Department of Physics

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    (A) Theoretical Study of Optical Constants of Two Dimensional Carbon Based Materials
    (2010) Meera, V.
    The electromagnetic response of two dimensional carbon based systems is studied using basic classical tools. The most interesting and important carbon based system known till today is the by now well-known two-dimensional material DGrapheneD. This material is special in several ways, indeed, its very existence is an enigma. Graphene and a few of graphene based systems are studied using theoretical techniques. The key point of this work is the theoretical formulation of monolayer free-standing graphene. This formulation acts as the reference for the subsequent studies done in various graphene based systems. In this formulation monolayer free standing graphene is modelled as a conducting medium of one atom thickness such that the media is non-refracting. The emergent electric field of the system has been calculated by solving the fundamental equations of electromagnetism, DMaxwellDs EquationsD which consists of reflected and transmitted fields. Three physically important and experimentally measurable quantities are derived analytically, viz. coefficient of reflection, coefficient of transmission and polarization of reflection. The energy conservation theorem has been derived for the system using which the loss also has been calculated to make sure of the correctness of the formalism. The results obtained for some of these quantities with fixed values of various parameters show an excellent agreement with the experimental observations available in this field. For most of the device and technological applications, graphene has to be deposited on a substrate. For this reason, we next study a related system - substrate graphene. Theoretical modelling of this system has been done by regarding graphene as a conducting medium deposited on top of a purely dielectric material characterized by a dielectric constant. MaxwellDs equations for this combined system has been solved using the boundary conditions and derived equations for the two parts of emergent field, reflection and transmission which has been made use to study the optical properties of this system. The linearly polarized limit of incident light in this system is important because of a well-known phenomenon in optics, DBrewsterDs phenomenonD. The most interesting feature of BrewsterDs phenomenon related to substrate graphene is the azimuthal angle dependence of the Brewsters minimum. The next immediate system related to graphene is the combination of two single graphene sheets separated by a distance of a few Angstroms. With the monolayer results as reference, the optical constants of the bilayer system has been solved using series summation methods used in many fundamental optics books (such as [142]). Presence of an extra layer leads to multiple reflection phenomena....
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    (A) Theoretical Study of Rabi Oscillations in Graphene
    (2014) Enamullah
    The nonlinear optical response of graphene has been studied by the newly developed technique which is an alternative to rotating wave approximation (RWA). This is referred to as asymptotic RWA (ARWA). is a single layer of graphite material possessing a degree of freedom known as pseudospin. It is a bridge between condensed matter and relativistic electrodynamics, since the low energy spectrum of graphene near some particular points called Dirac points is linear in momentum. The interaction between the charge carrier and the periodic potential of graphene leads to quasiparticles which obey the Dirac relativistic equation. At the Dirac points, the conduction band just touches the valance band due to which it is also called zero band gap semiconductor. These peculiar properties of graphene are the motivation behind our work and are studied using optical means, specifically through nonlinear optical response. As the title of the thesis itself suggests, this work is the study of the well-known coherent optical phenomenon in nonlinear optics viz. Rabi oscillation - a periodic exchange of energy between a two level system in case of atoms and two band system in case of semiconductors and the applied optical field. It is the oscillation in the population and polarization of carries (with a given wave-vector in case of a band) with a frequency !R determined by the intensity of the externally applied optical field. This frequency !R is typically much smaller than the optical frequency ! itself. This phenomenon is well described in various textbooks on quantum optics. The same phenomenon manifests itself in semiconductors, which have bands instead of energy levels. It is therefore appropriate to investigate the same effects in graphene where the bands are linear instead of parabolic and the system is two dimensional instead of three and there is pseudospin character. The phenomenon of Rabi oscillations in graphene has been studied by Mishchenko among others, near the particle-hole resonance using the well known rotating wave approximation (RWA).
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    Ab initio molecular dynamics studies of structural, dynamical and spectroscopic aspects of waterborne selenium and arsenic species
    (2018) Borah, Sangkha
    Water contamination is a global issue in most parts of the world. The present thesis deals with the microscopic nature of interaction of two major water contaminants, namely selenium (Se) and arsenic (As), restricting largely to their relevant inorganic forms, owing to their higher solubility in water, employing ab initio molecular dynamics simulation. Various microscopic properties including molecular structure, hydrogen bonding, vibrational spectra, etc. are examined in detail. The thesis is organized as follows. A review of the state-of-the-art research activities on solvation studies of ab initio molecular dynamics, bio-geochemistry of selenium and arsenic species as well as the motivation for the present work is outlined in Chapter 1. Chapter 2 of the thesis summarizes the theoretical techniques employed in the work. Chapters 3 and 4 of the thesis present ab initio molecular dynamics studies on various inorganic Se – VI and Se – IV species in water. In chapters 5 and 6 inorganic As – III and As – V species are examined. Chapter 7 gives the conclusion of the thesis.
