PhD Theses (Physics)

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Magnetic Field Induced Transitions and Exchange Interactions in Columbites
(2023) Maruthi, R
The studies on strongly correlated aspects of the Columbite family of compounds have become more intense in recent years because of their novel magnetic and electronic properties which provide impetus to the scientific community in searching for practical systems in the field of ‘Quantum Magnets’ that can be used to test the predictions of theoretically solvable models. Columbites are generally complex transition metal oxides with the chemical formula AB2O6, where both A and B site atoms are transition metal cations with divalent and pentavalent oxidation states, respectively. These systems are unique in the sense that they exhibit quantum critical behavior, magneto-electric coupling, tri-critical behavior, field-induced metamagnetic transitions, etc. The exotic physical properties of Columbites have been utilized in various industrial applications and they have huge commercial demand. In this thesis, we tried to establish the complete H-T phase diagram and understand the magnetic ground spin configuration of magnetic ions in the Columbite family of compounds. Nonetheless, we also focused on the determination of exchange constants between the magnetic cations by using different experimental results and theoretical models which is the strongest point of the current thesis. Also, the research work related to the dielectric response of columbites, mainly the analysis pertaining to the temperature and frequency dependence of ac-conductivity by using different theoretical models is unique. Magnetic measurements on MnNb2O6 reveal the robust anti-ferromagnetic ordering below TN = 4.33 K which is further confirmed by heat capacity measurements (TN = 4.36 K). The high-temperature paramagnetic susceptibility is fitted with modified Curie-Weiss law (χ = χ0 + C/(T-ϴ)) which yields ϴ = -17 K and C = 4.38 emu K mol-1Oe-1. Using these magnitudes, we further estimated the magnitude of effective magnetic moment μ which is ~ 5.920 μB per Mn2+ ion in MnNb2O6 system, and the corresponding g-factor 2.001 for Mn2+. Moreover, this compound shows magnetic field-induced spin-flop transition at Hsf = 18 kOe. We provide a clear and vivid picture of the H-T phase diagram of the MnNb2O6 system which shows the triple point at TTP (H, T) = (18 kOe, 4.06 K). Next, we employed the molecular field theory (MFT) and estimated the intrachain and interchain exchange constants whose magnitudes turn out to be J0/kB = -1.08 K and J┴/kB = -0.61 K, respectively. Furthermore, we presented the ac-conductivity (σ(ω,T)) analysis exhibiting the thermally driven, Arrhenius-like behavior which is predominant at temperatures above 300 K. However the Double power law-based explanation of the dispersive behavior of electrical conductivity σ(ω,T) studies provide evidence for the correlated-barrier hopping (CBH) conduction mechanism of charge carriers for temperatures between 173 K and 473 K. Moreover, the dynamical response of complex electric modulus spectra (M*(ω,T)) and the corresponding analysis using the Kohlrausch-Williams-Watts method shows the non-Debye type relaxation process is prevalent in the MnNb2O6 system with decay function exponent β lying between 0.794 and 0.840. Besides we presented the magnetic properties of tantalite Columbite MnTa2O6 which provide evidence of the AFM ordering with Néel temperature TN = 5.97 K consistent with the TN = 6.00 K determined from the peak in the Cp vs. T data. Further we estimated the critical exponents α = 0.10(0.13) for T > TN (T < TN) from experimental data of Cp vs. T near TN through the mathematical fits to the equation: Cp = A|T-TN|-α. Magnetic studies reveals μeff = 5.96 μB per Mn2+ ion and yields the effective spin S = 5/2 with g = 2.015. Finally, we mapped the H-T phase diagram using the M-H isotherms and M-T data measured at different H yielding the tricritical point TTP (H, T) = (17.0 kOe, 5.69 K) for MnTa2O6. Using the magnitudes of ϴ and TN and molecular field theory, the antiferromagnetic exchange constants J0/kB = -1.5 ± 0.2 K and J┴/kB = -0.85 ± 0.05 K are determined for the MnTa2O6 system. Further, we explored the magnetic ground state properties of CoNb2O6 which shows that the ground state of Co2+ has the effective spin S = 1/2 and not S = 3/2 expected from Hund’s rules, the S = 1/2 ground state resulting from the combined effects of non-cubic crystalline field and spin-orbit coupling. On the other hand, by means of the experimentally obtained g value with S =1/2 and the experimental critical fields for spin flips we calculated the interchain antiferromagnetic exchange constants J1/kB (= -0.42 K) and J2/kB (= -0.67K) along with intrachain ferromagnetic exchange constant Jo/kB = 6.2 K. Next, we further explored the rich magnetic properties of the NiNb2O6 system.
