PhD Theses (Physics)

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Circuit Quantum Electrodynamics (cQED) Hybrid Systems for Quanutm Technological Applications: A Theoretical Study
(2025) Nongthombam, Roson
In this thesis, we study various quantum phenomena and their applications in a hybrid system implemented on a circuit quantum electrodynamics (cQED) platform. cQED explores the interaction between nonlinear superconducting circuits—functioning as artificial atoms—and quantized electromagnetic fields in the microwave-frequency range. The artificial atom in these circuits is realized through a Josephson tunnel junction, formed by a thin insulating barrier at the interface between two superconductors. The non-linear properties of this junction transform the equally spaced energy levels of a simple LC harmonic oscillator into atomic-like energy levels that are unequally spaced. A notable feature of this artificial atom is its ability to tune the frequency of its energy levels by driving it with a microwave field.
Structure and Dynamics of the Spin-orbit Coupled Ultra-dilute Quantum Droplets
(2025) Gangwar, Km Sonali
In recent years, quantum droplets (QDs) have emerged as a new form of matter in ultracold atomic systems, with research still in its early stages. QDs typically form due to the interplay between attractive mean-field (MF) interactions and repulsive beyondmean- field (BMF) interactions caused by quantum fluctuations. In one-dimensional systems, however, QDs in binary mixtures are predicted to form through a different mechanism, where the balance between attractive BMF and repulsive MF interactions plays a crucial role in the droplet formation.
Quasi-BIC Resonance Modes in Metasurface: Excitation and Applications at Near-Infrared and Terahertz Frequencies
(2025) Bhowmik, Bhairov Kumar
Bound States in the Continuum (BICs) are states that remain perfectly localized despite lying in the continuum, with ideal BICs possessing an infinite Q factor. However, real scenarios exhibit quasi-BIC modes, observed as Fano resonances with high but finite Q factors. This thesis investigates symmetry-protected quasi-BIC modes in metasurfaces for terahertz (THz) and near-infrared (NIR) applications.
Development of Sol-gel Derived Bioactive Glass-ceramics for Bone Regeneration and Hyperthermia Applications
(2025) Nitu
Bioglass and glass-ceramics play a crucial role in regenerative medicine by facilitating bone regeneration and tissue engineering applications. Despite its invention over 50 years ago, 45S5 bioglass remains the benchmark for bioactive materials. However, maintaining its vitreous state in sol-gel-derived nanopowders remains a challenge. This work systematically investigates the effect of Na₂O content on the vitreous state and bioactivity of sol-gel-derived 45S5 compositions. The findings confirm that complete retention of the vitreous state is not feasible through the sol-gel route, shifting the focus to glass-ceramics while preserving high bioactivity. To develop an efficient thermoseed for magnetic hyperthermia, iron oxide and magnetic nanoparticles (MNPs) were incorporated into 45S5 glass-ceramics. While direct iron oxide substitution was cytotoxic above 10 wt.%, MNP-substituted glass-ceramics exhibited improved magnetic properties without compromising bioactivity. The optimal combination of bioactivity and induction heating performance was achieved when MNPs substituted CaO. A systematic heat treatment further optimized the material’s properties, yielding a thermoseed with superior performance compared to FluidMag-CT. This study provides new insights into sol-gel-derived 45S5 ceramics, including phase evolution, bioactivity, and magnetism, offering a significant advancement in bioactive and magnetic glass-ceramics.
