrix spectroscopy imaging and image analysists techniques for human cervical pre-cancer discrimination.
17.12.2015
11:00 AM onwards
FB 382 (Physics Department Seminar Room)
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Vandana Yadav
Prof. V. Subrahmanyam
Y9109082
Steady state properties of nonequilibrium growth and transport processes: A boundary layer based approach
14 December, 2015
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 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.


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Mr. Sourabh Barua
Prof. K P Rajeev
Y7109075
Electrical Transport Studies of Topological Insulator Materials Bi2Te3, Sb2Te3 and Bi2-xSbxTe3-ySey
Monday, 30 Nov 2015
10 AM
FB 382
Topological 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.


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Anirban Bagui
Prof. S. Sundar Kumar Iyer
Y7109861
Effect of electric-field annealing during solvent drying step of
active layer in organic solar cell devices
5th August, 2015
12 noon
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.


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Pranati Kumari Rath
Prof. Pankaj Jain
Y9109077
Large Scale Anisotropy in Cosmic Microwave Background Radiation and its Relationship with Primordial Power Spectrum
27th July, 2015
4 pm
FB-382
The 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.


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Shail Pandey
Prof. Sudeep Bhattacharjee
Y7209063
Pulse modulated microwave plasmas : self-excited instability
and plasma states
3rd July, 2015
4.30 pm
L-5
Waves 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.


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Subhash Mahapatra
Prof. Tapobrata Sarkar
10109069
Applications of the Gauge/Gravity duality : Superconductivity,
Optics and Entanglement Thermodynamics.
1st June, 2015
10 am
L-6
A 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.


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Anukul Prasad Parhi
Prof. S. Sundar Kumar Iyer
Y5209061
Annealing in the presence and absence of electric field of copper phthalocyanine based thin films and their applications in organic solar cells
18th May, 2015
6 pm
Samtel seminar room
The 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.


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Bahadur Singh
Y9209061
First-principles investigations of topological phases in materials
11th May, 2015
11 am
FB-382
Prof. Rajendra Prasad
A 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.


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Samit Paul
Y8209868
Micron Focusing, Diagnostics and Interaction of Multi-Element
Ion Beams with Matter
7th May, 2015
11 am
L6
Prof. Sudeep Bhattacharjee
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, 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.



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Prabwal Jyoti Phukon
Y8209867
Studies on the Physics of Brames from the bulk to the boundary
27th October 2014 (Monday)
3 pm
FB-382
Branes 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.


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Hemanadhan M.
Y8209866
Study of excited-state energy density functionals
constructed by splitting k-space for homogeneous electron gas
19th September 2014 (Friday)
9:30 am
FB-382  

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Prabhakar Tiwari
Y9109075
Observations of Large Scale Anisotropy and Cosmological Models
8th September 2014 (Monday)
12.00 pm
FB-382
The 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.  


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Mr. Shyam Lal Gupta
Y6209062
Dynamics of Laser Ablated ZnO Colliding Plasma Plumes
19th August, 2014 (Tuesday)
03:00 pm
FB-382
Irradiation 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 – 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.

 

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Mr. SK Firoz Islam
Y8109070
Theoretical Study of Electronic, Transport and Thermoelectric Properties of Spin-Orbit Coupled Two-Dimensional Electron Systems in Presence of Magnetic Field.
3rd June, 2014 (Tuesday)
6 pm
FB-382

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M. Hemanadhan
Y8209866
Study of excited-state energy density functionals constructed by splitting k-space for homogeneous electron gas.
2nd June, 2014 (Monday)
4 pm
FB-382

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Abhishek Chowdhury
Y7209061
Multi-element focused ion beamlets for localized low energy ion matter interaction.
30 May, 2014 (Friday)
4 pm
FB-382
In this thesis a low energy multiple ion beamlet system (MIBS) is developed and utilized for non-destructive investigation of ion matter interactions at micrometer length scales. Since low-energy ion beams (1-5 keV) cannot penetrate deep inside the substrate, therefore the excitations are mainly limited to surface and sub-surface layers. Until now researchers had mainly considered single, broad ion beams (diameter ~ 0.1-2 cm) which made local perturbations challenging. The focused multiple ion beamlets can be employed for tailoring and creation of localized resistive, conducting or optically active regions in matter in a patterned manner.

