PhD Theses (Physics)

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    Interaction and disorder effects on topology in one-dimensional lattices
    (2024) Padhan, Ashirbad
    The topological phases and phase transitions in condensed matter is subject of great interest in the field of condensed matter physics. Starting from the first observation of the quantum Hall effect in two dimensional electron systems, study of topological phases has attracted a great deal of attention in the last several decades. Topological phases are characterised by gapped bulk spectrum and gapless or localized edge states, non-local correlations and well defined topological invariants. A class of topological phases which is known as the symmetry protected topological phases where the bulk-boundary correspondence is protected by some underlying symmetries. In general the topological character is robust to perturbation although strong perturbation such as interaction and disorder breaks down the topological nature. However, in certain cases, these perturbations can drive a topological phase transition or may induce a topological character in the system. Due to the rapid progress in the field of quantum simulations of such systems in artificial experimental setups and their relevance as effective models to some of the real materials, these systems are explored in various different contexts. Motivated by this development, we focus on the study of interaction and disorder effects on the topological character of low dimensional lattice systems in this thesis.
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    Exploring Charge Storage Mechanism in MXene as Supercapacitor Electrode: A First-Principles Approach
    (2024) Das, Mandira
    The exfoliation of layered transition metal carbides/nitrides, MXenes (Mn+1Xn), from its 3D precursor MAX is remarkable event in the history of 2D materials. Ti3C2 , the first discovered MXene, caught everyone’s attention due to its excellent charge storage capacity as a supercapacitor electrode. It has been the most explored MXene in this 2D subfamily. 70% of the MXene research is on this compound only. Baring Ti3C2 , various other transition metal-based MXenes have been synthesized to date. This 2D subfamily exhibits diversity in structure and composition. MXene is enormously famous due to its performance as an energy storage device. The high electrical conductivity, hydrophilicity, surface redox activity, and mechanical stiffness make it a potential alternative to Graphene as an electrode in energy storage devices like batteries and supercapacitors. Experimental evidence suggests that diversity in structure, composition, and surface passivations affect the supercapacitive property of MXenes. However, there are a few scattered attempts to explore these aspects for varieties of systems to understand the mechanisms of charge storage in MXene-based supercapacitors. This thesis provides a systematic study on understanding structural and compositional effects on the electrochemical performances of MXene-based supercapacitors. Our investigations start with exploring the capacities of M 2 C and M 3 C 2 MXenes as supercapacitor electrodes. We consciously choose various 3d and 4d transition metals as M elements. We show the significance of quantum capacitance on energy storage performance. We also explain the effect of surface passivation on MXene capacitances. Further, we provide a comparative study of substitution and doping in enhancing the storage capacity of MXenes. An explanation of the influence of doping sites on the redox capacitance of Ti 3 C 2 is given. Next, we attempt the route of surface engineering to improve the energy storage capacities of MXenes. To this end, we construct Janus MM’C MXene and study their electrochemical performances. In the course of this study, we find that much superior capacities are obtained if one of the components of Janus is a magnetic element. We extend the study by considering solid solutions of one of these systems and investigating the effect of chemical and magnetic disorders on its supercapacitive performance.
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    Study of Negative Magnetization and Tunable Exchange Bias Behavior in Nanostructured Double Perovskites
    (2024) Patra, K Pushpanjali
    The recent scientific focus on R2CoMnO6 (R is trivalent rare earth element) stems from its intriguingly complex magnetic behavior, which has opened up a diverse range of potential applications. Such as metamagnetic behavior, magneto-electric coupling, multiferroic behavior, spin-phonon coupling, magnetization reversal (MR) and tunable exchange bias behavior, magnetocaloric effect, low-temperature magnetic frustration and large magneto-resistance like properties. Few research groups have been started working on Ho2CoMnO6, and reported a FM TC around 77 K, with a large magnetic entropy (Sm) of value ~ 12 J/kg.K at a 7 T field. This feature makes this material applicable for magnetic refrigeration. R2FeCrO6 are also an important family of magnetic double perovskites, while Fe-Cr based perovskites have been extensively studied, there has been relatively limited exploration of Fe-Cr based double perovskites. Hence, our current research is centred on exploring the magnetic DP materials based on Co-Mn (Ho2CoMnO6) and Fe-Cr (Y2FeCrO6), with the primary objective of tuning and comprehending their magnetic properties in this direction. To the best of our understanding, there hasn't been much study done on nanostructured Ho2CoMnO6 and Y2FeCrO6 DPs, despite their high interest. In this work we have synthesizing nanostructured Ho2CoMnO6 and Y2FeCrO6 DPs and emphasis on the investigation of their structural and magnetic characteristics.