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    Accretion-ejection mechanism from advective accretion disc around rotating black holes
    (2018) Aktar, MD Ramiz
    In the first work, we investigate the effect of spin on the mass outflow rates from a steady, advective, inviscid, geometrically thin accretion flow around a rotating black hole. For this purpose, we adopt pseudo- Kerr potentials to mimic the space-time geometry around rotating black holes. We calculate maximum outflow rates in terms of spin of the black holes. Interestingly, we observe a weak correlation between the spin and the maximum outflow rates from our model. Finally, we apply our accretion-ejection model to estimate the kinetic jet power for various black hole sources. In the second work, we consider a steady, dissipative, geometrically thin accretion flow around a rotating black hole. We find that the shock waves exist for a wide range of dissipative parameters. We also calculate critical limit of viscosity parameters in terms of spin of the black holes that permits shock solutions. We also calculate the maximum QPO frequency in terms of the black hole spin using our accretion-ejection model. Finally, we apply our formalism to constrain the spin parameter of the black hole source GRO J1655-40. In the third work, we perform a comparative study of a steady, dissipative, geometrically thin accretion flow around a rotating black hole using three pseudo-Kerr potentials. We observe that in case of weakly rotating black holes, the critical viscosity parameters for all the pseudo-potentials agrees quite well, but differs considerably from each other for rapidly rotating black holes. We also indicate that BH-XRBs along the ‘outliers’ track seems to be rapidly rotating based on our model. In the fourth work, we study the dynamics of advective inviscid accretion flow using time-dependent hydrodynamical simulation around rotating black holes. Due to the shock compression, the post-shock corona (hereafter PSC) usually becomes very hot and dense and therefore, acts as a source of high energy radiations. When PSC modulates, the high energy photon flux coming out from PSC is also oscillates that eventually exhibits Quasi-periodic Oscillations (QPOs). When PSC executes very fast oscillation, it demonstrates high frequency QPOs (HFQPOs). We carry out the numerical investigation of HFQPOs for various sets of input parameters and compare our results with the observation of LAXPC/AstroSat and RXTE for galactic black hole source GRS 1915+105. Based on this comparative study, we indicate the possible range of mass and spin values of this source
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    Analytical and simulation modeling of terahertz waveguides and sensors based upon plasmonic and metamaterial structures
    (2017) Islam, Maidul
    Plasmonics and metamaterials have emerged as one of the most fascinating areas in photonics because of their significance in developing next generation miniaturized high speed components and sensitive devices. In the thesis work, the focus has been made in investigating the potential of plasmonic and metamaterial structures in the design and construction of terahertz waveguides and thin film sensors. We have proposed a planar plasmonic terahertz waveguide comprising of one-dimensional array of sub-wavelength scale periodically arranged tilted pillars, where the propagation properties of the terahertz modes can be controlled through the bending of pillars. In plasmonic terahertz waveguides, we have also investigated the role of internal corrugations in altering the propagation properties of the guided terahertz modes. We further investigate the potential of planar plasmonic terahertz waveguides as thin film sensors. In this novel study, we designed a plasmonic waveguide comprising of periodic rectangular grooves and filled them with analytes of different refractive indices. The potential of planar terahertz metamaterials as thin film sensors has been widely investigated in last few years. In this context, several metamaterial configurations have been devised to effectively sense an analyte. We have examined the role of fundamental and higher-order resonances as thin film terahertz sensor. For this purpose, we have used a single split ring resonator (SRR) based terahertz metamaterial, which exhibits fundamental and higher order resonance modes subject to the polarization of electric field vector of incident terahertz radiation with respect to the gap of SRR. In order to understand and analyze the numerical observation in our study, we have modeled plasmonic and metamaterial structures with a semi-analytical transmission line approach.