Ultrafast, Highly Sensitive Optical Fiber Relative Humidity Sensor Employing Graphene-derived Nanomaterials and Other Novel Materials for Potential Applications in Human Breath Monitoring and Voice-print Recognition
(2025) Mohan, Sunil
This thesis focuses on developing an optical fiber relative humidity (RH) sensor that offers a linear response over a broad dynamic range with optimal sensitivity, ultrafast response times, high resolution, and exceptional repeatability. The goal is achieved using simplified optical fiber configurations and intensity modulation via evanescent-wave absorption. The research explores the feasibility of these sensors for real-time applications in human breath monitoring and voice print recognition. To achieve this, novel sensing materials such as GO-ZnO nanocomposite, GO-ZnO-Fe nanocomposite, rGO-TiO₂ nanocomposite, polyhydroxybutyrate (PHB), and ash derived from menthol plants are utilized for the first time in sensor fabrication. Comprehensive experimental studies are conducted to optimize parameters like chemical composition, reaction conditions, and film thickness, ensuring superior sensor performance. A sensor employing GO-doped silica sol-gel film demonstrates linearity across 15.0−95.3%RH, with a sensitivity of 0.1036 dB/%RH. Sensitivity is further enhanced to 56.3mV/%RH (0.1262 dB/%RH), with response/recovery times of 0.18 seconds, by incorporating reduced graphene oxide (rGO) into the silica film. Using graphene oxide quantum dots (GQDs), another sensor achieves a sensitivity of 0.2437 dB/%RH over a 3−70%RH range, with response/recovery times of 0.025 seconds. Further developments include sensors with GO-ZnO and GO-ZnO-Fe nanocomposite claddings. These demonstrate sensitivities of 33.6mV/%RH and 60.6mV/%RH over ranges of 17−91%RH and 17−80%RH, with response times of 0.33 seconds and 0.031 seconds, respectively. A rGO-TiO₂ nanocomposite-based sensor achieves an exceptional sensitivity of 103.5mV/%RH over 3−70%RH with response times of 0.025 seconds. The research culminates in the development of an ultrafast sensor employing GO-coated reduced-to-few-micron core-diameter (RFMCD) PCS fibers. This sensor achieves the widest linear dynamic range of 3−94%RH with a sensitivity of 0.0115%RH⁻¹, resolution of ±0.049%RH, and a groundbreaking response time of 50 milliseconds. Further innovations include a biodegradable PHB-doped PVA sensor with a sensitivity of 66mV/%RH over 58−98%RH and response times of 0.0125 seconds. Additionally, a biowaste-based sensor using menthol plant ash achieves a linear range of 4−90%RH, sensitivity of 9.7mV/%RH, and response/recovery times of 0.013 and 0.014 seconds, respectively. The developed sensors exhibit exceptional reversibility, stability, and resolution, establishing benchmarks in optical fiber RH sensing. Their innovative configurations and rapid response times demonstrate transformative potential for real-time health monitoring and biometric applications. This research underscores the capability of evanescent-wave-based optical fiber sensors to meet modern sensing demands with unparalleled accuracy, speed, and reliability.
Magnetohydrodynamic and Hydrodynamic Studies of Relativistic Accretion Flows Around Black Holes
(2025) Mitra, Samik
One of the most efficient energy sources in the universe is matter accretion onto compact objects, such as black holes (BHs) and neutron stars (NSs). With the presence of an ample amount of angular momentum, this convergent matter cannot fall radially onto the central object. Instead, it forms a swirling disk-like structure widely known as the accretion disk. One of the major questions in accretion theory is to explain the angular momentum and energy transport within the disk.
New Aspects of Quantum Light Generation and Propagation in Quantum Dots
(2025) Hazra, Samit Kumar
In recent times, semiconductor-integrated photonics have gained much research interest in making reliable quantum photonic integrated circuits (QPIC) required in quantum technology. Generally, a QPIC is densely packed with tiny optical elements such as quantum light sources, quantum communication channels, detectors, modulators, beam splitters, quantum gates, etc. This thesis addressed three problems in making such optical elements in quantum dots (QDs), satisfying the tiny footprint and low-power requirement criteria. We have addressed the self-induced transparency (SIT) phenomenon in a QD medium at ultra-cold temperatures. In this process, a specific light pulse can propagate through the medium with minimal distortion in pulse shape at resonance frequency for a long propagation length. Further, we have proposed a scheme for arbitrary vector beam generation in a thin disc-shaped QD medium. We have considered a four-level diamond system interacting with one weak probe field and two relatively strong control fields. The four-wave mixing (FWM) process produces a generated signal with orthogonal polarization, and vector superposition with applied field generates a vector beam(VB). We have found the feasibility of generating a nondegenerate two-photon laser for a single QD embedded in a two-mode microcavity. We have analyzed both the incoherent and coherent pumping mechanism and its turnout that coherent pumping provides better conditions for such laser generation. Finally, we have shown that the system exhibits continuous variable twomode entanglement generation for two-photon resonant pumping.
