Recursos de colección

ETD at Indian Institute of Science (3.241 recursos)

Repository of Theses and Dissertations of Indian Institute of Science, Bangalore, India. The repository has been developed to capture, disseminate and preserve research theses of Indian Institute of Science.

Physics (physics)

Mostrando recursos 1 - 20 de 171

  1. Tomographic Studies of Pulsar Radio Emission Cones and Searches for Radio Counterparts of Gamma-Ray Pulsars

    Maan, Yogesh
    Radio emission from pulsars is believed to originate from charged particles streaming along the open magnetic field lines, radiating within a narrow cone at each of the two magnetic poles. In each rotation of the star, the emission beam sweeping across the observer’s line of sight, is seen as a pulse of radio emission. Average pulse profiles integrated over several hundreds of individual pulses, along with polarization information, reveal the viewing geometry and various emission properties(e.g., emission in multiple cones, frequency dependence of the emission altitude, notches in the average profiles, etc.), and provide some clues about the possible emission...

  2. Spiral-Wave Dynamics in Ionically Realistic Mathematical Models for Human Ventricular Tissue

    Nayak, Alok Ranjan
    There is a growing consensus that life-threatening cardiac arrhythmias like ven- tricular tachycardia (VT) or ventricular fibrillation (VF) arise because of the formation of spiral waves of electrical activation in cardiac tissue; unbroken spiral waves are associated with VT and broken ones with VF. Several experimental studies have shown that in homogeneities in cardiac tissue can have dramatic effects on such spiral waves. In this thesis we focus on spiral-wave dynamics in mathematical models of human ventricular tissue which contain (a) conduction in homogeneities, (b) ionic in- homogeneities, (c) fibroblasts, (d) Purkinje fibers. We also study the effect of a periodic...

  3. Evolution of the Magnetic Ground States with Lattice Distortion and Chemical Inhomogeneity in Doped Perovskite Oxides

    Manna, Kaustuv
    The physics of doped transition metal perovskite has been an area of intense research in the last few decades due to their interesting magnetic and transport properties. Various exciting phenomena such as, colossal magneto resistance, high Tc superconductivity, multiferroicity, ferroelectricity, high temperature ferromagnetism, etc., have made these systems more fascinating in terms of fundamental study as well as technological applications. There are several intrinsic material characteristics in these perovskite oxides that can impact their magnetic properties. Lattice distortion and chemical in homogeneity are two important ones. Changes in valence and ionic radius in rare earth (A- site) and transition metal...

  4. Structure and Dynamics of Interfacial Molecular Membranes

    Bhattacharya, Rupak
    This thesis describes the study on structure and dynamics of various kinds of molecular membranes in general. We have studied the morphological transition of colloidal as well as biologically relevant membranes and qualitatively argued regarding the interplay between structure and dynamics. Systematic measurements have been performed to address the issue of ambiguous behavior of molecules under stress when its confined at the interface. The structural and dynamical effect on interfacial membranes have been studied for soft colloidal free standing langmuir monolayer as well as for the quasi two dimensional lipid membranes on solid supports. For organic nanoparticle monolayer we have...

  5. Preparation and Optical Properties of Hybrid Assemblies of Metallic Gold Nanoparticles and Semi-Conducting CdSe Quantum Dots

    Tripathi, Laxmi Narayan
    This thesis summarizes the methods of preparation and optical properties of hybrid assemblies of Au NPs and cadmium selenide (CdSe) QDs. First chap-ter deals with the literature survey and theoretical aspects of plasmonics and discussions on optical excitations of metal (plasmons) and semiconducting QDs (excitons). Variation of energy levels of CdSe QDs and its optical properties i e. absorption and emission properties under strong confinement regime have been discussed with respect to effective mass approximation (EMA) model. This is followed by the discussion on optical properties of Au NPs and rods, describing absorption properties, based on Mie theory. Size and...

  6. High Magnetic Field Neutron Stars : Cyclotron Lines and Polarization

    Maitra, Chandreyee
    This thesis concerns with the study of X-ray binaries which are gravitationally bound systems consisting of a compact object (either a neutron star or a black hole) and usually a non degenerate companion star, both rotating around the common centre of mass. The compact star shines brightly in the X-ray regime. Emission from these systems are powered by accretion which is the most radioactively efficient mechanism known in the universe by the release of gravitational potential energy when matter from the companion star falls on the compact object. Accretion onto high magnetic field neutron stars are special as the magnetic...