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    Aspects of Dark Matter Phenomenology and Connection to Neutrino mass
    (2021) Nanda, Dibyendu
    After the remarkable discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012, the standard model (SM) of particle physics has been established as the most successful theory of elementary particles and their fundamental interactions except gravity. However, there are convincing reasons to believe that there is new physics beyond the standard model (BSM) as several observed phenomena as well as theoretical questions remain unanswered in the SM. Among such observed phenomena, the presence of dark matter (DM) giving rise to around 26% of the present Universe is very appealing. Although the rst evidence for DM appeared in the 1930s, it was only in the early 1980s that astronomers were convinced of the fact that most of the mass holding galaxies and clusters of galaxies together is non-luminous. Since the nature of particle DM and its interactions with the SM particles, except gravitational interactions, are not yet known, there exist two broad scenarios: one in which DM couples to SM with couplings of the order of electroweak interactions and one in which DM-SM couplings are very feeble. The rst scenario, popularly known as the weakly inter-acting massive particle (WIMP) paradigm, is the most studied particle DM candidate in the literature. WIMP can be de ned as a new elementary particle whose interaction strength is as weak as or weaker than the weak nuclear force. The typical mass range for WIMPs can vary from a few GeV to a few TeV. WIMPs can be thermally produced in the early Universe through their interaction with the bath particles. Their relic abundance can be found from the very well known \freeze-out" mechanism. WIMPs can also leave observable signatures at several direct, indirect search experiments by virtue of their sizeable interactions. While no such observations have been made yet, the other scenario, known as the feebly interact- ing massive particle (FIMP) scenario, has also gained attention in the last few years. In this paradigm, DM will have very feeble interaction with SM particles, which prevents them from getting produced thermally in the early Universe. However, it is extremely difficult to produce or detect such particles directly in our ongoing experiments. Another observed phenomena is the existence of tiny but non-zero neutrino mass and large mixing, which have been confirmed by several neutrino experiments for more than two decades till now. Specially, the more recent experimental results have not only confirmed the results from earlier experiments but also discovered the non-zero reactor mixing angle 13. The two most important unknowns at present in the neutrino sector are the mass hierarchy: whether it is normal (m3 > m2 > m1) or inverted (m2 > m1 > m3), and the leptonic Dirac CP phase . Apart from neutrino oscillation experiments, the neutrino sector is constrained by the data from cosmology and rare decay experiments. For example, the combined data fProm the Planck 2018 and BAO experiments constrains the sum of absolute neutrino masses ijmij < 0:12 eV at 95% CL. If there are no right-handed neutrinos, the Higgs field, which 2 lies at the origin of all massive particles in the SM, can not have any Dirac Yukawa coupling with the neutrinos. If we include right-handed neutrinos by hand without a Majorana mass for the right-handed neutrinos, the required Yukawa couplings are extremely small, around 10􀀀12 or smaller, usually considered as unnatural. Additionally, a bare mass term of the right-handed neutrinos is allowed by gauge symmetries, introducing a new scale outside the purview of the SM. The purpose of this thesis is to study minimal models of scalar or fermion DM, where light neutrino mass can also arise naturally with a non-trivial connection to the DM sector. To do that, we have extended the particle content as well as the symmetry of the SM either by gauge or global symmetries and studied their phenomenological consequences. We have shown that such symmetries not only stabilize DM but also play a crucial role in generating neutrino mass and mixing. We elaborate upon direct and collider search prospects of the models both in the context of WIMP, and in some speci c scenarios, we have also discussed the possibilities of constraining the model from the cosmological observations like e ective relativistic degrees of freedom (DOF) in cosmic microwave background (CMB). We have also discussed the interplay of thermal and non-thermal contribution to DM relic density. The thesis has been divided into ve chapters. We have started with a brief introduction of the DM physics and the neutrino mass generation mechanism in chapter 1. In chapter 2, we have studied a class of a very well motivated BSM framework based on the gauged B 􀀀 L extension of the SM with B, L being baryon and lepton numbers, respectively. This minimal and economical model generating nonzero neutrino mass has been studied for a long time. The most interesting feature of the minimal version of this framework is that the inclusion of three right-handed neutrinos, as it is done in the type I seesaw mechanism of generating light neutrino masses, is no longer a choice but a necessity due to the requirement of the new U(1)B􀀀L gauge symmetry to be anomaly free. We have discussed three di erent versions of the model where neutrino mass (either Majorana or Dirac) and DM physics, namely the stability, type and number of DM candidates are dictated by the anomaly cancellation requirements. In chapter 3, we consider the possibility of probing the left-right symmetric model (LRSM) via the CMB. LRSM has been one of the most popular BSM frameworks studied in the literature. Here the gauge symmetry of the SM is extended to SU(3)c SU(2)L SU(2)R U(1)B􀀀L so that the right-handed fermions (which are singlets in SM) can form doublets under the new SU(2)R. This not only makes the inclusion of right-handed neutrino automatic, but also puts the left and right-handed fermions on equal footing. Due to the presence of gauge interactions of the right-handed neutrinos, they can be thermally produced in the early Universe and contribute to the total e ective relativistic DOF and can be signi cantly constrained. The chapter 4 discusses the role of discrete symmetries in physics of DM and neutrinos. It is based on two di erent scenarios where the relic abundance of DM is generated either from a hybrid of thermal and non-thermal mechanisms or from purely non-thermal mechanism. While in the rst scenario, Z2 symmetry plays the role of stabilizing DM and generating radiative neutrino mass, in the latter scenario, the non-abelian discrete symmetry group A4 is implemented to generate light neutrino mass and mixing. In the rst case, we have shown that a thermally under-abundant DM candidate can get non-thermal contribution to satisfy relic density bounds. The second section is dedicated to providing a common framework explaining the origin of tiny couplings required for light Dirac neutrinos and non-thermal DM. In chapter 5, we have summarized our discussion and mentioned the future directions.