The beamlets are extracted from a compact wave driven plasma source developed in the laboratory. Two types of beamlet "shower" electrodes are employed in the experiments: one with a honeycomb structure with notched apertures and another a 5×5 array of through apertures. Detailed experiments are performed to optimize the plasma source parameters for obtaining uniform beamlet current, as confirmed by angular measurements of the ion saturation current near the extraction electrode. A computer controlled switching technique is developed that can control motion of individual beamlets for patterned irradiation.

The MIBS is employed to systematically investigate low-energy ion irradiation induced changes in sheet resistivity and Debye temperatures in metallic nano-films of Ag, Cu and Al of thickness d/lo ~ 2-5, where d is the film thickness and lo is the bulk mean free path. The number of defects and impurities in the nano-film is varied in a controlled manner by varying the ionic mass number and beam fluence. Both atomic (Ne, Ar, Kr) and molecular (H2, N2) gases are employed in the investigation. Low temperature measurements are carried out for pristine and irradiated films to determine the residual sheet resistance (RRS). An empirical formula relating RRS with ionic mass number and beam fluence is proposed for the first time. Large diffusion of impurities into the film driven by thermal gradients at the interface is found to give rise to the unexpectedly large observed values of sheet resistance, as confirmed from energy dispersive x-ray spectroscopy. The Debye temperature as determined from Bloch-Grüneisen fitting of sheet resistance versus temperature data, is found to decrease with both ionic mass number and fluence, primarily due to the change in bulk modulus of the nano-film. The surface morphology of the films are investigated using atomic force microscopy (AFM) measurements. The experimental results are verified using well known ion matter interaction simulation tools such as SRIM, TRIM and TRIDYN_HZDR.

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Mihir Sarkar
Y7209062
Charge particle beam interaction with matter from a perspective of lithography.
27th May, 2014 (Tuesday)
2 pm
FB-382
The interaction of energetic charged particles with matter results in a variety of events which are interesting from the point of view of science, and offers exciting possibilities for exploring novel processing applications. In that context, we study two loosely connected aspects of charged particle matter interaction, one of which relates to the charge exchange process that occurs to high speed ions passing through the stripper medium of a tandem accelerator. The other one is electron and ion beam induced lithography which occurs by ionization of a target resist medium. The investigations are motivated by the unique possibilities of charge particle beam lithography for prototyping organic devices.

The charge exchange process is studied by using negative carbon ion (12C-) beam and analyzing the effect of the stripper gas pressure on the average charge (q_avg), charge state fractions, and transmission of the incident beam. It is shown that the equilibrium charge state istribution in the outgoing beam is slowly altered by increment in the stripper gas pressure. For particular incident ion energy the transmission shows a maximum with variation in stripper gas pressure due to the interplay between populations of charge changing target atoms and elastic scattering centers.

The charge particle based lithography process was taken up with MeV roton beams extracted from the tandem accelerator and keV electron beams of a dual-beam SEM/FIB. The effects of proton beam exposure dose and post exposure processing conditions on the quality of the developed micro-structures are analyzed. The exposure characteristics of 30 keV electron beam lithography in a thick chemically amplified resist SU-8 are discussed. The widening of the developed structures in excess of a particular equienergy deposition density contour follows non-monotonic variation. The sidewall profiles indicate that the crosslink profile of the resist proceeds in a reaction-diffusion environment in which the photoacid diffusion itself is controlled by crosslinking density.

Next, wetting behavior of an electron beam written 3D micro-structure pattern on ITO/Glass substrate by two functional inks used for inkjet printing material deposition is examined. The patterned substrate facilitates all additive deposition of an organic semiconductor over the micro-structure pattern. The possibility of convenient processing f array of thin film transistors and similar potential applications are discussed.