  7. Magnetic and Magnetotransport Studies in Transition Metal Oxides : Role of Competing Interactions

    Sow, Chanchal
    There was a fame time for silicon in condensed matter physics, then the graphene era came and now topological insulators are gaining lot of attention, but magnetism in condensed matter physics has remained always fascinating starting from the ancient days up to now and it will remain as one of the core topic in basic or applied physics. The improvement in the modern techniques allows one to explore magnetism in different length scales as well as in different time scales. As an effect of the improvement in experimental techniques, different magnetic anomalies are unearthed. As a result theories are getting...

  8. Investigations of Phase Change Memory Properties of Selenium Doped GeTe and Ge2Sb2Te5

    Vinod, E M
    GeTe and Ge2Sb2Te5 alloys are potential candidates for non-volatile phase change random access memories (PCRAM). For electrical data storage applications the materials should have stable amorphous and crystalline phases, fast crystallization time, low power to switch, and high crystallization activation energy (to be stable at normal operating temperatures). Phase change memories can be tuned through compositional variations to achieve sufficient phase change contrast and thermal stability for data retention. Selenium is one of the attractive choices to use as an additive material owing to its flexible amorphous structure and a variety of possible applications in optoelectronics and solar cells. GeSb2Te3Se alloy, in which...

  9. Pulsed Laser Ablated Dilute Magnetic Semiconductors and Metalic Spin Valves

    Ghoshal, Sayak
    Spintronics (spin based electronics) is a relatively new topic of research which is important both from the fundamental and technological point of view. In conventional electronics charge of the electron is manipulated and controlled to realize electronic devices. Spintronics uses charge as well as the spin degree of freedom of electrons, which is completely ignored in the charge based devices. This new device concept brings in a whole new set of device possibilities with potential advantages like higher speed, greater efficiency, non-volatility, reduced power consumption etc. The first realization of the spintronic device happened in 1989, owing to the discovery...

  10. Electron Filed Emission Studies of Nanostructured Carbon Materials

    Ivaturi, Sameera
    Field emission is the emission of electrons from a solid under an intense electric field, of the order of 109 V/m. Emission occurs by the quantum mechanical tunneling of electrons through a potential barrier to vacuum. Field emission sources offer several attractive features such as instantaneous response to field variation, resistance to temperature fluctuation and radiation, a high degree of focusing ability in electron optics, good on/off ratio, ballistic transport, and a nonlinear current-voltage relationship. Carbon nanotubes (CNTs) are potential candidates as field emitters since they possess high aspect ratio and are chemically inert to poisoning, and physically inert to...

  11. Role of Excess Fe in Pristine and Substituted Fe Chalcogenide Superconductors

    Cherian, Dona
    Fe chalcogenides : The discovery of superconductivity in Fe based compounds trig- gered an intense research activity in this field with significant importance given to material synthesis. As a result, numerous materials falling into four major classes and sharing similarities in physical properties were synthesized and investigated. In spite of subtle differences, all of them share many common features like crystal symmetry, magnetic ground state, close resemblance in phase diagram etc. Fe super- conductors are broadly classified into Fe pnictides (with Fe − pnictogen layer) and Fe chalcogenides (with Fe − chalcogen layer) in which the binary Fe chalcogenides possess the simplest crystal structure. The distinct magnetic and...

  12. Development of a Spin-Polarized Low Energy Electron Diffraction System and Investigation on Spin-Orbit and Exchange Interactions on Ir(100) and Ultrathin Fe(100) Grown on Ir(100)

    Pradeep, A V
    Spin-polarized electron beam has not yet been produced from an unpolarized electron beam using Stern-Gerlach type spin filter, because of the Lorentz force and Heisenberg uncertainty principle. At present, electron spin detectors and filters work on the basis of spin-dependent scattering of an electron beam from crystal surfaces. Single channel efficiencies of all the spin detectors for electrons are orders of magnitudes lower than the ideal one. Specular reflection type spin-polarized low energy electron diffraction (SPLEED)-spin detectors are having higher single channel efficiencies compared to the conventional Mott detectors. Moreover, multichannel detection can be realized from specular reflection type SPLEED-spin...

  13. The Many Facets of Variabilities in X-ray Binaries

    Islam, Nazma
    More than half a decade of X-ray astronomy with various balloon borne and space orbiting X-ray instruments, have led to discoveries and detailed studies of X-ray binaries. An important property of X-ray binaries is intensity variations of different magnitudes in a wide range of timescales from milliseconds (quasi-periodic oscillations, millisecond pulsations), to a few weeks (orbital and super-orbital modulations) or longer (outbursts etc). In this thesis, different types of variabilities of X-ray binaries are considered in X-ray binary population studies and to investigate certain aspects of some individual systems. In Chapter 1, we provide an introduction to various types of variabilities...

