# List of Ph.D. Theses

 Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Mr. Ravi Pratap Singh R K Thareja10109068Time resolved investigations of laser ablated single and colliding carbon plasma/plumes.May 20, 2016 (Friday)11:00 AM onwardsFB 382 (Physics Department Seminar Room)--- Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Shraddha SharmaAmit DuttaY9209067Fidelity and Loschmidt echo: quenches, non-analyticities and emergent thermodynamics2nd May (Monday), 201610 AMFB 382 (Physics seminar room)Our work focuses on relating quantum information theoretic measures such as the ground state fidelity and the Loschmidt echo (LE) to quantum phase transitions (QPTs) and dynamics. Fidelity is the modulus of the static overlap of the two ground states of a system at different parameter values; whereas, the LE is the modulus of the overlap of two states evolved from the same initial state with two different Hamiltonians. Therefore, one can map a connection between the two; both the tools effectively detects the quantum critical point (QCP) and follows universal scaling behavior close to it. We shall discuss the marginal behavior of fidelity and LE and explain how logarithmic scaling behavior enters in the scaling of fidelity in such situations. Furthermore, we shall relate LE to dynamical fidelity (obtained for a periodically driven quantum Hamiltonian), dynamical phase transitions. Exploring the finite temperature LE, we shall further study the emerging thermodynamics and analyze the behavior of the average irreversible work and the irreversible entropy in a many body quantum system following a sudden quench. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Mr. Dushyant Kumar Prof. R.C. Budhani and Prof. Z. HossainY9109064Spin and Electronic Charge Diffusion in Superconducting NbN and Disordered SrTiO3 Under the Influence of Magnetic Field and LightApril 28, 2016 (Thursday)10:00 AM (Tea at 09:45 AM)FB 382 (Physics Seminar Room)Spin-based electronics generally called “spintronics” encodes and processes information in the quantum-mechanical spin of the electrons leading to reduced power consumption with faster operation. However, the generation of pure spin currents, its transport and detection remain a challenge. A comprehensive understanding of these phenomena requires study of spin diffusion in metals, semiconductors, and superconductors. It has been of fundamental interest to search for the sources of pure spin currents and the materials which can allow long spin diffusion lengths. In this thesis work we have addressed both these issues. We used the full Heusler alloy Co2MnSi (CMS) as a source of polarized spins. This Heusler compound is a half-metallic ferromagnet. Such a band structure suggests 100% spin polarization, thus making the ordered cubic phase of Co2MnSi a good candidate for spintronic devices. As for the material for efficient transport of spin current, we have concentrated on SrTiO3, a cubic band insulator well known for its useful dielectric and optoelectronic properties. The high mobility (∼2 x 103 cm2/Vs at 15 K) electron gas was induced in the few nanometer thickness of STO by irradiating its surface using Ar+-ions. The process converts ~4 nm thick surface layers of STO into a metal while over the remaining thickness of a 0.5 mm wafer remains insulating. The unperturbed base material has been used as a gate dielectric in a back gated geometry. For device perspective of the surface electron gas of reduced STO, it is desirable to study its response to photon and electrostatic fields. In this connection, we have studied the photoconductivity (PC) and photoluminescence (PL) of ion irradiated STO and tuned them using electrostatic gate field. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Nikhil Kumar Prof. Anjan GuptaY9109074Controlling thermal hysteresis in superconducting weak links and micron size superconducting quantum interference device.April 27, 201611:00 AM onwardsFB 382 (Physics Department Seminar Room)--- Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Gangadhar Behera S. Anantha Ramakrishna Y9109065Nanostructured plasmonic thin films for enhanced optical 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. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Ms. Reeta Pant Krishnacharya Y9209066Investigation of Tunable Wetting and Slippery Behavior on Hydrophilic Surfaces519 April 2016 (Tuesday)11:00 AM (Tea will be served at 10:45 AM) FB382 (Physics Seminar Room)Interaction of a liquid with a solid surface is one of the most fundamental surface/interfacial phenomenon which primarily governs the wetting behavior of the liquid-solid system. The surface wettability can be controlled by manipulating the surface free energy of the solid and/or liquid-solid interfacial energy with passive or active methods. Reversible switching of surface wettability using polystyrene/titania nanocomposite based responsive surfaces upon ultraviolet (UV) exposure and annealing will be discussed. As prepared nanocomposite coating shows hydrophilic behavior which become superhydrophobic upon annealing at 180°C for 90 mins. This happens due to