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    Properties of Band-Engineered Topological Systems in Two Dimensions
    (2023) Mondal, Sayan
    We study the evolution of the topological properties of Chern insulators subjected to band engineering for a variety of systems, such as, the honeycomb lattice, dice lattice, bilayer honeycomb lattice etc. Since topology is inextricably related to band properties, engineering band deformities results in the evolution of the topological invariants and may even induce phase transitions. In particular, a honeycomb lattice under deformation demonstrates vanishing of the Dirac electrons, and eventually yields a scenario where the electronic dispersion is linear in one direction and quadratic along the other one. This is known as the semi-Dirac dispersion. A variety of materials and cold atomic systems, demonstrate such a dispersion. Further, the inclusion of the Haldane flux breaks the time-reversal symmetry (TRS) and creates an energy gap in the spectrum which makes the system a topological (Chern) insulator. The topological gap vanishes in the semi-Dirac limit, which, however reopens upon further deformation. The nature of the gaps prior to, and beyond the semi-Dirac limit have distinct features (TRS remains broken all the while), and have been elaborately studied in the thesis. Going a step ahead of the traditional Haldane model, we have considered a third neighbour hopping, in presence of which the system exhibits higher Chern number C, such as, C=±2, along with C=±1. Further, a bilayer Haldane system with Bernal stacking exhibits differential behaviour of the bands that are closer to the Fermi level than the ones further away from it. Multiple topological phase transitions are realized for such a bilayer model. Moreover, a dice lattice, which not only is an interesting extension of the honeycomb structure of graphene, it also hosts a flat band that is in general relevant for studying strong electronic correlations. As in the earlier case, the system possesses topological regions with higher Chern numbers, however, it shows topological phase transitions straight from C=±2 phases to a C=0 phase (trivial insulator). In all of these cases, we have depicted the phase diagrams to support the topological phase transition occurring therein via the presence or the absence of edge currents in semi-infinite ribbon geometries, and evolution of the plateau in the anomalous Hall conductivity in presence of band engineering. The corresponding scenario in a quantum spin Hall insulator described by a Kane-Mele model has been explored and the spin resolved bands respond in no different manner to the band deformation as shown via computing the Z2 invariant. The evolution of the spin Hall response shows vanishing of the quantum spin hall phase in the semi-Dirac limit.
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    Study on CVD Growth of non-van der Waals 2D Bi2O2Se and its Hybrid Integration for Optoelectronic Applications
    (2023) Hossain, Md Tarik
    This thesis focuses on investigating the CVD growth of 2D non-van der Waals Bi2O2Se semiconductors and their structural, optical, electrical, and thermal properties, including photodetector applications. The thermal conductivity of CVD-grown ultrathin Bi2O2Se layers is calculated through an optothermal Raman measurement technique. The optical properties of 2D Bi2O2Se are thoroughly studied on various growth substrates. We discovered room-temperature exciton formation resulting in broadband absorption and photoluminescence in ultrathin Bi2O2Se established through spectroscopic studies and theoretical DFT calculations. We prepared heterostructures of Bi2O2Se with perovskite (CsPbBr3) nanocrystals as well as with 2D van der Waals type semiconductor MoS2 and investigated the effect of charge transfer on the luminescence and photo-conducting properties of the heterostructure. A photodetector is also fabricated based on the heterojunction, and the hybrid photodetector show superior photo-responsive properties compared to the bare Bi2O2Se-based devices. Free-standing ultrathin nanosheets of Bi2O2Se are chemically synthesized for photoconductivity study that discovers defect-induced negative persistent photoconductivity in highly defective Bi2O2Se that can convert into positive photoconductivity through vacuum annealing. These results are important for developing non-van der Waals heterostructures for ensuing applications