Finally, 30 keV electron beam lithography is investigated in a novel multilayer of resist which includes an intermediate metallic layer in the stack. The feasibility of the process and the possible line width resolution has been analyzed by obtaining the spatial distribution of electron energy deposition density profile through the multilayers of resist. As a test case application, the lithography process is shown to facilitate fabrication of a vertical organic transistor based on poly (3-hexylthiophene) (P3HT) as the active material.

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P. K. Rout
Y7209866
Interface superconductivity and giant proximity effect in La-based cuprate heterostructures
23rd May, 2014 (Friday)
3 pm
FB-382
A range of novel physical phenomena like high mobility two-dimensional electron gas, quantum Hall effect, magnetism, and interface superconductivity are observed at interfaces between complex oxides, which completely differ from the ones of the constituent compounds. Recent observation of superconductivity at the interface of various cuprate heterostructures is one such exciting phenomenon. Here, we will present the observation of interface superconductivity in the bilayer systems composed of various combinations of cuprates like La2-xSrxCuO4 and La2-x-yNdySrxCuO4. The nature of the interfacial layer in such composite systems will be discussed. The second part of the seminar will deal with the superconductor (S)-normal metal (N)-superconductor (S) Josephson junctions based on high temperature cuprate superconductors. Unlike conventional SNS junctions, the supercurrent in these junctions can flow through barriers as thick as 50 nm, which is order of magnitude larger than the coherence length. We will discuss our results in the context of "giant proximity effect".

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D. N. Patel
Y5209062
Spectroscopic investigations of brass plasma and synthesis of nanoparticles.
19th May, 2014 (Monday)
12 noon
FB-382
Laser ablation of materials has applications in thin films, laser etching, micromachining and nanotechnology. A comprehensive study of laser ablation of brass is carried out in different ambient conditions viz. vacuum, air and liquid to understand composite/alloy target plasma and formation of nano particles in plasma plumes. The brass plasma is created using Nd:YAG
laser operated at 10 Hz with 1064 nm and 266 nm. Optical emission spectroscopy (OES) measurement at threshold of brass shows the
preferential vaporization of Zn. The influence of magnetic field on plasma plume dynamics shows the splitting, oscillations and rotations of the
plume. The behavior of the expanding plume in various ambient; vacuum, air and liquid, shows distinct confinement of plume in water ambient. The high temperature, density and pressure of plume in water ambient facilitate the formation of metastable phases. Nanoparticles formed in the plumes show a compositional change with the size of nanoparticles in the vapor phase. The nanoparticles are characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) measurement, photoluminescence (PL) and Raman spectroscopy. The smaller size particles (nanoparticles size ~55 nm and size distribution FWHM ~ 7.5 nm) are reported in water ambient, however, the larger size particles (micron size particles ~ 2.5 µm and size distribution FWHM ~ 1.7 µm) are observed in air ambient. Smaller size particles are mostly Zn enriched. The photoluminescence (PL) measurement of the particles deposited in water ambient shows the peak at 380 nm and the PL peaks at 380 nm, 415 nm and 440 nm are observed when the particles are deposited in air ambient. The measurement of particles size shows the decrease in size away from the crater periphery in air however, in the case of ablation in water ambient nanosized particles are observed away from the crater periphery and nano rod shaped structures are reported at the crater periphery. The Raman measurement on the deposited structures confirms the ZnO rod at the crater periphery.

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Pabitra Mandal
Y6109067
Anomalous magnetic response of CaFe1.94Co0.06As2 superconductor and nonlinear response of the driven vortex state in NbS2 superconductor.
17th May, 2013
11:15 am
FB-382
In recent times studies on the new class of Iron-Pnictide superconductors have revealed a complex doping phase diagram. It is found that in a certain doping range, superconductivity in these system emerges via the suppression of magnetic order. As a part of the thesis, we have developed a high sensitivity magneto-optical imaging technique for sensitively imaging small changes local magnetic field distribution in underdoped 122- CaFe0.94Co0.06As2 single crystals. Our investigations reveal evidence of an unusual coexistence of magnetic and superconducting property in this material. Details of this will be discussed. From our work we suggest that there exists an the inherent coexistence of magnetism along with superconductivity may also be responsible for the anomalously strong pinning found in this class of superconductors.
I will also present our investigation into novel phase transformations in the driven vortex matter in NbS2 superconductor. We investigate the scaling of the current-voltage characteristic of the superconductor using universal scaling ansatz proposed by Meng-Bo Luo et al., PRL 98, 267002 (2007). This study attempts to understand the depinning transition and what is the nature of the driven vortex phase.