increase in surface roughness during annealing. Subsequently upon UV exposure, the superhydrophobicity slowly decreases and the samples become superhydropholic with almost 0° water contact angle. The UV exposed superhydrophilic samples recover their superhydrophobicity upon annealing again at 180°C. Detailed investigation of static and dynamic wetting transition will also be presented. Subsequently superhydrophobic and lubricating fluid infused slippery surfaces will be discussed. Based on energy minimization, stability criteria for lubricating fluid infused slippery surfaces will be established. Initially we observed that slipping water droplets sink into lubricating silicone oil layer due to strong polar interaction between hydrophilic substrate and water molecules. This sinking can be prevented by using hydrophobic surfaces. Annealing silicone oil coated substrates makes the surface hydrophobic. Optimization of annealing parameters will be discussed in detail. Characterization of the stable slippery surfaces using contact angle hysteresis and slip velocity measurements will be presented. Subsequently, I will discuss the effect of substrate roughness on the stability of slippery surfaces. It was found that small nanoscale roughness enhances the stability of the lubricating fluid as well total slippery behavior. Fabrication of stable slippery surfaces based on nanostructured formed by nanoparticles enable us to get rid of the hydrophobic surface requirement. Hydrophilic silica and titania nanoparticles were used to cast nanostructured film which after coating with silicone oil show stable slippery behavior. Towards the end, I will discuss about the effect of surface wettability on the lubricating film thickness underneath a test drop. Electrical measurements were used to measure the thickness of the lubricating film underneath a test drop. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Rohit KumarProf. Mahendra K. Verma10109875Energy transfers in dynamos with small and large magnetic Prandtl numbers03.11.2015 2:30 PMFB 382 (Physics Seminar Room)The presence of magnetic field in celestial bodies is explained by dynamo mechanism in which a conducting fluid in motion generates a self-sustained magnetic field. In dynamo, energy is transferred form velocity field to magnetic field and consequently the growth of magnetic energy takes place. The magnetic Prandtl number (Pm), the ratio of kinematic viscosity and magnetic diffusivity, is an important non-dimensional parameter for dynamos. We study energy transfers in dynamos with small- and large-Pm by performing direct numerical simulations (DNS) in a 3D periodic box on 10243 grid, using a pseudo-spectral solver Tarang. Energy fluxes and shell-to-shell energy transfers show that for dynamo with large-Pm, the growth of magnetic energy takes place due to nonlocal energy transfers from large-scale velocity field to small-scale magnetic field. For dynamo with small-Pm on the other hand, the magnetic energy grows due to local energy transfers from large-scale velocity field to large-scale magnetic field. We also use a shell model of dynamo to understand the energy transfers for extreme values of Pm, which are otherwise inaccessible to DNS due to the computational constraints. The energy fluxes in our shell model simulation are in qualitative agreement with the DNS results. We also construct a low-dimensional model to study the dynamo transition for very small to very large Pm and observe that the critical magnetic Reynolds number for dynamo transition, Rmc, saturates to constant values in the two limiting cases of Pm. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Mr. Shubhankar DasR.C. Budhani and Zakir HossainY9109078Magnetoresistance and Magnetothermopower studies of delta-doped 2-dimensional electron gas at the interface of LaTiO3/SrTiO31st April (Friday), 20168:45 AMFB 382 (Physics Seminar Room)Studies of the 2-dimensional electron gas (2DEG) at the interface of LaAlO3 (LAO) or LaTiO3 (LTO) and TiO2 terminated SrTiO3 (STO) has attracted much attention in recent years due to its interesting properties like metal-insulator transition, magnetism, superconductivity and strong spin-orbit interaction effects in the electronics transport. We have used a new approach to bring about modification in the electronic properties of this 2DEG. This involves delta (δ) doping at the interface of LTO/STO by an iso-structural antiferromagnetic perovskite (TN = 298 K) LaCrO3 (LCO). This δ-layer dramatically alters the properties of the 2DEG. The changes includes increase in room temperature sheet resistance (R□), drop in the sheet carrier density (n□) almost exponentially with the layer thickness, and emergence of new features in the temperature dependence of R□ at T ≤ 50 K. Our spectroscopic measurement along with density functional theorem (DFT) calculations show that the Cr-ions at the interface act like a trap for electrons which are transferred from the LTO to STO surface. Extensive measurements of out-of-plane and in-plane magnetoresistance (MR) have been carried out on all the samples to address issue such as weak antilocalization