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    Production of Dark Matter and Baryon Asymmetry in Non-standard Cosmologies
    (2023) Das, Suruj Jyoti
    This thesis is devoted to the study of two long-standing problems of particle physics and cosmology: the origin of dark matter and the baryon asymmetry of our Universe, in the presence of a non-standard cosmological history in the first few seconds of the evolution of our Universe. We focus especially on early matter-dominated eras, considering two possible origins of them, one due to a long-lived particle (LLP), and the other from primordial black holes (PBH). Apart from investigating the standard dark matter (DM) and baryon asymmetry production mechanisms in the presence of these non-standard epochs, the thesis mostly touches upon dark matter scenarios beyond the conventional WIMP paradigm which has been searched for several years now in dark matter direct detection experiments like XENON, LUX etc. and also in collider search experiments like the large hadron collider (LHC), with no positive results so far. While the DM candidates we study are unlikely to show up in these conventional DM search experiments, we propose an alternative and novel probe to look for these DM candidates, which is through stochastic gravitational waves generated in the early Universe. The shape of such gravitational wave spectrum is determined by the non-standard cosmological background which in turn dictates the dark matter phenomenology. The setups we consider can also generate the baryon asymmetry of the Universe through leptogenesis (baryogenesis), which occurs at a very high scale that is out of direct reach from any current experiments, but can be probed indirectly through gravitational waves. The amplitudes and frequencies of these gravitational wave spectra are within reach of near-future gravitational wave detectors such as LISA, DECIGO, CE etc. In addition, the particle physics setups we have considered also have detection prospects on their own, which get modified in the presence of a non-standard cosmological epoch.
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    Thermodynamic and fluid interpretations of gravitational field equations: General relativity and beyond
    (2023) Dey, Sumit
    The intriguing connections between gravity and thermodynamics have been a long standing subject of study. The conventional laws of black hole mechanics have often provided deep insights into the nature of gravity. In this thesis, we explore the picture of emergence of the gravitational field equations from a classical stand-point and test its validity to theories beyond general relativity. In this thesis, we develop in detail the geometrical construction of a general integrable null hypersurface in the Riemann-Cartan spacetime. The Riemann-Cartan spacetime is a generalization to the usual (pseudo)-Riemannian spacetime (equipped with the Levi-Civita connection) in the sense of allowing non-trivial torsion in it. We develop in detail the evolution equations of certain geometric data established on the null surface. In the thesis, we try to interpret the physical nature of the gravitational field equations on the null surface in the light of the evolution equations constructed. Our first study is the general case of gravitational theories described on spacetimes equipped with the Levi-Civita connection. We show in a covariant fashion that the field equations on the null surface under the process of virtual displacement take up a thermodynamic structure without taking recourse to any explicit coordinate system adapted to the null surface. Next, we take the specific case of scalar-tensor theory and show such a thermodynamic interpretation of the field equations allow us to shed some light on the issue of the physical (in)equivalences between the Einstein and Jordan frame. We also provide a proof of the zeroth law for Killing horizons in the scalar-tensor theory. Next, we take the explicit case of Einstein-Cartan gravity and show similar thermodynamic interpretation exists for the gravitational field equations on the null surface. We also study the dynamics of a geometrical data called the Hajicek 1-form on the null surface in Einstein-Cartan gravity and show that under suitable conditions, it looks like a Cosserat fluid. This strengthens the analogy of the horizon or null surface dynamics to that of a viscous fluid flow, for theories even beyond general relativity. Finally, we conclude the thesis with a brief discussion of the conclusions and potential future directions.