Geometrical Frustration in Jahn-Teller Active Spinel Pyrochlores
(2023) Jena, Suchit Kumar
Geometrical frustration (GF) in magnetic materials has attracted researchers of current era owing to their unusual physical properties, in which the origin of frustration is two-fold. The first and foremost requirement is the peculiar arrangement of the crystal lattice, while the second most important requirement is the nature of the magnetic ions occupying the specific lattice sites. Such GF phenomena is inherent in the specific lattice having corner-shared tetrahedral geometry, commonly known as the ‘Pyrochlore lattice’, where the fragile magnetic ground state leads to novel physical phenomena such as: Reentrant spin-glass state, Quantum spin-liquid/ice nature, Bipolar exchange-bias, and giant magneto-caloric-effect. In this context, spinel oxides (AB2O4) are considered to be the well-known systems which are prone to exhibit unusual magnetic properties because of their special features like competing exchange interactions (JAB, JAA and JBB) and the topology of B sublattice. However, in the spinel-Pyrochlores only the B sublattice formulates a pyrochlore arrangement whereas A forms a diamond lattice. Therefore, tuning the magnetic interaction on the B site have shown much higher degrees of magnetic frustration. The GF phenomena in the spinel-Pyrochlore ZnFe2O4 (ZFO) was first predicted by Anderson in 1956, which was then experimentally unveiled to exhibit magnitude of frustration index f = |ΘCW|/TN as high as 12 (where the antiferromagnetic Néel temperature TN ≃ 10 K). In this research work, we attempt to lift the GF in cubic ZFO by compelling it to stabilize in the lower crystal structural symmetry (tetragonal) with incorporation of the Jahn-Teller (JT) active spin‒1 Mn3+ on B site and additional dilution effect from another JT active spin‒1/2 Cu2+. Here the divalent Cu is capable of occupying both A and B sites and is expected to alter the exchange coupling significantly. The role of weakly magnetic Ru3+ substitution on the magnetic exchange interactions of ZFO has also been investigated. A systematic comparison of the change in magnetic ordering with the substitution of Ru3+ and Cu2+ are thoroughly studied in terms of the associated exchange interactions (J) along with the magnetic Field-Temperature (H‒T) phase diagrams. Alongside, this work aims to probe the variations occurring in JAA, JAB and JBB with increasing the Cu content in ZFO. Such Cu substitution in ZFO introduces complexity in the cationic distribution which leads to very high ferrimagnetic (FiM) ordering TFiM ~ 743 K in the tetragonal CuFe2O4. Further, a comprehensive study of the magnetic and dielectric properties of the investigated systems are discussed considering their importance in the field of microwave and spintronic devices.
Laser Cooling and Trapping of Rubidium using a Narrow Transition
(2024) Das, Rajnandan Choudhury
Laser cooling and trapping serve as a crucial gateway, offering insights into fundamental physics and opening the way for diverse quantum technologies. Among the various elements, Rubidium (Rb) is one of the most extensively studied elements in atomic physics. The cold atom community has mainly used Infrared lasers to cool and trap Rb atoms in a Magneto-optical trap (MOT), usually through the 5S1/2 → 5P3/2 transition at 780 nm. In this thesis, we explore the laser cooling and trapping of Rb atoms in MOT using the 5S1/2 → 6P3/2 narrow-line transition at 420 nm (blue MOT). Despite its large branching ratio, we observe efficient cooling with the 420 nm transition, achieving around 108 atoms in the blue MOT at a typical temperature of 54 μK. We also present a method for the continuous loading of Rb atoms in the blue MOT and a theoretical framework for cooling atoms with two simultaneous transitions. We also describe the direct spectroscopy of Rb at 420 nm, which is challenging due to its weak transition strength. Furthermore, we numerically analyze the role of spontaneously generated coherence (SGC) in polarization gradient cooling with F = 1 → 2 transition and investigate the feasibility of blue-detuned cooling at this transition in the absence of SGC. We experimentally demonstrate blue detuned cooling in type-I and type-II MOT. Additionally, we study various configurations of red-detuned as well as blue-detuned blue MOT, achieving temperatures as low as 24 μK in D1 MOT and 31 μK in D2 MOT. Our studies may find applications in quantum technologies based on the narrow-line cooling transitions
Search for the Decay B0s → J/ψπ0 at Belle Experiment
(2024) Kumar, Devender
We report the first search for the decay B0s → J/ψπ0 using 121.4 fb−1 of data collected at Υ(5S) resonance state by the Belle detector at the KEKB asymmetric energy e+e− collider located at the High Energy Accelerator Research Organisation, Japan. We observe no signal and report an upper limit on the branching fraction, B(B0 s → J/ψπ0) of 1.21 × 10−5 at 90% confidence level. This result is the most stringent, improving the previous bound by two orders of magnitude.