and Kondo scattering. We have also observed a gradual crossover from positive out-of-plane MR to negative in-plane MR when magnetic field is titled with respect to the film surface. The MR measurements are augmented by the measurement of thermopower (S) which increases dramatically with δ-layer thickness at ambient temperature. The linear temperature dependence of S in the temperature range 100 to 300 K is indicative of diffusion thermopower. We also observed a large enhancement in thermopower in the temperature range where a minimum in R□ is observed. This enhancement is attributed to Kondo scattering. The thermopower is suppressed in the presence of a magnetic field and the suppression is isotropic with respect to the field direction. We will also present a tunable Rashba S-O interaction in these interfaces by δ-doping with another iso-structural ferromagnetic perovskite LaCoO3 (LCoO). In LCoO-doped sample, the inelastic scattering time varies as 1/T and the S-O scattering time remains constant in temperature, which suggests that the spin relaxation follows the D’yakonov-Perel mechanism. The δ-doping also results in 3 orders of magnitude decrease in τso whereas the inelastic scattering time increases very slowly with doping. A detailed analysis of anisotropic MR when the field is applied in the plane of the sample displays the effect of mixing of spin-up d-states and spin-down d-state at Fermi level due to spin-orbit interaction. Reference: 1. Shubhankar Das et al., Phys. Rev. B 90, 081107(R) (2014). 2. Shubhankar Das et al., Phys. Rev. B 90, 075133 (2014). 3. Shubhankar Das et al., Under review in Phys. Rev. B. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: GopalPankaj JainY9109066Cosmological and Colliders Implications of Conformal Symmetry21-3-2016 (Monday) 4.00 p.m. (Tea @ 3.45 p.m.)FB 382 (Physics Conference room) Symmetry has always played an important role in understanding Nature. At present most of our theoretical knowledge comes from the symmetry principles. In this talk I will discuss the implications of scale or conformal symmetry. In particular, I will propose a possible solution to the fine tuning problem of cosmological constant within the framework of softly broken conformally symmetric model. Cosmological constant is potentially a source of Dark energy, which constitutes about 70% of the energy density of the Universe. A major problem with the cosmological constant is that it gets very large quantum contributions from the matter sector at each order in perturbation theory. So we need to cancel these large contributions in order to maintain the small value of observed dark energy density, leading to an acute fine tuning problem. In our proposed solution we have shown that the matter sector will not contribute to the cosmological constant and hence we have solved the fine tuning problem. Here we have not considered the quantum gravity effects since quantum theory of gravity is not well formulated. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Dheeraj PratapProf. S. Anantha RamakrishnaY9109063Anisotropic and Hyperbolic Metamaterials in the Cylindrical Geometry15 Mar. 2016 (Tuesday)11 a.m. (Tea will be served at 10:45) L6, Lecture hall complexMetamaterials are artificially designed composite materials that are structured on subwavelength lengthscales. They are often inherently optically anisotropic, having direction dependent properties, due to their structure. Acid anodization of an aluminium micro wire was used to fabricate a nanoporous alumina microtube with non-branching nanopores aligned along the radial direction. This is essentially a metamaterial fiber with anisotropy in the cylindrical geometry. These nanoporous alumina micro tubes show enhanced optical and thermal properties, of which a study of the optical properties is presented here. Not only are these nanoporous alumina structures anisotropic, but they are radially inhomogeneous as well due to the radius dependent size of the nanopores. These metamaterials were homogenized to obtain the electromagnetic effective medium parameters (dielectric permitivity tensor)using techniques of transformation optics and Maxwell-Garnett homogenisation theory. Nanowires of silver were deposited electrochemically within the nanopores of the cylindrical nanoporous alumina to yield a metamaterial with hyperbolic dispersion. These metamaterial fibers exhibit novel optical modes that are described by Bessel functions of fractional and imaginary orders solely due to the anisotropy. Particularly for the high transverse momentum modes (whispering gallery modes), these modes are confined increasing near the centre of the waveguide in sheer contrast to conventional fibers where they localise to the edge. The inhomogeneous nature of the metamaterial fibers also cause extremely strong confinement of the modes near the center. The nanoporous alumina microtubes coated with active dye molecules of Rhodamine-6G show highly enhanced fluorescence, which is highly quenched in the presence of silver nanowires within the nanopores. Other studies, such as the possibility of manipulating the structure of the nanopores and a study of fluorescence from molecules deposited on nanoporous alumina, in the flat geometry were also carried out. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Jhuma DuttaProf. S. Anantha Ramakrishna and Prof. Harshwardhan WanareY9109069Photonic and Plasmonic properties of Periodically Patterned Columnar Thin Films 19 Feb. 2016 (Friday) 11 a.m. (Tea will be served at 10:45) L8, Lecture hall complexColumnar thin films are known to be deposited by making incident a collimated vapor flux at large oblique angles to a substrate. Using a substrate that is periodically patterned at micro/nanoscales, new kinds of periodically patterned columnar thin films (PP-CTFs) have been fabricated. The extremely anisotropic optical properties offered by the nanocolumnar structure with larger scale structures for photonic bandgap or diffractive effects provide for new physical phenomenon. These structures were also found to be extremely reconfigurable giving rise to enormous flexibility in applications. PP-CTFs are shown to have enhanced photonic and plasmonic properties. Dielectric PP-CTFs are shown to function as blazed diffraction gratings with large asymmetric diffraction efficiencies. A CTF made of plasmonicmetals like silver renders it an effectively biaxially anisotropic continuum. PP-CTFs of silver showed strong blazing action and unidirectionally coupled optical radiation to surface-plasmon-polariton(SPP) waves for both p- and s-polarizations. The blazing effect of the gratings of dielectric CTFs as a result of the spatially linear phase shifts caused by prismatic air cavities was understood using Kirchhoff-Fresnel diffraction theory. Homogenization of the metallic CTFs using the Bruggeman formalism revealed them to display hyperbolic dispersion, and the dispersion of SPP waves on both 1D and 2D gratings of such anisotropic hyperbolic media was found to be adequately described thereby. Detailed electromagnetic simulations of the grating structures reveal large changes in the photonic properties with the slant angle such as diffraction efficiencies and the electromagnetic near fields. Furthermore, these slanted nanocolumnar structure can be uniformly reconfigured by ion beam irradiation method and gives rise to reconfigurable blazed gratings, thereby maximizing the diffraction efficiencies for different wavelength bands by changing the blazing (angle) condition. A novel application of CTFs to fingerprint visualization was developed. Visualization of latent fingerprints is enhanced by deposition of columnar thin films at large oblique angle of CaF2 and SiO2 on fingerprint marks on nonporous surfaces and further enhanced visualization is obtained by treating the deposited CTFs with a fluorescent dye and fluorescence imaging. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Jagtap J Manihrao Prof. V. Subhramanyam and Prof. Asima PradhanY9109074Mueller matrix spectroscopy imaging and image analysists techniques for human cervical pre-cancer discrimination.17.12.201511:00 AM onwardsFB 382 (Physics Department Seminar Room)--- Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Vandana YadavProf. V. SubrahmanyamY9109082Steady state properties of nonequilibrium growth and transport processes: A boundary layer based approach14 December, 20152:30 PMFB 382Microtubules are important components of the cytoskeleton of a cell. These are hollow,tube shaped biopolymers made of alpha, beta-tubulin heterodimers. Experimental studies reveal an interesting growth dynamics of microtubule which play an important role in various processes such as intracellular organization, transport, exerting pushing and pulling forces etc., it is crucial to understand the growth dynamics of microtubules and its regulations by various regulatory proteins. It has been predicted experimentally that motor proteins belonging to Kinesin-5 and Kinesin-8 family can regulate the dynamics of microtubule in a length dependent manner. Here I discuss a two-state model of microtubule polymerization with a length-dependent dynamics and obtain the length distributions of microtubules through a discrete formulation. These investigations suggest that the length dependent regulation leads to a much faster decay of distributions as compared to an exponential decay. In the latter part, I shall discuss about the boundary layer technique and show how it can be used to understand phases and phase transitions of driven many-particle exclusion processes. We develop the fixed point based boundary layer method and apply this to two distinct driven exclusion models to show how the shape of the particle distribution profile, the location of the boundary layer etc can bepredicted from the stability properties of the fixed points of the boundary layer equation. Further, I shall discuss the applicability of boundary layer method in order to understand the polymerization dynamics in the presence of diffusing tubulins. Since various rates associated with the polymerization dynamics are expected to be influenced by the number of available tubulins, the growth dynamics and the diffusive dynamics of tubulins are coupled in a nonlinear way. The boundary layer analysis emerges as a powerful method that allows us to obtain analytical solutions for the length distributions in a systematic way. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Mr. Sourabh BaruaProf. K P RajeevY7109075Electrical Transport Studies of Topological Insulator Materials Bi2Te3, Sb2Te3 and Bi2-xSbxTe3-ySey Monday, 30 Nov 201510 AMFB 382Topological insulators have generated a lot of interest because of the revolutionizing properties predicted for these materials. These materials are supposed to have a conducting surface and an insulating bulk and the surface carriers have spin locked to their momentum. The surface conduction is furthermore topologically protected from disorders as the surface bands traversing the bulk band gap cross at the time reversal invariant momenta and these crossings are guaranteed unless time reversal symmetry is broken by application of a magnetic field or magnetic impurities. However, experimental verification of the surface conduction from these topological surface states in electrical transport studies has been difficult because of the bulk conduction in these materials due to crystalline defects. The focus of this thesis is the electrical transport studies of single crystals of topological insulator materials: Bi2Te3, Sb2Te3 and Bi2-xSbxTe3-ySey. In case of Bi2Te3, even though the sample was metallic, we were able to obtain signatures of the topological surface states from the Shubnikov-de Haas (SdH) oscillations in the magnetoresistance, such as a non-zero Berry phase, magnitude of surface conduction similar to that expected for topological insulators and also large and linear magnetoresistance. We also performed angle dependent magnetoresistance studies which have revealed that the SdH oscillations do not show the angle dependence expected for a two dimensional Fermi surface of the topological surface states. Further, the angle dependence of the frequency of the oscillations does not adhere to the behavior expected for the conventional three dimensional bulk Fermi surface in this material. We also observe asymmetry in the magnetoresistance in positive and negative fields, which has been attributed to anisotropic bulk transport earlier, but we show that such asymmetry can arise due to the mixing of Hall signal with longitudinal resistance in this material. In Sb2Te3, we observed prominent SdH oscillations although the Berry phase extracted was inconclusive in determining whether these were from the surface states. In Bi2-xSbxTe3-ySey, the single crystals obtained were metallic and these showed linear magnetoresistance. We also made a simple model of the dependence of resistivity on thickness for a topological insulator and compared it to data for various topological insulators published in the literature. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Anirban BaguiProf. S. Sundar Kumar IyerY7109861Effect of electric-field annealing during solvent drying step ofactive layer in organic solar cell devices5th August, 201512 noonSCDT seminar roomThe performance of organic electronic devices can often be improved by improving the nano-morphology of organic active layers in those devices. In this thesis, the role of thermal annealing of polymer layers during solvent drying in the presence of a constant electric-field to improve the the nano-morphology of the films thus formed was studied.Comparative studies of morphological and electro-optical properties of the P3HT:PC61BM based blend film and bulk hetero-structure solar cell devices were carried out to understand the role of 'electric-field annealing'. The external quantum efficiency (EQE) and power conversion efficiency (PCE) of the solar cells made showed significant enhancement due to better charge transport in the case of electric-field annealed devices.In order to study this process step on the hole mobility in the P3HT layer, devices with holes as the primary charge carriers were fabricated. The hole mobility in P3HT was found to increase monotonically for annealing field strengths up to 2000 V/cm. The current density – voltage (J–V) data corresponding to the space charge limited currents (SCLC) at various low temperatures for P3HT based hole-only devices were fitted with the empirical model and the Gaussian disorder model (GDM) to interpret the data. X-ray diffraction (XRD) measurements confirmed increase in crystallinity and crystallite size of the films for electric-field annealed samples. The observed changes in the charge transport properties due to electric-field annealing of the films at the time of their fabrication have been explained based on the above measurements and analysis.Solar cells in inverted structure have also been made with appropriate modification in device structure to get better performing devices by electric-field annealing. Finally, the effect of electric–field assisted treatments was studied on devices fabricated with PTB7 (a new promising polymer for organic solar cells) as the device active layer. The experimental results presented in this thesis confirm that application of electric-field during annealing of organic films during their formation is a useful processing step to fabricate higher mobility polymer films for building improved organic electronic devices. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Pranati Kumari RathProf. Pankaj JainY9109077Large Scale Anisotropy in Cosmic Microwave Background Radiation and its Relationship with Primordial Power Spectrum27th July, 20154 pmFB-382The Cosmological principle says that the universe at large distance scale is assumed to be statistically homogeneous and isotropic. The Cosmic Microwave Background Radiation (CMBR) is considered to be one of the best experimental evidence supporting this principle. However, there exists considerable evidence in cosmological data which suggests violation of this principle.In this talk, I will revisit two of these anomalies observed in the CMBR temperature data provided by WMAP and recently by PLANCK team. These includes the alignment of the quadrupole (l=2) and octopole (l=3) and the hemispherical power asymmetry. To explain the low-l alignment, I will discuss some anisotropic inflationary models within the framework of the Big Bang cosmology. In these models, the anisotropy decays very quickly during the inflationary phase of expansion. The resulting direction dependent power spectrum in this anisotropic background leads to violation of isotropy and hence explain the alignment of the low l CMBR modes. I will also discuss an inhomogeneous power spectrum model in order to explain the hemispherical power asymmetry, observed in the CMBR data. The hemispherical asymmetry can be parametrized in terms of the dipole modulation model. Alternatively, I will show that, an anisotropic dipolar imaginary primordial power spectrum, which is possible within the framework of noncommutative space-times, also provides a good description of the observed dipole modulation in CMBR data. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Shail PandeyProf. Sudeep BhattacharjeeY7209063Pulse modulated microwave plasmas : self-excited instabilityand plasma states3rd July, 20154.30 pmL-5Waves and instabilities are inherent to plasma and have been a widely studied subject owing to its importance in electron acceleration, wave absorption, and plasma heating. While most of the earlier studies discuss these in a preformed steady-state plasma, limited studies exist on excitation of waves and instabilities in pulsed plasmas. In the pulsed regime, large amplitude electromagnetic pulses can interact with a self-generated growing plasma, leading to the formation of instabilities. The present thesis carries out an experimental investigation of the plasma state created by pulsed electromagnetic waves in the microwave regime and excitation of instabilities due to counter streaming charged particles in the ionization front.Pulsed microwaves of 2.45 GHz are launched in a low pressure argon (0.2 – 2.5 mTorr) gas to generate a temporally growing ionization front which subsequently develops into a full fledged plasma. Depending upon the peak power and time duration of the pulses, plasmas are characterized by an afterglow state (AGP) or an interpulse state (IPP). It is found that : (i) the AGP state characterized by a temporally decaying plasma with a low value of electron temperature ~ 1 eV, occurs at larger pulse duration > 50 microseconds and (ii) the IPP state, where the plasma grows beyond the pulse duration, often forming a quasi-steady state in the power off phase, is realized at shorter pulse duration. The distinction between the two is further investigated by measurements of the electron energy probability function (EEPF).The IPP state is governed by plasma growth within the pulse, where strong interaction with the injected microwaves can be realized leading to formation of instabilities, which is investigated next. The measured particle current profile shows two phases of plasma development: (i) self-excited wave (SEW) in the ionization front, followed by (ii) rapid plasma growth. The growth rate of the SEW comes out to be ~ (1.0 – 4.5) × 10^7 s^-1, depending upon the pressure, which is further confirmed with optical emission spectroscopy measurements. Time – Frequency Analysis (TFA) indicates the presence of two frequency bands (~ 4 MHz) centered around 3.8 MHz and 13.0 MHz.The profile of electron thermal velocity v_th(t) and relative charged particle drift velocities V_D(t) in the SEW, obtained from the measured EEPF, indicate V_D > v_th – a condition necessary for excitation of Buneman instability (BI). The calculated growth rate and oscillation frequency of BI further confirms its existence in the SEW. Finally, the energy transfer from the instability leading to the heating of the plasma during the pulse is investigated. Speaker:Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Subhash MahapatraProf. Tapobrata Sarkar10109069Applications of the Gauge/Gravity duality : Superconductivity,Optics and Entanglement Thermodynamics.1st June, 201510 amL-6A remarkable connection, in the context of string theory, has emerged in last few years between gravity and the strongly coupled field theories. This