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Mr. Ajeet Kumar Sharma
Y8109061
Distributions of first passage times in stochastic processes in a living cell
8th May, 2014 (Thursday)
4 pm
FB-382

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Mr Suman Banerjee
Y5209865
Opto-electronic analysis of planar, bulk and hybrid planar-mixed heterojunction organic solar cells
8 May, 2014 (Thursday)
11 am
Samtel Centre Seminar Room
he electrical response to incident light on a thin film stack system was analysed with an opto-electronic model by considering interference effect in a thin film stack system and solving the basic semiconductor equations to describe various characteristics of organic solar cells. The model was then used to describe incident light wavelength dependent photocurrent response (spectral response) of Schoottky diode, planar and bulk-heterojunction solar cells. Important parameters of organic materials. Such as, exciton diffusion length, exciton dissociation
efficiency and exciton blocking efficiency are estimated. Quantitative analysis of the role of different thin film layers and optimised film
thicknesses of efficient solar cell devices are estimated using the model.

The model is used further to explain the current-voltage (I-V) characteristics of organic solar cell. A general model for planar mixed-heterojunction solar cell is developed and used for planar and bulk heterojunction solar cell by considering some special cases. The role of mobility and contact property (extraction rate) on the I-V characteristics of planar and bulk-heterojunction solar cells are explained using the model. Many devices were made and characterised using the experimental setup designed in the lab. Device characteristics were also explained using the model developed.

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Mr. Samar Layek
Y6209862
Structural and Magnetic Properties of Transition Metal based Oxides/hydroxides Synthesized Using Specific Methods
31st March, 2014 (Monday)
6 pm
FB-382
Transition metal oxides/hydroxides are important in many applications. Three oxide systems namely iron oxide/hydroxides, Nickel oxide and Barium ferrite are investigated. Interesting results are obtained as the properties can be controlled by the synthesis method and parameters.

Hematite nanoparticles synthesized by using different combustion methods show variety of magnetic properties depending of the particle size and specific choice of fuel used for preparation. Maghemite nanorods of length 20 nm show preferential magnetic ordering.

Mixed iron oxide/hydroxides are one of the potential candidates for the fluoride and heavy metal ions purification. The effect of dilute amount of Ca, Mg substitution on the phase formation, structural and magnetic properties is studied in details. Increasing Ca doping leads to phase formation. Simultaneous incorporation of Ca, Mg leads to completely amorphous phase.

The effect of transition metal and rare earth metal substitution on the structural and magnetic properties on NiO nanoparticles is studied. Mn
doping changes the antiferromagnetic ordering in NiO to completely ferromagnetic ordering whereas weak ferromagnetic nature is found for other dopants. The value of magnetization depends on the concentration of doping and the particular dopant.

Enhancement of the magnetic properties in single phase doped BiFeO3 ceramics is found by mechanical activation. The magnetization is seen to increase either by doping in Bi/Fe site or by decreasing crystallite size below a certain limit.

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Durgesh Chand Tripathi
Y5109066
Carrier Transport in Organic Semiconductor Diodes with Doped/Undoped Interfaces
29th March (Saturday), 2014
5 pm
SCDT Seminar Room
Organic semiconductors are currently being used for many large area electronic applications. With rising complexity of multilayer structures such as in organic light emitting diodes, the doped/undoped (high-low) organic interface has become an important determinant of device performance. However, underlying mechanisms of charge transport across the doped/undoped interface is not understood.