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    Aspects of Fermi arcs and surfaces from the perspective of Gauge -Gravity Duality
    (2022) Wahlang, Wadbor
    Despite several efforts, strongly correlated systems still pose as one of the most challenging conundrums in theoretical physics. The difficulty in modelling and solving the problems has grown even more after the discovery of high-temperature superconductors, topological insulators, Dirac and Weyl’s semimetals. Experimental results on these materials have revealed many intriguing phenomena such as pseudo-gap phase and Fermi arcs, which we have very little understanding theoretically. These systems are not suitable to be described by conventional perturbative approaches. In the present thesis, we used an alternative method with a non-perturbative tool provided by the gauge-gravity duality, allowing us to access the strongly coupled regime. With the motivation to understand the underlying mechanism that gives rise to these peculiar phenomena, we proposed several holographic models by exploring the symmetries of either the background geometry in the bulk or at the boundary. By computing the fermions spectral function, we primarily explore the properties of the Fermi surface and Fermi arcs in certain strongly coupled systems of interests. Our results revealed some interesting features that are closely related to those that are seen in real materials from condensed matter experiments
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    Aspects of Flavor Leptogenesis in Particle Physics and Cosmology
    (2023) Datta, Arghyajit
    The dynamical generation of cosmological baryon asymmetry is one of the leading problems in the field of Particle physics and Cosmology, which the standard model of particle physics can not explain. In this thesis, we have analyzed in detail the generation of baryon asymmetry by out-of-equilibrium decay of heavy particle states responsible for neutrino mass. The relevant mechanism is known as baryogenesis via leptogenesis. In this process, heavy particle states decay out-of-equilibrium to produce lepton asymmetry, which is then converted to baryon asymmetry by sphaleron processes before they decouple. It is shown in the literature that the individual charged lepton Yukawa interactions could influence the lepton asymmetry generation when they become dominant over the expansion of the Universe at a low energy scale. In this thesis, we have investigated some beyond standard model scenarios which can influence such flavor leptogenesis setups. Firstly, we have investigated the impact of an additional flavor symmetry on charged lepton, neutrino Yukawa, and Majorana Right handed neutrino mass matrices. This eventually leads to interesting results not only in the neutrino sector but also in terms of the flavor leptogenesis scenario. The scenario predicts the normal hierarchical scheme in the neutrino mass and falsifies the leptonic sector’s maximal CP asymmetry. On top of that, a successful low scale leptogenesis scenario is constructed, evading the important Davidson-Ibarra bound. Then, we propose two scenarios which not only explain the existence of dark matter, baryon asymmetry, and neutrino mass simultaneously but also provide a platform where the early universe dynamics of the dark matter can impact the lepton asymmetry of the Universe, sometimes leading to low scale leptogenesis scenarios. Considering the low scale nature of the lepton asymmetry generation, individual lepton flavors have played an important role in determining the correct amount of asymmetry in all these scenarios. Finally, we have studied the impact of prolonged reheating scenarios on the charged lepton equilibration temperatures that eventually affect the so-called individual lepton flavor regimes of flavor leptogenesis setup. As a result, allowed parameter space significantly gets altered if leptogenesis occurs during the reheating period.
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    Aspects of Low Scale Leptogenesis and Connection to Dark Matter
    (2023) Mahanta, Devabrat
    The observed baryon asymmetry and dark matter (DM) in the universe have been two longstanding puzzles in particle physics and cosmology. While the standard model (SM) of particle physics can neither satisfy the required criteria to generate the observed baryon asymmetry of universe (BAU) dynamically nor offer a viable DM candidate. Among several popular mechanisms put forward to explain these observed phenomena, leptogenesis is one of the most popular one to explain the origin of BAU whereas particle DM of thermal or non-thermal origin having mass around the electroweak scale has been a popular DM paradigm. In this thesis, we aim to study a few leptogenesis scenarios which can also shed light on the origin of DM. A common framework for explaining both the BAU and DM is motivating due to its minimal and predictive nature. We consider a few realistic particle physics models where there exist new particles and symmetries beyond those in the SM. While canonical neutrino mass models, also known as seesaw models, predict high scale leptogenesis out of reach from direct search experiments, we focus on leptogenesis and DM scenarios where scale of leptogenesis can be brought down to TeV corner such that these scenarios can be tested at near future experiments. After giving introduction to the observed evidences and popular theoretical mechanisms for BAU and DM in chapter 1, we consider a radiative seesaw, known as the minimal scotogenic model in chapter 2, to study the possibility of thermal as well as non-thermal fermion singlet DM with the heavier singlet fermions being responsible for successful leptogenesis. In chapter 3, we study a novel scenario where lepton asymmetry is generated from three-body decay of a heavy fermion with DM as one of the final states. While phase-space suppression and involvement of new parameters independent of neutrino mass lead to sub-TeV scale leptogenesis, the DM sector naturally emerges as a two-component type. In chapter 4, we study the possibility of having successful TeV scale leptogenesis with light Dirac neutrinos in a gauged B-L model. The symmetry and particle content of the model allow for lepton number violation by more than two units while keeping light neutrinos as purely Dirac. Apart from successful TeV scale leptogenesis and other phenomenological aspects of gauged B-L model, we also show that the model can be probed at future cosmology experiments capable of measuring additional relativistic degrees of freedom affecting cosmic microwave background power spectrum. Finally, in chapter 5, we consider the impact of non-standard cosmological histories on the production of BAU and DM in two different setups; one where lepton asymmetry arises from two-body decay similar to the minimal scotogenic model and the other where it arises from DM annihilations. Depending upon the type of non-standard epoch, the scale of leptogenesis can be lower compared to the standard cosmology in some cases, making the detection prospects more promising.