connection, which is generally called as the gauge/gravity duality or the AdS/CFT correspondence, maps a quantum theory of gravity in (D+1) dimension Anti de sitter (AdS) spacetime to D dimensional quantum field theory - living at the boundary of AdS spacetime. An important feature of this duality is that in certain approximation, the strongly coupled limit of field theory corresponds to the weakly coupled limit of the gravity theory. This strong-weak nature of the correspondence has provided a unique approach to address some questions in strongly coupled condensed matter systems which otherwise would be intractable.In this thesis, we discuss important applications of the gauge/gravity duality, on three main directions. It includes the phenomenon of superconductivity, optics and entanglement thermodynamics.In this thesis work, we have studied and developed various generalized models for holographic superconductors and have computed response functions of these generalized superconductors using the gauge/gravity duality. We have been able to show that the occurrence of negative refractive index is a generic feature of holographic superconductors for specified ranges of frequencies. For these superconductors, we also showed that the holographic entanglement entropy is a good measure to probe different phases of the system. We further found a indication of the first law of thermodynamics like relation in the context of entanglement entropy for holographic superconductors. Speaker: Supervisor: Roll no:Title: Date: Time: Venue: Abstract: Anukul Prasad ParhiProf. S. Sundar Kumar IyerY5209061Annealing in the presence and absence of electric field of copper phthalocyanine based thin films and their applications in organic solar cells18th May, 20156 pmSamtel seminar roomThe ability to tailor nano morphology by influencing the growth and crystallization process of organic thin films is desired for various applications. Annealing is known to help in modifying thin film textures and properties and is easier to implement during device fabrication. In this thesis, films of copper phthalocyanine (CuPc) and their blends have been subjected to thermal annealing in the presence and absence of electric field. The films were then characterized by various methods including x-ray diffraction (XRD), scanning electron microscope (SEM) and by taking the film’s absorption spectrum. These were analysed to explain the effect of the annealing of the films.Initially, pristine CuPc films were annealed at different temperatures, electric fields and annealing ambiance - in vacuum as well as in nitrogen. It was observed that there was enhanced re-crystallisation by annealing at higher temperatures, in higher electric fields and in vacuum.Blends of CuPc with buckminsterfullerene (C60) films of different volume ratios (3:1, 1:1 and 1:3) were then annealed at different temperatures both in the presence and absence electric field. While the blending inhibited re-crystallisation (when compared to pristine films of CuPc), above certain temperatures, the two constitutes, CuPc and C60, were phase-segregated. The phenomenon of abrupt phase segregation was observed in 1:1 and 1:3 blends under electric field. Analyses of the absorption spectra carried out confirm blending and phase-segregation of the two constitutes in the film during annealing. Using density functional theory (DFT), the poloarisability of the constituent molecules were calculated and the trends in the intermolecular attraction and segregation were explained.Bi-layer organic solar cells were fabricated with CuPc and C60 films. The effect of annealed CuPc films and CuPc-C60 bi-layer films on the solar cell performance and their current-voltage characteristics were analysed. It was observed that annealing of the bi-layer stack resulted in improved solar cell performance. However, the application of electric field during annealing of the bilayers does not significantly add to the improvement in performance of the solar cells observed due to thermal annealing. Speaker: Roll no:Title: Date: Time: Venue: Supervisor: Abstract: Bahadur SinghY9209061First-principles investigations of topological phases in materials11th May, 201511 amFB-382Prof. Rajendra 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. Speaker: Roll no:Title: Date: Time: Venue: Supervisor: Abstract: Samit PaulY8209868Micron Focusing, Diagnostics and Interaction of Multi-ElementIon Beams with Matter7th May, 201511 amL6Prof. Sudeep BhattacharjeeA focused ion beam (FIB) system is an inevitable tool for research and applications in nano-science and technology. However, the availability of only gallium (Ga) ions as projectile, in liquid metal ion source (LMIS) based FIB systems limits its applicability. Moreover, metallic Ga has contamination issues and provides less milling yield. In order to ameliorate the above problems, an emerging area in this field is development of focused ion beams of a variety of elements utilizing plasmas. A compact, high current density (~ 1 A/cm^2), microwave plasma based multi-element focused ion beam (MEFIB) system has been developed in the