In this thesis, we study charge transport across the doped-undoped interface using capacitance-voltage (C-V) and temperature dependent current density-voltage characteristics. The specially designed device structures for this purpose have been fabricated using a state-of-the-art 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 carrier transport mechanism at doped/undoped interface is sensitive to the barrier height and carrier accumulation at the interface. It is found that the charge transport across homo-interfaces is controlled by tunneling through alignment of local density of states on the two sides; whereas at hetero-interfaces it is controlled by the offset.

The characteristic peak observed in C-V in such structures is found to depend on device configuration, and the peak height is sensitive to the layer thickness. A competition between the diffusion dominated and drift dominated transport is responsible for occurrence of the peak. A functional relation between small signal capacitance and voltage has been established in the diffusion regime prior to the C-V peak. It has also been shown that the measured mobility by impedance technique in space charge limited regime is dependent on device structures needing careful interpretation of its field dependence.

Electroluminescent transient technique has been used to measure diffusivity and mobility simultaneously in organic diodes. In contrast to mobility, field dependence of diffusivity showed a specific minimum at a critical field, which suggests a convenient normalization procedure, called "shift normalization", to annihilate temperature dependence. This method has been used for a unified description of mobility in prototypical small molecule Alq3 from which all the parameters of Gaussian disorder transport models can be inferred independently.

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Md. Shamim
Y6209861
Construction of exchange and correlation energy functionals for
excited-states in time-independent density functional theory.
27th December, 2013 (Thursday)
10:00 am
FB-382

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Debaprasad Sahu
Y7109067
Physics of negative ion containing plasmas: volume generation, measurement and wave induced phenomena.
16th December, 2013 (Monday)
10:30 am
FB-382In this thesis, a novel microwave plasma based volume negative ion source is developed and detailed experiments are performed to optimize negative ion generation for hydrogen (H-)and measurement of negative ion density. Electromagnetic waves of 2.45 GHz (angular frequency 1.53×1010 rad/s) are launched directly into a multicusp (MC) plasma device in the k perp B mode, where k is the wave vector and B is the magnetostatic field. Localized wave induced resonances are created at the periphery of MC (inner radius = 60 mm), depending upon the magnetic field and plasma density. These resonance zones are identified experimentally. The electron cyclotron resonance (ECR) occurs close to the boundary at r = 42 mm, whereas the upper hybrid resonance (UHR) lies r = ~ 22 mm away from MC boundary.The H- source is divided into two sections namely production (PR) and attachment (AR), separated by a transverse magnetic filter, which deflects the hot electrons (7 - 15 eV) trying to enter from PR and allows only the cold electrons (~ 1 - 2 eV) into AR. The hot electrons in PR are favorable for the generation of vibrationally excited molecules of hydrogen, which pass into AR, where negative ions are produced by their dissociative attachment with the cold electrons. The plasma frequency at the center is ~ (1.2 - 1.4)×1010 rad/s and is comparable to the angular wave frequency indicating sustenance of close to cutoff density plasmas (~ 7.44×1010 cm-3) at the wave frequency. The electron mean free path lies in the range ~ 25 - 30 cm and is comparable to the system size, therefore the plasma is almost collisionless. Measurements of electron energy distribution function (EEDF) in both AR and PR confirm the nature of the distribution and suitability for the aforementioned processes. The H^- density measured using two methods, viz. the extracted current and the second derivative beat method is found to be ~ 5×109 cm-3. The source has also been operated in the pulsed mode. Here, temporal filtering generates negative ion rich plasmas in the afterglow phase where cold electrons (~ 1 eV) are generated. The H- density in the afterglow is measured to be ~ 2×1010 cm-3, using the saturation current ratio method and is higher than that of the continuous mode case.Finally, a steady state and a time dependent model, using particle and charge balance equations are developed to estimate the negative ion density in both the downstream region (continuous mode) and in the temporal afterglow (pulsed mode). The results of the two models agree reasonably well with the experimentally measured H- density. The compact negative ion source would be useful in a variety of physics studies and applications such as dusty plasma with negative ions, neutral beam injection in fusion, modification of material surfaces, semiconductor fabrication, and ion beam lithography.