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    Asymptotic symmetry and its role in black hole thermodynamics
    (2022) Maitra, Mousumi
    The astonishing connection between the asymptotic symmetries and the thermodynamics of the black hole was realized by the scientific communities about fourty years ago. One of the unsolved tasks in the study of the black hole thermodynamics is to uncover and understand the microscopic degrees of freedom which are responsible for the entropy of the black hole. In this context, the Noether charges and current related to the asymptotic symmetries play a pivotal role in resolving this issue. In 1962, surprisingly, an infinite dimensional group of symmetries near null infinity of the asymptotically flat spacetime was discovered as the asymptotic symmetry group (BMS), which has the flat space Poincaré group as the subgroup. Initially, it was revealed that the asymptotic symmetry analysis near null boundaries could shed light on the gravitational scattering phenomena, but later the symmetry analysis was well extended to the near-horizon region of the black hole. In the present thesis, we investigate the various aspects of the asymptotic symmetries in gravitational theories. At first, we try to analyze the asymptotic symmetries and the conserved charges near a generic null hypersurface having electromagnetic charge, in higher-order theory of gravity with the presence of the non-linear gauge field. We hope that this result will illuminate the physical importance of the charges in a more general context. However, by these lines of work, consequently, it is found out that the supertranslation and superrotation parameters modify the macroscopic parameters of the black hole. We argue that this can be treated as the breaking of the symmetry of the arbitrariness of the solutions, by the black hole backgrounds. In Rindler and Schwarzschild black hole backgrounds, we study the Lagrangian dynamics of the Goldstone modes and interestingly the Fourier modes of the supertranslation parameter come out to be the unstable ones. In the semi-classical regime, we found that this instability can lead to the thermalization of the horizon. Next, the same analysis is further performed in the rotating black hole background. Finally, We investigate the asymptotic symmetries near a timelike hypersurface at a finite distance outside the horizon of the black hole, following the standard procedure.
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    Atomic Coherence and its Spectroscopic Applications
    (2021) Nyakang'o, Ogaro Elijah
    Spectroscopy and stabilization of the laser at a particular transition is key in the modern area of atomic and optical physics. The present thesis details the mechanism of induced atomic coherence between multi-level atomic system driven by lasers and their role in the absorption and fluorescence spectrum. The theoretical study presents a frame work to identify the nature and the role of interference between the excitation paths associated with the Autler-Townes (AT) peaks (i.e., a pair of transitions from the ground state to the dressed states) in a multi-level system. In three-level system the excitation paths associated with the two AT peaks interferes pair-wise and the nature of interference is very simple which can be constructive, destructive or no interference depending upon the decay rate of the states coupled by the strong control lasers. In four-level system the nature of interference is more complicated but again the excitation paths associated with all the three AT peaks interferes pair-wise. For any system, if the decay rate of the levels coupled by the control lasers are equal then there is no interference between any of the excitation paths associated with the AT peaks. We further studied the saturated fluorescence spectroscopy which is a very useful spec- troscopic technique for weak transitions. In the study, the Doppler-free fluorescence dip in the fluorescence spectra is caused by velocity selective saturation (VSS) effect and the dip is further modified by velocity induced population oscillation (VIPO) effect. The VIPO effect is caused by the beating of two counter-propagating elec- tromagnetic fields in a moving atomic frame (due to opposite Doppler shift for a given velocity). The line-shape of the saturated fluorescence dip is sensitive to the laser beam misalignment in the case of the atomic beam. The shifts of the fluores- cence dip is dependent on the average velocity of the atomic beam and the angles of misalignment of the laser beams. The theoretical study is experimentally utilized for double resonance spectroscopy at infrared (5S1=2 ! 5P3=2) strong transition and blue (5S1=2 ! 6P1=2) weak transition in Rb atom. The double resonance (at 780 nm and 421 nm) is implemented using EIT effect in a V-type system and enhanced absorption (EA) effect in optical pumping system. The scan non-linearity of the blue laser (which is the dominant source of error in the experiment) is minimized by using acousto-optic modulator to shift the laser frequency within a small range of frequencies around the neighboring hyperfine level. The hyperfine splitting of the 6P1=2 state is measured with a precision of < 400 kHz and the magnetic dipole hyperfine constant is also calculated. We further utilized VIPO at infrared transition, VSS at blue transition and the combi- nation of the two effects to resolve closely spaced hyperfine levels of a weak transition by eliminating the residual (or partial) two-photon Doppler broadening in a wave- length mismatched double resonance spectroscopy. The double resonance experiment is conducted on 5S1=2 ! 5P3=2 strong transition (at 780 nm) and 5S1=2 ! 6P3=2 weak transition (at 420 nm) in Rb atom at room temperature. The residual Doppler broad- ening is caused by the thermal motion of the atoms in the vapor cell. The elimination of the partial Doppler broadening using the VIPO and VSS effects is followed by the subtraction of the broad background of the two-photon spectrum. Since the VIPO and VSS effects are phenomena for near zero velocity group atoms, the subtraction gives rise to Doppler-free peaks and the closely spaced hyperfine levels of the 6P3=2 state in Rb are well resolved. The resolved peaks with narrow linewidth, are impor- tant for stabilizing the blue laser at a particular transition (i.e., for tight laser locking) for future goals of laser cooling and trapping at this transition. The spectroscopy and stabilization of the blue laser is also beneficial for quantum information processing with coherent excitation of Rydberg states in Rb atoms.