Novel Bosonization Techniques in One Dimension
(2024) Babu, Nikhil Danny
In this work, the most singular contribution to the density density correlation functions (DDCF) of strongly inhomogeneous Luttinger liquids is derived and is shown to be expressible as compact analytical functions of position and time with second order poles and involving the scale independent bare reflection and transmission coefficients. The results are validated on comparison with standard fermionic perturbation theory. The DDCF is a crucial input to the powerful non-chiral bosonization technique (NCBT) that has been successfully used to obtain the correlation functions of inhomogeneous systems in one dimension whilst treating the impurity backscattering non-perturbatively, unlike conventional methods. The exact dynamical non-equilibrium Green functions (NEGF) for a system of noninteracting chiral quantum wires coupled through a point-contact is obtained analytically. The system considered is isomorphic to integer quantum Hall (IQHE) edge states coupled through a point-contact constriction. The tunneling I-V characteristics is obtained for an arbitrary time-dependent bias in the case of infinite bandwidth in the pointcontact. The case of finite bandwidth in the point-contact is also studied and non-Markovian transients in the tunneling current is observed upon sudden switch on of a bias voltage. The transient phenomena is consistent with numerical simulations and is observed to be a consequence of the appearance of a short distance cutoff in the problem when a finite bandwidth is considered. In a subsequent work, an unconventional bosonization procedure similar to NCBT is introduced, and is used to reproduce the exact NEGF of noninteracting chiral quantum wires coupled through a point-contact driven out of equilibrium by application of a bias. The novel unconventional bosonization scheme is shown to be internally consistent with Wick’s theorem used to obtain four-point functions. The proposed bosonization procedure can be extended to the case of fractional quantum Hall (FQHE) edge states with a point-contact wherein interparticle interactions become important. The FQHE edge states with single channel edge modes like in the Laughlin series, are modelled as chiral Luttinger liquids. In subsequent works, the DDCF for chiral Luttinger liquids with an impurity is computed using a generating functional method and is shown to be consistent with fermionic perturbation theory. The obtained interacting DDCF is used in conjunction with the unconventional bosonization procedure to derive the tunneling density of states (TDOS) at the point-contact for electron tunneling and quasiparticle tunneling cases, and the results agree with the accepted literature, thereby demonstrating the utility of the novel bosonization procedure in obtaining non-perturbatively, the correlation functions (most singular part) of inhomogeneous systems in one dimension.
Novel aspects of radiation pressure in hybrid quantum systems
(2024) Das, Sanket
The study of radiation pressure involves a wave's average force on a surface or a particle. This force arises from the transfer of linear momentum when the wave interacts with the surface or particle. The application of radiation pressure ranges from the universe's development to modern laser applications, such as optical tweezers and cavity optomechanics. In cavity optomechanics, when a laser is reflected from a surface, it generates various types of elastic waves that travel through the object but are generally delicate. In simple terms, light can cause a slight movement within the material, and it is observed in whispering-gallery-mode resonators and optomechanical crystals with photonic and phononic modes. The force of radiation pressure leads to various physical phenomena stemming from semi-classical lightmatter interactions, one of which is the mechanical analog of electromagnetically induced transparency (EIT). In EIT, an initially opaque three-level atomic medium becomes transparent due to applying strong control and weak probe fields. The destructive interference between two excitation pathways creates a narrow transparency window, resulting in anomalous dispersion and slow light effects. An optomechanical system with a single cavity mode and an acoustic mode closely resembles an atomic three-level system when a strong drive field and a weak probe field are present. This setup allows for the observation of an effect analogous to EIT.