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Amit Banerjee
Y7209864
Resonance behavior of FIB grown nanomechanical systems and the role of microstructure.
28th October, 2013 (Monday)
8:00 AM
FB-382
In recent time discovery of novel material properties at nanodimension has motivated studies pertaining to investigating the mechanical properties of materials at nanometer dimension. Due to limitations in detection capability, most of the studies on anomechanical systems have been on systems fabricated out of semiconducting materials. In this thesis we discuss ways to fabricate metallic nanomechanical systems and also develop ways to detect resonance behaviour in these systems. We have fabricated using the focused ion beam induced milling process metallic nanocantilevers by milling self-supporting polycrystalline thin films of Au and Ag. The resonance frequencies and the amplitude of vibration of the metallic nanocantilevers have been experimentally measured. We study the resonance behaviour of our nanocantilever systems within the Euler-Bernoulli formulation and investigate the deviations from the expected behavior. We show the existence of a characteristic dimension in the metallic nanocantilever system at which the dissipation in the system is minimum and the resonance frequency approaches the ideal Euler-Bernoulli limit. We shall discuss issues related to this characteristic dimension and its relationship with the novel microstructure observed in the metallic thin films from which the nanocantilevers are fabricated. We have also fabricated a set of elastically coupled Au nanocantilever system and studied their resonance behaviour. In the second part of this presentation, we shall demonstrate the fabrication and resonance behaviour of a focused electron and ion beam grown C-Pt core-shell type bilayer nanowire resonator system. The resonance frequency of a bilayer system depends on densities and Young's moduli of the constituting materials. We have experimentally measured the densities and Young's moduli of the materials that we have used for the present study. The resonance frequency behaviour of the bilayer system is compared with the theoretical predictions. Finally towards the end of the presentation we discuss a few novel method we have developed for sensing nanomechanical vibration.

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Nitul Singh Rajput

Studies of issues involved in Focused ion beam based Nanostructures and developing newer methodologies.
19th June, 2013 (Monday)
11:00 am
FB-382
FIB technology is gradually becoming one of the most popular techniques for micro/nano engineering materials. In FIB, the maskless processes (deposition & milling) are used as bottom-up and top-down fabrication tool to meet the challenges at nanoscale. Despite its successful venture from precise engineering to manufacturing for the study of basic physics, the ion irradiation processes have number of secondary effects. However, in some cases, the secondary effects viz., implantation, damage formation can be exploited fruitfully to modify the sample locally.

In this thesis work, the FIB processes are used for novel structure fabrication and several newer methodologies are developed. A metallic cantilever, fabricated using a very unique technique, tends to change its shape and geometry on ion irradiation.

Investigation shows that ion-matter interaction at nanoscale has a prime role in the bending process. The phenomenon is exploited to create several 3D micro/nano structures as well as used to sense mass of the order of femtogram, lifting a heavy mass... etc.

FIB is also used to fabricate metallic nanowires and nanogap electrodes and their characterizations are made. It is found that insulating base substrate has the prime role in the failure of the nanogap structures. Using milling and deposition, several overhanging 3-D structures are created and their thermal characterization is made.

And lastly we will discuss about the role substrate and the ion beam heating in the FIB-CVD process. It is found that, the ion beam heating is not significant to cause any disrupt change in the growth of the structure.

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Dhirendra Kumar Sinha
Y4109071
Carrier Transport and Electroluminescence in Polyfluorene embedded with CdSe/ZnS Quantum Dots
17th June, 2013 (Monday)
12:00 Noon
Samtel Centre for Display Technologies (Seminar Room)
We simultaneously study the carrier transport and luminescence processes in the model system of a blue-emitting conjugated polymer, Polyfluorene (PFO), embedded with core-shell type CdSe/ZnS Quantum Dots (QDs). The test devices used for these studies have conventional structure of ITO (transparent anode)|PEDOT:PSS (hole-injecting layer)|PFO:QDs
(active layer)| Ca-Al (Cathode), wherein the concentration of the Qds varies between 0-80% by weight. The study of the electroluminescence transients at varying temperatures (10-300 K) is the principal technique which we use in this study.
The thesis focuses primarily on the following three issues:

•The simultaneous determination of the field and temperature dependence of
the diffusivity and the mobility of the charge-carriers in PFO;

•The differences between the mechanisms of PL and EL at QDs and the role
of the charge-carrier mobility in controlling the EL;

•Modifications in the mechanisms controling charge-carrier mobility for a
wide range of QD-loading conditions, and field and temperature variation.