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    Atomic Coherence Based Electromagnetic Wave Interferometry
    (2023) Shylla, Dangka
    This thesis reports on the theoretical and experimental studies of closed loop multi-level systems, where electromagnetically induced transparency (EIT) is dependent on the phase difference between the electromagnetic fields forming the loop. We first theoretically investigate a scheme to develop an atomic-based microwave (MW) interferometry in Rb, based on a six-level loopy ladder system involving the Rydberg states in which two excitation pathways interfere constructively or destructively depending on the phase between the MW electric fields closing the loop. Then we compared the field strength sensitivity to previous demonstrations on MW electrometry employing Rydberg atomic states, this is two orders of magnitude more sensitive to field strength. Because previously investigated atomic systems are only sensitive to field strength but not to phase, this scheme offers a great opportunity to characterize the MW completely, including the propagation direction and wavefront. Currently, we do not have the experimental facility for Rydberg excitation so we cannot conduct the experiment of the above theoretically proposed work. However, we could demonstrate the phase-dependent EIT in the different configurations of a closed loop double-lambda system at 780 nm and 420 nm transitions in 87Rb at room temperature. For the MW field measurements, the sensitivity can be improved by employing the cold atoms because cold atoms reduce the Doppler mismatch between the 780 nm probe and 480 nm control fields and also minimizes the collisions and transit time dephasing effect. Taking this into consideration, we have also set up the cold atom experiments and so far, we have characterized the 85Rb atoms in the MOT using the 5S1/2(F = 3) → 5P3/2(F = 4) broad cyclic IR transition at 780 nm where we trap around 1.5×108 number of atoms at a typical temperature of 500 μK. In laser cooling and trapping experiments, the temperature of the cold atoms is sensitive to the lock point of the laser fields. The laser locking can have an offset from the line center of the transition which depends upon the linewidth of the transition. In order to determine the laser lock o set on a particular atomic transition, we also present an experimental study on the effect of detuning on a velocity-induced population oscillation (VIPO) dip which is used to precisely determine the lock point with an uncertainty of around 100 kHz.
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    Atomistic simulation of point defects in ionic crystals
    (2005) Sahariah, Munima B
    In this thesis, we have presentrd a comprehensive study of modeling point defects in high and low symmetry crystals with Polarizable Point Ion (PPI) model. We worked out an alternate scheme to estimating energies of point defects through finite size calculations. This new method is tested for cubic NaCl crystal and is found to agree well with other existing results. Next, we studied in detail the effect of quadrupoles in the formation energy of vacancies in alkaline earth oxides using the approximate Mott-Littleton (ML) scheme. In agreement with earlier results on AgCl and AgBr, the quadrupoles are found to have substantial effect on defect energies raising a challenge to the existing dipolar models. We then worked out the PPI firmulations for low symmetry crystals, both in perfect and defect environment. A suitable set of short range potential parameters are deduced for monoclinic...