Ultra High Energy Neutrinos and Gamma Rays in Astrophysics: A Multi-messenger Study
(2024) Sarmah, Prantik
The advancement of diverse high-energy neutrino and gamma-ray observatories has recently introduced a fresh perspective in multi-messenger astrophysics, enabling us to delve into the high-energy realms of our universe. This thesis embarks on a comprehensive examination of ultra-high energy (UHE) neutrinos and gamma-rays emitted by a variety of astrophysical sources, encompassing both Galactic and Extra-galactic entities such as young supernovae, supernova remnants, etc., spanning energy ranges from Giga-electronvolts to Zeta electronvolts. These particles emerge as secondary products resulting from the interactions of UHE cosmic rays with various targets such as protons and photons. The thesis investigates the propagation of these UHE secondary particles through space, while also scrutinizing their detectability, a crucial aspect for unraveling the origins and mechanisms behind high-energy cosmic ray production.
Enhanced energy storage performance of BaTiO3 and BiFeO3 - based dielectric ceramics and their thin films for capacitor applications
(2024) Balmuchu, Shashi Priya
The rising global demand for energy consumption for meeting the commercial and economic growth concerns for sustainable energy resources, generation, harvesting, and storage. The energy storage devices include electrochemical capacitors, batteries, fuel cells, and dielectric capacitors. Among these storage devices, dielectric ceramics offer inherently high power density and ultrafast charging/discharging time ranging from micro-nano to second time. This thesis mainly focuses on lead-free dielectric ceramics and thin films for energy storage applications. A detailed investigation of two systems, (1-x)BaTiO3-xBiFeO3 (BTBF) and (1-x)BaTiO3-xBi[Zn2/3(Nb0.85Ta0.85)1/3]O3 (BT-BZNT) from BaTiO3-BiMeO3 (where Me is trivalent or average trivalent metal ions) solid solution family in the bulk form is carried out. The ferroelectric studies for both compositions are analyzed to evaluate energy storage performance. The BTBF3 composition displayed a Wrec and η (%) of ⁓ 40 mJ/cm3 and 60.92%, respectively, at 25 kV/cm applied field, which is sufficiently lower as compared to other BaTiO3-BiMeO3 systems. A significant improvement in energy storage performance is observed in BT-BZNT ceramics. A remarkably high Wrec of 2.06 J/cm3 with an η (%) of 78% is achieved for BT-BZNT3 ceramics at an applied field of 180 kV/cm. The ultrahigh-energy efficiency of up to 96% is observed for higher concentrations in BT-BZNT ceramics. Herein, BT-BZNT3 and BT-BZNT4 ceramics exhibited a comparable energy storage performance at a lower applied field of 180 kV/cm, demonstrating the excellence of BT-BZNT ceramics over other BaTiO3-BiMeO3 family.
Molecular Dynamics Studies of ZrO2 and CeO2 based Solid Oxide Fuel Cell (SOFC) Materials
(2024) Madhual, Sudeshna
Solid oxide fuel cells (SOFCs) are a class of portable primary energy devices, that stand the potential as a green and sustainable alternative to fossil fuels. However, widespread commercial adoption of these electrochemical devices demands significant advancements in their design and efficiency as well as the development of infrastructural ecosystem. The electrolyte, that is sandwiched between the electrodes, is a crucial component of an SOFC device, having a critical impact on its performance. For the efficiency, operational safety, and lifespan of SOFC devices, the electrolyte material has to meet several criteria, such as high ionic conductivity, high chemical and mechanical stability, compatibility with potential electrodes, etc. As argued in the Introduction of this thesis, zirconia (ZrO2) and ceria (CeO2) based ceramic solids are among the most promising choices for practical applications, though their commercial viability demands further improvements. A thorough understanding of the microscopic nature of ion transport and the factors governing it is crucial to the necessary advancement of SOFC devices.
Investigation of propagation effect on laser beams carrying user defined phase profiles and turbulence resilient sensing scheme
(2024) Chauhan, Akanshu
The phase profile or the wavefront of a laser beam is the most important parameter so far as the information content in the beam is concerned. How propagation affects a laser beam carrying different phase profiles is important in a number of applications involving short to long range propagation of the laser beam. In this thesis we first carry out numerical simulation to understand how a laser beam diverges, by considering the diffraction induced divergence on the Zernike modes present in the beam. We come up with a functional form that can predict the effective size of a laser beam propagating a given distance and carrying a given phase profile. We also carry out numerical simulations to investigate the propagation effect on the phase profile in terms of the same on the constituent Zernike modes. We then develop an experimental arrangement to demonstrate the propagation of a laser beam carrying user defined phase profile and validate the important findings of the numerical simulations. Long distance propagation of a laser beam may also be affected by atmospheric turbulence, which may degrade the performance of a wavefront sensor. In this thesis we also carry out experiments to generate the effect of atmospheric turbulence on a laser beam and to investigate how various wavefront sensors perform in the presence of atmospheric turbulence. From our results, we come up with sensing schemes which are more resilient to the presence of atmospheric turbulence.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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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