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Gorky Shaw
Y6209863
Static and driven phases of vortex matter in superconductors with intrinsic and nanopatterned pins
06-05-2013 (Monday)
10:00 AM
FB-382
Preamble: The vortex state in superconductors exhibits competition and interplay between effects of interactions, thermal fluctuations and quenched random disorder (natural or artificially generated). Consequently the static and dynamic phases of the vortex state are quite complex. In this seminar, I will present results of my studies on the static and driven phases of vortex matter in high-Tc and low-Tc superconductors with intrinsic as well as artificially patterned pinning centers.
Static phases of vortex matter in sample with intrinsic pinning centers: Using high sensitivity magneto-optical imaging (MOI) on a Bi2Sr2CaCu2O8 (BSCCO) single crystal we have identified signatures of a novel interaction driven freezing transition from a dilute vortex liquid to solid phase along with a solid-liquid phase coexistence regime. We have constructed a phase boundary for the low field vortex liquid - solid transition line in the H - T vortex phase diagram. We have studied the effect of pinning strength on this phase boundary. We have also evaluated the entropy change associated with the low field phase transformation.
Driven phases of vortex matter: By measuring the velocity time series response of a vortex state in response to a continuous drive applied via a transport current, we have uncovered a new non-equilibrium driven jammed vortex state into which the vortex state organizes. The entry into a jammed vortex state we find is driven either by attempting to steadily accelerate the vortex state or by waiting for long time at constant drive. De-pinning of the jammed vortex state is also highly unusual, and is associated with giant vortex-velocity fluctuations with life-times exhibiting critical divergence on approaching the threshold depinning force value. Our study is compared with recent studies on similar effects found in driven colloidal systems.
Effects of artificial pinning on static phases of vortex matter: We have used Focused Ion Beam (FIB) to generate array of nanopins (blind holes) on surfaces of single crystals of 2H-NbSe2 and BSCCO. Bulk magnetization measurements in these samples provide evidence of a driven weak to strong pinning crossover. Using these artificial pinning centers we show how the local static configuration of vortices can be significantly altered with nanopatterning.

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Neeraj Shukla
Y6109861
Ion beam induced nanostructuring and magnetic ordering
22nd January, 2013
11:00 AM
FB-382
Ion irradiation has a variety of effects on various substrates such as sputtering of materials, adhesion of thin films, intermixing at the interface of multilayer's, scattering effects, ripple formation, defect formation. Low energy ions(few 10s of keV) are utilized for surface modification purposes and the high energy ions (few MeV) are utilized for ion implantation and for inducing defects of various kinds.
In the present work, we have used low energy (10-30 keV) focused Ga ion beam to fabricate and study micro/nano structures on a substrate and high energy(1-2 MeV) 1H+ and 12C+ ion beams to modify properties of highly oriented pyrolytic Graphite(HOPG).
By using focused ion beam (FIB) of 30 keV Ga ions, a variety of 1-D and 2-D micro/nano structures are prepared. A 2-D array of metallic patches has been fabricated using improved adhesion of metallic film on glass substrate caused by the ion beam bombardment. Similarly 1-D gratings with nanometer sized grating element have been fabricated by milling selective areas on silver film using Ga FIB. The forward and backward scattering of the incident Ga ions from different material structures is used to modify the nearby FIB fabricated structure. The effect has been investigated in three different geometries/material combinations.