  14. Growth and Physical Properties of Biaxial Nonlinear Optical Crystals of Ascorbic Acid Family

    Raghavendra Rao, K
    Saccharides, a class of organic materials, are potential candidates for nonlinear optical applications. Ascorbic acid is a sugar acid and is classified as a monosaccharide. The molecule of ascorbic acid has two chiral centers and, therefore, four stereoisomers. Among them, two are naturally occurring compounds; L-ascorbic acid and D-isoascorbic acid. From these two acids various salts and other derivatives could be synthesized. In this thesis, four compounds of the ascorbic acid family were selected for detailed study based on their nonlinearity, chemical and physical stability and their crystallization characteristics. The thesis is organized into seven chapters. The first chapter covers...

  15. A Study of Slow Modes in Keplerian Discs

    Gulati, Mamta
    A rich variety of discs are found orbiting massive bodies in the universe. These could be accretion discs composed of gas around stellar mass compact objects fueling micro-quasar activity; protoplanetary discs, mainly composed of dust and gas, are the progenitors for planet formation; accretion discs composed of stars and gas around super-massive black holes at the centers of galaxies fueling the active galactic nuclei activity; discs in spiral galaxies; and many more. Structural and kinematic properties of these discs in several astrophysical systems are correlated to the global properties; for example, over a sample of thousands of galaxies, a correlation...

  16. Exchange Spring Behaviour in Magnetic Oxides

    Roy, Debangsu
    When a permanent magnet is considered for an application, the quantity that quantifies the usability of that material is the magnetic energy product (BH)max. In today’s world, rare earth transition metal permanent magnets like Nd-Fe-B, Sm-Co possesses the maximum magnetic energy product. But still for the industrial application, the ferrite permanent magnets are the primary choice over these rare transition metal magnets. Thus, in the present context, the magnetic energy product of the low cost ferrite system makes it unsuitable for the high magnetic energy application. In this regard, exchange spring magnets which combine the magnetization of the soft phase...

  17. Magnetisation, Phases & Phase Transitions in Frustrated and Unfrustrated XY Model

    Maji, Maheswar
    Through our whole work we study the XY model with all its entirety, a particular spin model where spins are confined in a plane. We try to bring out a good understanding of this model with all different types of phases and phase transition, it undergoes in critical situations. We conceive of these external conditions from very different physical models like High Tc Superconductor, Ultracold atoms in optical lattice which are in focus of recent research. Firstly we model high Tc Superconductors with very simple 2D XY model to get an idea about the diamagnetic response exhibited by these materials...

  18. Charge Transport in Semiconducting Polymer Devices

    Anjaneyulu, Ponnam
    Understanding the fundamentals of Organic semiconductors is crucial aspect towards the technological applications. Conjugated polymers have shown many interesting physical properties. Especially the electronic and optical properties of these materials have great impact on the daily life. Much work has been devoted to gain the knowledge on the electrical and photo physical properties of these materials. Despite the large number of studies in fabrication and characterisation on these devices some of the fundamental properties like charge transport, carrier generation and its control by doping are not well accomplished. The Thesis consists of 6 chapters. First chapter is a brief introduction...

  19. Modelling, Fabrication and Characterization of HgCdTe Infrared Detectors for High Operating Temperatures

    Srivastav, Vanya
    In this work, we have designed, simulated, fabricated and characterized homojunction Hg1-xCdxTe detector for high operating temperature in the MWIR region. The IR photon detectors need cryogenic cooling to suppress thermal generation. The temperature of operation in narrow gap semiconductor devices is limited by the noise due to statistical nature of thermal generation-recombination in narrow gap semiconductors. To make IR systems affordable they have to be operated without cooling or with minimal cooling compatible with low cost, low power and long life. Several fundamental and technological limitations to uncooled operation of photon detectors have been discussed in Chapter-1 of this...

  20. Insights into Substrate Specificity in Sortase Enzymes from Structural Studies on a Novel Class of Housekeeping Sortase (SrtE) Identifying Functionally Important Cis-Peptide Containing Segments in Proteins and their utility in Molecular Function Annotation

    Das, Sreetama
    Understanding protein function is fundamental to the fields of protein engineering and drug design. While most of the previous efforts in this direction have focused on the sequence-structure-function paradigm, recent studies have pointed to protein dynamics as being integral to its activity. The work in the current thesis follows this overall theme of obtaining insights into protein function from its structure and dynamics. It can be broadly divided into two sections. In the first section, the thesis candidate has tried to elucidate the residues modulating the substrate specificity of a particular family of enzymes, known as sortases, through structural and...

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