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    Excitation of Toroidal Resonances in Meta-surfaces and their Applications at Terahertz Frequencies
    (2023) Bhattacharya, Angana
    he terahertz (THz) range of frequencies has become an important field of research in recent times, with the advent of electromagnetic metasurface technologies. This thesis focuses on the study of the excotic toroidal excitations in metasurfaces and their applications in terahertz frequencies. The toroidal dipole excitations, dominated over by electric and magnetic dipole excitations in natural materials, can be examined and utilzed in carefully designed metasurfaces. This thesis examines, both numerically and experimentally, the excitation of toroidal resonances in metasurfaces, their modulation and electromagnetically induced transparency effects in a toroidal metasurfaces. Further, the thesis discusses the applications of toroidal metasurfaces for broadband terahertz polarization conversion and in exciting polarization independent resonances via a lattice-coupled toroidal mode. In this thesis, the toroidal excitation has been discussed in carefully designed metamaterials with special toroidal symmetries in the terahertz range. The bilayer near-field oupling between two toroidal resonators was analysed and the passive modulation of he dual toroidal resonance has been discussed. In an effort to explore the possibility of active modulation in terahertz metasurfaces, the active tuning of toroidal resonances in a graphene based metasurface has been studied in this thesis. Further, several concepts and applications for the study of toroidal resonances in metasurfaces were examined. A study has been made on the excitation of single and dual-band electromagnetically induced transparency (EIT) via near-field coupled toroidal metasurfaces in this thesis. Such toroidal dual-band EIT could be impactful in the study of slow light systems. The thesis also examines the possibility of terahertz polarization conversion using toroidal excitations in a metasurface. Through the rotation of the meta-atom, nearly 40% cross-polarization conversion was achieved for a 45 degree rotation angle of the meta-atom. The thesis has also examined the possibility of further enchancement of the quality factor of a toroidal resonance by exploring the effect of coupling the toroidal excitation to the first-order lattice mode of the metasurface. The coupling between a toroidal mode and a first-order lattice mode resulted in the enhancement of quality factor in a simple metasurface geometry. The designed metasurface ensures polarization independence, such that the sharp toroidal mode is excited for both the orthogonal polarizations of incident THz radiation. The toroidal excitation and its applications, as discussed in this thesis, can have immense significance in high speed terahertz components for low-loss communication devices.
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    Develppment of Hydroxyapatite Based Ceramic Composites and thin Films for Biomedical Applications
    (2022) Das, Apurba
    Hydroxyapatite (Ca10(PO4)6(OH)2, HAP) is an immensely favored biomaterial in the biomedical industry, known for its extensive applications in orthopedics and dentistry. Naturally, it appears as an inorganic constituent of natural bones, with substitutions like Mg, Sr, and other trace ions occupying the Ca sites. To distinguish it from HAP, it is often designated as bone-like apatite or bone apatite in brief. Consequently, it can be perceived that the Ca=P ratio of bone apatites di ers from HAP (1.67 for HAP, whereas a ratio < 1.67 is obtained for bone apatite). The last two decades have seen an exponential rise in the biomedical industry related to HAP primarily due to its incredible biocompatibility, a nity to bond to living bones when used as implants, and its ability to promote ingrowth of new bones through osteoconduction without having any toxic or in ammatory response to the surrounding tissues. As a testimony to the claim, the recent reports published in Yahoo nance claimed that the HAP market is projected to grow from USD 2.2 billion (currently in 2020) to USD 3.1 billion in 2025 at a compound annual growth rate (CAGR) of 6.8%. It has been forecasted that the Asia-Paci c HAP market will grow at the highest CAGR precisely due to the increase in medical tourism in countries like India, South Korea, and Japan. It has led to the rapid development of the domestic health care sector and is an indicator for exhaustive research in HAP to develop devices and technologies to cater to growing medical tourism.