Ferromagnetic ordering has been shown to occur in Highly Oriented Pyrolytic Graphite (HOPG) due to proton irradiation. By varying ion beam parameters, magnetic moment of the order of 10-6 emu has been obtained in optimum conditions. We have obtained 2 orders higher magnetic moment in HOPG by bombarding 12C+ ions. The results are analyzed in terms of defect densities induced in different layers along the thickness traversed by ions. Magneto-resistance studies have been made on selected irradiated HOPG sample at 20 K in the in-plane and out-of-plane orientations of unirradiated and 1H+ and 12C+ ion irradiated HOPG flakes. The results correlate well with the
findings of the VSM studies of irradiated sample. 

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Dibyendu Hazra
Y5109065
Hysteresis in superconducting weak links and micron size superconducting quantum interference devices.
31 January, 2012
11 am
FB-382

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Mr. Victor Mukherjee
Y6209864
Non-Equilibrium Quantum Critical Quenches: Defects, Entropy and Fidelity
Wednesday, 9th November, 2011

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Indranuj Dey
Y5209863
Wave Interaction with Plasmas Confined in Multicusp Magnetic Fields
02-11-2011 (November 2, 2011 Wednesday)

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Mr. Devendra Kumar
Y280962
Non equilibrium features in the solid state: a case study of the transition metal oxide NdNiO3
Friday, 21 Oct, 2011

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Mr. Sunil Kumar Mishra
Y4109075
Size-dependent magnetization fluctuations and slow dynamics in NiO nanoparticles
Wednesday July 27,2011

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Jose V Mathew
Y5109069
Multi-element focused ion beams from intense microwave plasmas
22-12-2010 (December 22, 2010 Wednesday)

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Mr. Bhaskar Kamble
Y4809061
An effective quantum expansion parameter for metallic ferromagnets: Role of orbital degeneracy, hund's coupling, and quantum corrections.
Thursday, 4th November 2010

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Jaivardhan Sinha
Y4209862
Properties of magnetic materials under extreme conditions
07-10-2010

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Uma Divakaran
Y4209864
Slow quenching dynamics in quantum critical systems
4th June 2010 (Friday)

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Mr Bhupendra Nath Tiwari
Y220961
Correlations, Stability and Black Holes in string theory and M-Theory
18 May 2010

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Mr. Sarvesh Kumar Tripathi
Y6109861
Investigation of focused ion beam induced processes and their utilization for nanofabrication
15th April (Thursday), 2010

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Mr. Prasanta Kumar Muduli
Y3109063
Spin Polarized Transport in Planar Structures and Tunnel Junctions of Perovskite Oxides
12th April (Monday), 2010

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Mr. Ashok Garai
Y4109078
Low Molecular motor transport and traffic: effects of individual mechanochemistry and steric interactions
6 April, 2010

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Mr. Udai Raj Singh
Y4109077
Low Temperature Scanning Tunneling Microscopy and Spectroscopy Studies of Magnetoresistive Manganite Thin Films
3rd February, 2010

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Vinod Chandra
Y3109068
Hot QCD equations of state and quark-gluon-plasma in heavy ion collisions
Tuesday, Nov. 24, 2009

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Shyam Mohan
Y4109074
Instabilities in the vortex state of a type II superconductor
16th November, 2009

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Subhayan Mandal
Y2809066
Response functions of quark-gluon plasma
24 September, 5 PM, 2009

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Prasanta Kumar Muduli
Y3109063
Spin Polarized Transport in Planar Structures and
Tunnel Junctions of Perovskite Oxides
12th April, 2010

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Soumen Mandal
Y3809061
Magnetic interactions and electron transport in hole-doped
manganite-superconducting cuprate heterostructures
March 20, 2009

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Rajib Saha
Y280963
Angular Power Spectrum Estimation using Linear
Combination of Multi-Frequency CMB Maps
August 20, 2008

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Dasari Durga Bhaktavatsala Rao
Y210963
Spin Decoherence in Quantum Dots: A model Study
August 18, 2008

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Akhilesh Ranjan
Y020961
Response functions of quark-gluon plasma
August 4, 2008