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    Atomistic Simulation Studies of Alkali Ion Conducting Superionic Conductors
    (2020) Pramanik, Krishnanjan
    Designing of all-solid-state-battery by replacing the currently used liquid/gel electrolytes with solid ones is the prospect of next generation energy storage devices. This advancement promises higher energy density, safety and longer operating cycles. However, the progress towards the commercial realization of such devices demands microscopic insights on various factors, including the mechanism of ion transport in solids. This calls for computational investigations complementing experimental studies. In this thesis, computational studies of some of the promising classes of Li/Na-ion conducting solids are presented. The first chapter of the thesis is devoted to the review of the ongoing research on Li/Na-ion conducting inorganic solids. In the second chapter, the theoretical background of the computational methods, such as classical and ab initio molecular dynamics, metadynamics, nudged elastic band method, etc. employed in the studies are discussed. The third chapter presents fresh atomic-scale insights on the role of framework dynamics on ion transport in Li-substituted NASICONs (LiM2P3O12 where M = Zr, Hf, Sn, Ti) based on classical molecular dynamics study. The fourth chapter of the thesis addresses the ‘time scale’ issue of standard molecular dynamics simulation in the study of slow diffusing systems. Plugged in with classical molecular dynamics, the utility of metadynamics technique has been demonstrated by taking the low diffusing phosphate and silicate end members of the true-NASICON family Na1+xZr2SixP3– xO12 (0 · x · 3) as the prototype systems. This utility is further explored in the fifth chapter by using metadynamics interfaced with ab initio molecular dynamics, to understand the Li migration mechanism in °-Li3PS4. The sixth chapter summarizes the results.
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    Augmentation of ray-pencil model to calculate optical trapping force for arbitrary beam profiles and its experimental validation
    (2022) Malik, Karuna Sindhu
    Single beam optical trap comprises a tightly focused laser beam that exerts a pico-Newton level force on suspended microscopic particles. Since last couple of decades optical traps have become an important tool in areas such as biological and biomedical sciences. For proper design and efficient working of the optical traps it is important to know the various forces acting on a trapped particle using an appropriate force calculation model. Since majority of the recent applications involve trapping and manipulation of large biological particles hence for such particles force calculation model in the ray optics regime such as the ray-pencil model is more appropriate. However existing form of the ray-pencil model can compute optical forces due to plane wavefronts with cylindrically symmetric beam profiles only. On the other hand there are optical traps that use complex beams such as vortex beams. Moreover, often the light beam used in an optical trap does not have a plane wavefront due to aberrations present. For all such cases the present form of the ray-pencil model is not sufficient. In this thesis we propose an augmentation in the ray-pencil model to be able to compute optical forces due to an arbitrary beam profile. We develop a dynamic holographic optical trap to experimentally measure the various optical forces acting on a spherical bead. We validate the proposed augmented force calculation model by comparing the numerical results with the experimental results. We also compute the optical forces on a trapped bead due to a number of vortex beams using t he proposed model. We then use our holographic trap to perform trapping experiments using the vortex beams. We again find that the experimental results agree well with the theoretical predictions.
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    Branching fraction measurement for the decayB0s ! and search for the decay B0s! at high energy e+e_ collisions at energy
    (2014) Dutta, Deepanwita
    We measure the branching fraction for the decay B0 s and search for the decay B0 s using 121.4 fb of data collected at the resonance with the Belle detector at the KEKB asymmetric energy B-factory located at the High Energy Accelerator Research Organization (KEK), Japan. B0 s branching fraction is computed to be (3.7+0.6 0.7) स 10 with a signal significance of 10.6 including the systematic uncertainties. This result is in good agreement with the theoretical predictions and a recent indirect estimate from LHCb. We do not observe any significant signal for the B0 s and have thus set the 90% confidence level upper limit on its branching fraction to be 3.1 This result improves on the previously published 90% confidence level upper limit by a factor of about 3 and provides the most stringent limit till date.
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    Characterizing Dark Matter Dynamics and Collider Implications
    (2021) Chakraborti, Sreemanti
    After the Higgs discovery, the Large Hadron Collider (LHC) did not find any evidence of new physics. Despite the Standard Model (SM) being “the” theory for the elementary particles, there are still many unanswered questions that give a strong motivation for beyond the SM (BSM) theories. A particle candidate for dark matter (DM) is one such motivation, as its existence is beyond any doubt from various cosmological and astrophysical observations. In its pursuit, many theories have been proposed for DM origin, the most popular till date being the freeze-out mechanism, featuring Weakly Interacting Massive Particle (WIMP). Although WIMP can explain the observed DM relic density well, it fails to fit the direct detection constraints. This motivates the need for alternatives, featuring the non-WIMP cases. This thesis focuses on three such non-WIMP scenarios having interesting implications in the DM dynamics and collider probes. In the first part, we discuss two multipartite WIMP scenarios of electroweak scale DM mass. Here, the interplay of stable DM components of different spins and isospins successfully evade the direct detection constraint, which is otherwise not possible in similar single component WIMP models. In the context of collider probe, we show that as a contrast with traditional cut-based MET analysis strategies in the literature, which was initially developed for SUSY analysis, a hard MET cut at low DM mass is not required in the non-SUSY