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    Tunable Ordered Assembly of MXene and Related Nanomaterials for Scalable Energy Applications
    (2023) Dutta, Pronoy
    Development of functional macrostructures form two-dimensional materials while addressing the key fundamental challenges in supercapacitive energy storage systems remains a monumental task in the field of energy storage. Meanwhile MXene, a new family of two-dimensional materials has emerged as a highly attractive material in supercapacitive applications due to their high conductivity, hydrophilicity and pseudocapacitive nature. This thesis aims towards development of functional macrostructures from MXene and related nanomaterials in the form of hydrogels while ensuring scalability and low-cost of synthesis. MXene hydrogels are an interconnected network of these two-dimensional materials which offers highly restacking controlled porous structure that enables facile accessibility of the bulk of the electrodes to the electrolyte ions. Such restacking controlled assembly enables high utilization of active mass ensuring high specific capacity and rate performance. This thesis introduces a room-temperature self-assembling strategy with the help of a small amount of graphene to induce gelation in a system of MXene and graphene for hybrid hydrogel development. It is shown that such room temperature induced assembly not only protects the intrinsic properties of MXene by preventing synthesis induced oxidation, but also enables state-of the-art performance even in commercial scale mass loading electrodes. Furthermore, development of hydrogels of pristine MXene has been a great challenge due to the relative small sheet size and intrinsic stiffness of MXene. Here, in this thesis, for the first time a critical-density induced gelation strategy is introduced which enables the development of self-supporting pristine MXene hydrogels. It is established that liquid crystallinity induced ordering in MXene dispersion can lead to higher crosslinking of MXenes into the hydrogels which leads to better stability in such systems. The as developed hydrogels were used as supercapacitive electrodes having mass loading as high as ~ 15 mg cm-2, which simultaneously deliver excellent gravimetric capacity of 337 F g-1 and a very high areal capacitance of 5042 mF cm-2. Further, in order to mitigate the high concentration requirement with critical-density controlled self-assembly, an electric-field guided forced assembly of MXene is also introduced which enables large scale assembly of MXene into hydrogels in seconds and also allows facile controllability over the MXene sheet orientation in 2D sheet like and 3D hydrogel monoliths. Such orientational controllability of MXene sheets in hydrogel enables their use in a plethora of applications which is not otherwise easily achievable with conventional assembling strategies. The electric-field guided assembly is also demonstrated to enable co-assembly of nanomaterials with MXene. As an example, co-assembly of cellulose nanofiber (CNF) and MXene is performed to develop flexible electrodes that are used in asymmetric devices as well as wearables. The MXene-CNF hybrids were paired with a reduced graphene-carbon nanotube-polyaniline electrode to develop asymmetric supercapacitor which delivers a high energy density of 23 Wh kg-1 at a power density of 501 W kg-1. Furthermore, the possibility of developing wearable devices with the help of electric-field guided assembly is also demonstrated which shows excellent stability after 5000 bending cycles at extreme 90º and 180º and retains over 80% of their initial capacity after the bending tests. The high scalability, low-cost and a highly optimized electrode structure in all these works makes the MXene based electrodes, as developed in this thesis, ready for practical adoptability and commercialization.
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    Search for the decay B_s^0 → π^0 π^0 at high luminosity electron-positron collider experiment
    (2023) Borah, Jyotimoi
    We report the results of the first search for the decay B_s^0 → π^0 π^0 using 121.4 fb^{-1} of data collected at the Υ(5S) resonance with the Belle detector at the KEKB asymmetric-energy electron-positron collider. We observe no signal and set a 90% confidence level upper limit of 7.7×10^{-6} on the B_s^0 → π^0 π^0 decay branching fraction. Our result constitutes the first stringent upper limit set by the Belle experiment for this decay channel.
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    Some Studies of the Semileptonic B-meson Decays via the Neutral and Charged Current Exchanges
    (2023) Ray, Ipsita
    This thesis focuses on the study of the semileptonic B meson decays via the neutral and charged current exchanges. The charged current transitions occur at tree level in the Standard Model (SM) and the branching fractions of the semileptonic decays of B mesons to lighter leptons (electron and muon) in the final state as considered in this thesis have been determined to be mostly SM-like. In such cases, the charged current transitions provide an important avenue for the clean extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements which is very crucial for understanding the CKM structure of the Standard Model and for precise theoretical predictions of several observables. In this thesis, we have studied the extraction of the CKM elements |Vub|, |Vcb|, and the ratio |Vub|/|Vcb| which are relatively less precisely known.