laboratory, that is capable of delivering beams of different ions (H_2 , Ar, Kr, Ne) of energies up to 18 keV and spot size of ~ 25 µm, using a 1 mm plasma electrode (PE) aperture. The beam has an emmittance of ~ 0.1 mm-mrad and brightness of ~ 10^5 Am^-2 sr^-1 V^-1, thereby making it an excellent candidate for FIB. In this thesis our target has been to reduce the beam to submicron spot size and make it operate at higher energies (30 keV) comparable to that of conventional FIBs. In parallel, we developed a new diagnostics (spider probe) for measuring micron size beams and have investigated the time dependent physics of the interaction with matter, particularly, the milling capability to make micro-pores and microstructures on metallic foils and thin films. In order to achieve the above, first the plasma has been optimized to maximize the extracted beam current density maintaining tolerable ion energy spread at the meniscus (~5 eV). The utilization of two Einzel lenses in conjunction with a PE and a beam limiting (BL) aperture, is found to have better control over the beam energy and current density. Controlled sculpting of micron scale pores in aluminum foil with Ar and Kr ion beams is demonstrated. The temporal evolution of the micro pores caused by beam interaction on metallic foils has been studied and smallest pore diameter of ~ 3 µm has been successfully created. This shows that with proper control of the irradiation time, submicron pores can be fabricated. Further experiments, using smaller PE apertures of 45 µm and 31 µm, created by MEFIB, have been used to obtain a focused beam size ~ 1 µm. A time-dependent exposure method to determine the exact location of the focal point of the beam has been invented that takes care of the over irradiation issue. Extensive use of Lab-View software is made for manipulating the beams to create several microstructures such as circular and rectangular trenches, gratings, symbols and alphabetical letters on 50 nm Cu thin films. In order to reduce the beam spot size further, guiding and transmission of extracted Ar ion beams with the help of straight and tapered micro-glass-capillary have been investigated. The beam current through the capillary is found to have a threshold extraction voltage and observed to exhibit hysteresis, with a unique self-focusing capability. The temporal and dimensional dependence of the hysteresis have been studied. The guiding capability of the tapered capillary is found to be more effective where beam size reduction is desired without compromising total beam current unlike electrostatic beam limiters. For further understanding the phenomena, Particle-In-Cell simulations that solves Poisson's equation and equation of motion self consistently are carried out, and the experimental results are found to have a reasonable agreement with simulations. Speaker: Roll no: Title: Date: Time: Venue: Abstract: Prabwal Jyoti Phukon Y8209867Studies 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. Speaker: Roll no: Title: Date: Time: Venue: 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 Speaker: Roll no: Title: Date: Time: Venue: Abstract: 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. Speaker: Roll no: Title: Date: Time: Venue: Abstract: 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 â€“ ion mean free path depends on relative collision velocity of two seed plasmas, the ionization state, and the ion density of the plasma plumes. In the present work we report the dynamical behavior of collinearly colliding ZnO plumes and formation of interaction zone / stagnation layer. The plasma plume dynamics is studied using fast imaging and optical emission spectroscopy. Two seed plasmas initially evolve independently and then start interacting at ~15 ns along their lateral dimensions. The interaction of seed plasma plumes results in a stagnation layer having abundance of the higher ionic states, high electron number density and temperature with slower decaying rate as compared to the single plume that lasts for longer time delays as compared to single plume. At the later stages of plasma evolution the vapor phase is populated by nanoparticles /nanoclusters of ZnO. The presence of nanoparticles/clusters of ZnO in the vapor phase for longer time delays in colliding plumes is confirmed using dynamic laser light scattering (Rayleigh) and photoluminescence (PL) techniques. The configuration is used for deposition of ZnO thin films on glass substrates using 1064 nm irradiation. The deposited films are characterized using X-ray diffraction, AFM, optical transmission in the UV-visible range and photoluminescence measurements. IR ablated colliding plasma plumes results in deposition of a-plane ZnO thin films with better optical properties as compared to c-axis oriented thin films obtained using conventional UV (355 nm) single plume deposition. The observed differences in the quality and properties of thin films are attributed to the flux of mono-energetic plasma species with almost uniform kinetic energy and higher thermal velocity reaching the substrate from interaction/stagnation zone of colliding plasma plumes.