models. The second part includes two chapters. The first one focuses on reviving the very constrained scalar singlet DM (SDM) model of electroweak scale by extending the dark sector with additional vector-like dark lepton doublet. This induces DM coannihilation and adds additional channels to pair annihilation and all these alleviate the severe direct detection constraints on standard SDM. We also discuss the possible collider probe with multilepton final states and τ-tagging at the LHC using the ROOT based Boosted Decision Tree (BDT) classifier Toolkit for Multivariate Analysis (TMVA). In the second chapter, we add an extra vector-like singlet in the dark sector of the previous chapter. This simple extension helps to do away with ad-hoc addition of new scale of spontaneous symmetry breaking to lift the degeneracy of the lepton doublet components. Furthermore, the mixing between the same-charge dark leptons become a key factor to distinguish between the dark leptons of different isospins, which has not been discussed much in the literature. We discuss how the mixing leaves a positive impact on various search prospects of DM such as indirect detection and collider probe. The proper choice of final states in the LHC features this mixing effect substantially and a significance study is done for future luminosities. In the third part, we discuss a minimalistic BSM scenario which identifies different areas of the parameter region for electroweak scale DM mass, where DM relic density is controlled by either non-thermal (freeze-in) or thermal (freeze-out) DM dynamics or their admixture. For suitable parameters, it gives a quantitative estimate of the limiting case where the transition between the above regimes takes place. As a natural consequence of the model arrangement, one or more long-lived particles (LLP) are produced. The DM dynamics often dictates the LLP characteristics and the signatures substantially change from one case to another leading to exotic signatures such as cascade decays of LLPs and kinked charged tracks. The model also features right-handed neutrinos which account for neutrino mass through Type-I seesaw mechanism, that requires it to be long-lived as well. The search prospects for such neutral LLP giving displaced vertex signatures are also discussed for the LHC and future detectors.
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    Classical and Quantum Aspects of Near-Horizon Physics
    (2023) Dalui, Surojit
    In recent years, researchers' attention has been sparked by the thermal and geometrical characteristics of black hole horizons as well as their intimate relationship with the dynamics of particle motion surrounding them. Because of this, research into near-horizon physics has received a lot of interest recently. Over time, systems have begun to exhibit some intriguing behaviours whenever they come under the dominance of this mysterious one-way membrane, according to scientists. One of these traits is the appearance of chaotic dynamics in a system in the vicinity of the horizon. It has been found that the influence of horizon on a system can introduce chaos within the system. Research on chaos in the presence of horizons has been ongoing for a long time, but the reason for this special feature of the horizon is still not apparent. Similarly, it is crucial to take into account in this context why all horizons (whether static or stationary) express the same phenomenological quality. Contrarily, the idea of black hole thermodynamics has been around for a while and is based on an analogy between the laws governing black holes and those governing typical thermodynamical systems. However, no one has ever really addressed why these thermodynamical quantities are connected to the horizon. In actuality, we still don't fully understand the underlying physical process that generates temperature in the horizon system. For instance, the kinetic theory of gases explains that the temperature of a gas contained in a cylinder is caused by the kinetic energy of the gas particles. However, it is unknown at this time whether a similar mechanism will operate in the scenario of a horizon. As a result, it is also unknown which microscopic degrees of freedom (MDOF) are in charge of such a property. Despite numerous tries, there are currently no conclusive explanations.
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    Coherent Control and Manipulation of atoms using femtosecond pulses
    (2013) Kumar, Parvendra
    Coherent control and manipulation of atoms is one of the central themes of research in atomic and optical physics owing to many potential applications including laser cooling, optical lattices, Bose-Einstein condensate, quantum information processing, high precession spectroscopy, and quantum computing. For many of these applications, it is highly desirable to produce samples of cold atoms whose population resides almost entirely in a particular quantum state. Primarily, two methods of coherent control are used to transfer the complete population to a particular state of atoms. These are stimulated Raman adiabatic passage (STIRAP) and adiabatic rapid passage (ARP). On the other hand, since the seventies of the last century, the laser induced forces, the so-called optical dipole force and the dissipative force, have been used routinely for the manipulation of atoms. The optical dipole force and the dissipative force have been used for the trapping of particles and cooling of atoms respectively. This particular area of research, i.e. coherent control and manipulation, is getting tremendous boost due to the recent technological developments in the generation of femtosecond pulses. In the context of coherent control, femtosecond lasers may be advantageous to realize ultrafast population transfer that is decoherence-free and robust. On the other hand, concerning manipulation of atoms or molecules using lasers, the magnitude of the dissipative force induced by commonly used CW lasers is limited due to spontaneous decay process. This limitation may be surmounted by using picosecond or femtosecond pulses, thereby realizing a very strong optical force. In this thesis, a detailed theoretical study is carried out on the manipulation and coherent control of atoms beyond the so-called rotating wave approximation (RWA). In the context of coherent control, we have studied the coherent population transfer (CPT) in two-, L -like three- and Y-like four-level atoms. This thesis also reports a study on the femtosecond pulse induced optical force on two- and three-level atoms in an atomic beam.