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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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    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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    Phases and Critical Analysis of Quantum Systems in presence of Quasiperiodic Potential
    (2023) Roy, Shilpi
    Disorder is universal in quantum systems. It has an enormous effect on electronic motion in solids. Consequently, the system undergoes a localization transition from a metal to an insulator under the drive of random disorder strength in three dimensions. On the contrary, another type of deterministic potential, namely, the quasiperiodic (QP) potential allows exhibiting of the localization transition in one-dimensional systems. In this thesis, our primary motivation is to study the localization properties corresponding to a dimer model in presence of the QP potential. More specifically, the dimer model has been chosen for the purpose of breaking the self-dual symmetry, thereby aiming to introduce mobility edge (ME) in the one-dimensional system. The combination of these two, such as the dimerization and the QP potential, reveals a noteworthy observation. In particular, the results which are analyzed via computing the participation ratio, eigenenergies, density of states and finite-size scaling analysis etc., imply a reentrant localization transition that transcends the existing conventional theory in literature. Further, we analyzed the critical behavior corresponding to this reentrant localization transition using a multifractal analysis, followed by a critical state analysis, and via computing the Hausdorff dimension. Moreover, this dimer model, aided by $p$-wave superconducting pairing in presence of QP potential, preserves topological properties, which was missing in the purely dimerized model with QP potential. Thus by exploring the localization and the topological properties, we have reported significant results on a series of phase transitions occurring in the system. While studying the localization properties via participation ratio, fractal dimension and level spacing, we inferred two different types of anomalous MEs in addition to realizing a multifractal phase. Further, by examining the topological properties via calculating the real-space winding number and the number of Majorana zero modes, we observe several phase transitions from topologically trivial to topologically Anderson to Anderson localized phase. In the end, in addition to the non-interacting one-dimensional quantum systems, we have also studied a two-dimensional interacting system in presence of a QP potential. Using the site-decoupled mean-field approximation, percolation analysis and the finite-size scaling analysis, we have explored the phase diagram, and subsequently investigated the critical properties of the various phase transitions occurring in the system.
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    Development of lithium ferrite-based ceramics for microwave applications
    (2023) Mohapatra, Prajna Paramita
    Spinel ferrites have gained attention for a long period of time due to their unique electrical, optical, and magnetic properties. They are also very promising for applications such as circulators, phase shifters, memory, magnetic recording devices, gas sensing, etc. The present thesis is focused on the synthesis of lithium ferrite-based ceramics and thin films. Lithium ferrite exhibits high Curie temperature, square hysteresis loop, high saturation magnetization, excellent dielectric properties, high resistivity, etc. The solid-state reaction method is used to prepare substituted lithium ferrites and composites. The effect of alkaline earth elements such as Sr and Mg on the structural, microstructural, dielectric (1 MHz – 1 GHz), and magnetic response is analyzed. Enhanced dielectric response (εr = 3034, tanδ = 0.001 at RT, 1 MHz) is observed for the Mg composition, x = 0.005, whereas in the case of Sr series, the best dielectric response is observed for x = 0.003 (εr = 5986 and tanδ = 1.17 at RT, 1 MHz). The obtained EA for LMFO and LSFO is 1.39 – 0.35 eV and 0.124 – 0.077 eV, respectively. The LMFO with x = 0.007 exhibited the best permeability (μr = 29) and magnetic properties (Ms = 55 emu/g) at room temperature. Also, in the LSFO series, x = 0.007 showed the highest magnetization among all samples (MS = 61 emu/g). Improved dielectric response with low magnetic as well as dielectric loss is observed for Mg substituted lithium ferrite as compared to Sr. The combined magnetic, dielectric, and permeability response made the Mg substituted lithium ferrite more suitable for circulators and phase shifters. Again, the lithium ferrite/carbon black and Dy substituted lithium ferrite/carbon black composites are prepared, and EMI shielding effectiveness is analyzed in the X (8.2 – 12.4 GHz) and Ku (12.4 – 18 GHz) frequency bands. Permittivity and permeability are also analyzed. Shielding effectiveness is enhanced with the carbon black as well as Dy content. The maximum shielding effectiveness of 24 dB was obtained for LD10FO/CB ~ 17 ‒ 18 GHz. The Dy substitution enhances the magnetic as well as dielectric loss. Further, 99.68 % of Aeff is achieved with 20 wt % of CB reinforcement in LFO, whereas the maximum absorption efficiency of 99.6 % is obtained for LD10FO/CB ~ 17 ‒ 18 GHz.This renders majorly absorption-based shielding rather than reflection-based shielding. The enhancement in the shielding efficiency is attributed to the synergetic effect of the dielectric loss and magnetic loss. Various contributions of magnetic loss, such as natural resonance, domain wall resonance, eddy current loss, hysteresis loss, and spin polarization, are discussed. Further, lithium ferrite and Dy substituted lithium ferrite in the form of thin films are synthesized by PLD having different film thicknesses. The strain-induced structural, microstructural, magnetic, dielectric, and electrical response is analyzed. The magnetization is reduced with the enhancement in film thickness which is explained on the basis of magnetoelastic energy density. The dielectric constant is enhanced, whereas the dielectric loss is reduced with the enhancement in the film thickness. The electrical conduction mechanism is also analyzed, which is in good agreement with Mott’s VRH mechanism. The varying thickness of a film is an effective parameter for tuning the physical properties of the film. The observed results suggest that LDFO films are promising for magnetic oxide semiconductor applications.
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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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    Development of Electronically Tuned Nanomaterials for Electrocatalysis
    (2023) Majumdar, Abhisek
    A green energy-dependent sustainable future can be promised by converting and storing renewable energies in terms of chemical fuels like hydrogen through electrochemical water splitting. However, the requirements of high overpotential to overcome the energy barriers of both hydrogen and oxygen evolution reactions (HER & OER) restrict the overall efficiency of hydrogen generation by electrocatalysis of water. Noble metal based electrocatalysts (platinum, iridium, ruthenium) have been believed as ideal electrocatalysts due to their high activity, selectivity and optimal adsorption ability for HER, OER reaction intermediates. However, their high cost and scarcity compelled scientists to search for new, cost-effective and simple strategies for the development of efficient electrocatalysts. In this regard, rational design of hetero structures and anchoring single-atom catalysts (SAC) on adequate support are the two successful strategies to lower the overpotentials for HER and OER processes. Though substantial work has been presented in the literature based on efficient heterostructure and SAC development, the used conventional methods are extremely time-consuming, energy inefficient and complex. In addition, high quality atomistic interfacing in heterostructure development is difficult to realize due to the multi-step process requirement of the conventional strategies. Stabilization of SACs over a proper support is also very challenging yet important to synergistically enhance catalytic activity of the system specially in the dynamic OER environment where usually reconstruction of the catalyst happens. These challenges drastically reduce the efficiency of the catalysts and increase the required overpotential for HER and OER and thus realization of catalysts with high current density for practical application become very difficult. Therefore, for practical adaptation of these catalysts we need to focus not only on the optimization of performances but also in new technologies to do the processing of the catalyst at low cost. The current thesis thoroughly addresses the described challenges by inventing radically new processes for the development of heterostructures and SACs and by providing theoretical understanding of synergistic electronic coupling for the enhancement of catalytic activity. For instance, atomic interfacing between molybdenum selenide (MoSe2) and nickel cobalt selenide (NiCo2Se4) has been achieved by selenization induced dealloying process. This results in vertical orientation of inter-spaced MoSe2 on conducting NiCo2Se4 support which drastically enhances the HER catalytic activity due to its unique structural configuration and synergistic heterostructure formation as confirmed from density functional theory (DFT) thus requires only overpotential of 89 mV to get a current density 10 mA cm-2, and a Tafel slope of 65 mV dec-1. Further, interfacing between crystalline and amorphous structure has been presented here to achieve an advanced crystalline-amorphous core-shell hetero structure of amorphous molybdenum sulfide (a-MoSx) and crystalline molybdenum tungsten oxide (MoWO) via microwave induced rapid surface amorphization process. From the DFT analysis we found that the core not only provides sufficient conductivity and increases the HER activity of the active site of amorphous a-MoSx but also substantially increases the number of HER active sites. This results in excellent catalytic activity for HER and exhibits an overpotential of 136 mV at 10 mA cm-2 in the acid electrolyte, which is much lower than the overpotential of parent oxide (356 mV) and its fully sulfurized crystalline counterpart (163 mV), and the same catalyst can be extended to operate in various pH conditions as well. In addition, we have realized a rapid and energy efficient recrystallization strategy based on microwave irradiation for the universal development of nickel-iron based chalcogenide and phosphides. This strategy results in biphasic structure of iron doped Ni3S2/NiS which shows exceptionally high OER activity requiring only 187 mV for 10 mA cm-2 and commercial level current density of 500 mA cm-2 at 289 mV. The comprehensive analysis indicates the phase evolution of NiS to amorphous Ni-(oxy)hydroxide during OER process to generate iron doped Ni3S2/NiOOH heterostructure is the reason for its high activity. Furthermore, single atom iridium has been photochemically decorated on the surface of MoSe2@NiCo2Se4 heterostructure which on electrochemical surface reconstruction displays outstanding OER activity, requiring only 200 mV overpotentials for 10 mA cm-2. A series of post-OER characterizations have been done to understand how iridium single atoms stabilize over the surface of base material and the structure realized from these findings has been used in DFT to understand the origin of high activity of this catalyst. We believe, this present thesis work will provide a new direction for the development of electrocatalysts via new efficient strategies and lay the platform for the development of highly active practical electrocatalysts not only in water electrolysis application but also in diverse fields like sea water splitting, fuel cell and metal-air batteries.
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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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    Efimov Universality in Exotic Strange and Charm Nuclei: A Low-energy Effective Theory Framework
    (2023) Meher, Ghanashyam
    The present thesis deals with the investigation of low-energy two- and three-body universality that could manifest in exotic strange and charm nuclei. To supplement the plethora of existing works based on potential models on such systems, the main objective of this thesis is to employ a model-independent effective field theory (EFT) framework as a modern systematic computational tool for understanding the underlying binding mechanism without reference to inherent (microscopic) short-distance details. In particular, pionless EFT or its variant, so-called the Halo/Cluster EFT, provides a versatile theoretical technique to specifically search for the feasibility of Efimov mechanism in halolike nuclear clusters. Here we presented leading order EFT investigations of the putative S-wave bound hypernuclear cluster states, such as the iso-doublet mirror partners (Ʌ 5 ɅH, Ʌ 5 ɅHe) in the (J=1/2, T=1/2) channel, as well as the Ξ- nn cluster in the (J=1/2, T=3/2) channel, in the strange sector. The mirror clusters are studied as 2Ʌ (double-Ʌhyperon) halo systems with a composite core, identified either as a triton (t) or helion (h). Whereas, the Ξ-nn system is studied as a 2n-halo system with a Ξ-hyperon elementary core. Furthermore, in the charm sector, we studied the putative 2n halo-bound D0nn system in the (J=0, T=3/2) channel invoking an idealized zero-couplinglimit ansatz which excludes all effects of decay and coupled channels dynamics. The general EFT formalism involves the diagrammatic construction of a system of Faddeev-like three-body integral equations embodying the re-scattering dynamics in the momentum-space representation. Using momentum cut-off regulators in the integral equations which are significantly larger than the hard scale of the EFTs, the three-body contact interaction becomes cyclically singular indicating the onset of renormalization group (RG) limit cycles with discrete scale invariance. Thus, our results formally indicate the manifestly Efimovian nature of each of the cluster systems leading to ostensible Efimov states. However, the paucity of current empirical information to determine various free EFT parameters precludes definitive conclusions on the feasibility of such systems being realistically Efimov-bound. Nevertheless, despite phenomenological limitations, the thesis amply demonstrates the predictability of the EFT analyses by illuminating various remnant features of Efimov universality at a qualitative level. Constraining the cut-off dependence of doubleɅ separation energy and the corresponding three-body scattering lengths of the (Ʌ 5 ɅH, Ʌ 5 ɅHe) mirrors, predicting the Phillips-line correlation curves for the Ʌ 5 ɅH, Ʌ 5 ɅHe and Ξ-nn systems, and finally, demonstrating the structural universality of the ground state of a plausible D0nn halo-bound cluster by determining its geometrical features (e.g., matter density form factors, mean square radii, etc.), were some of the predictable features emphasized in this thesis.