Field theory in time dependent background: Superradiance and Sonoluminescence
(2024) Karmakar, Rajesh
The dissipative nature of time dependent systems is well known to have an intriguing feature of superradiance like phenomena when interacting with radiation either classically or quantum mechanically. In this thesis we have considered two particular dynamical systems arising in two different realms namely, oscillating black holes (BH) in the sky and oscillating bubbles in the laboratory. In the first part of the thesis, we studied the classical scattering of massless fields from oscillating BHs and calculated the absorption cross-section. In the second part of the thesis, we have considered a laboratory system of an oscillating gas bubble, where we have proposed a framework and studied the phenomena of quantum mechanical particle production mapping the system into an analog geometric background.
Study of the decay B° → γ γ at Belle and Belle II
(2024) Maurya, Shubhangi Krishan
We report the result of a search for the rare decay B0 → γγ using a combined dataset of 753 × 106 B ¯B pairs collected by the Belle experiment and 387 × 106 B ¯B pairs collected by the Belle II experiment at the Υ(4S) resonance produced in electron-positron collisions. A simultaneous fit to the Belle and Belle II data sets yields 11.0+6.5 −5.5 signal events. We determine the branching fraction B(B0 → γγ) = (3.7+2.2 −1.8(stat) ± 0.5(syst)) × 10−8 with a signal significance of 2.5σ and set a 90% confidence level upper limit of B(B0 → γγ) < 6.4 × 10−8. This result improves on the previously published 90% confidence level upper limit by BABAR by a factor of about five and provides the most stringent limit to date.
Synthesis and characterization of silver nanoparticles for light trapping application in thin film solar cells
(2024) Gangwar, Manvendra Singh
In the present thesis, silver nanoparticles have been synthesized by solid-state dewetting of the precursor silver films deposited at different deposition conditions (deposition time, rf power, substrate temperature) using rf sputtering technique. Surface morphology and growth dynamics of silver nanoparticles are studied using atomic force microscopy (AFM) with advanced statistical analysis. Height-height correlation function (HHCF) and power spectral density function (PSDF) are extracted from AFM data, and scaling exponents 𝛼𝑙𝑜𝑐𝑎𝑙, , 1/z and 𝛼 are determined using analysis. A direct correlation between morphology and the localized surface plasmon resonance (LSPR) properties is observed. The microstructure influence on dielectric function and plasmonic properties of silver nanoparticles (Ag NPs) is studied using spectroscopic ellipsometry (SE). Dielectric function and plasmonic properties of Ag NPs are investigated from spectroscopic ellipsometry (SE) data using a quite unique model in terms of the combination of different oscillators: Drude-Lorentz model along with two Gauss oscillators to account for intraband, interband transitions and different modes of localized surface plasmon resonance (LSPR) of Ag NPs. The influence of the substrate temperature on the growth of Ag NPs and their several properties like localized surface Plasmon resonance (LSPR), photoluminescence, and Raman spectroscopy is studied. Enhancement in PL peak intensity and Raman peak intensity is found to be in accordance with the LSPR of Ag NPs. Both simulation and experimental studies on single junction hydrogenated amorphous silicon (a-Si:H) thin film solar cells is done prior to the implementation of Ag NPs as a plasmonic back reflector in (a-Si:H) thin film solar cells. The effect of emitter layer (a-Si:H (p)) doping and absorber (a-Si:H (i)) layer thickness is studied. Further, simulation results are compared with the experimental results. A good match between simulation and experiment results is obtained. As an application part of this thesis, the role of silver nanoparticles as plasmonic back reflector for light trapping application in hydrogenated amorphous silicon (a-Si:H) thin film solar cells is explored. Excellent light trapping by plasmonic back reflector in solar cells is observed. A broadband enhancement in quantum efficiency is shown by a-Si:H thin film solar cells fabricated on the plasmonic back reflector with a gain of 47% in short circuit current density (𝐽𝑠𝑐) as compared to the flat back reflector. As a result, the improvement in the performance of the a-Si:H thin film solar with plasmonic back reflector is observed corresponding to an efficiency (η) of 8.4% against 5.6% efficiency of the a-Si:H thin-film solar cell with flat back reflector respectively.
Magnetic and Electronic Structure of few Perovskites in the form of Bulk and 2D-Superlattices
(2023) Kiran, Dokala Ravi
Complex oxide systems with the perovskite structure are becoming highly significant for modern day magneto-electronic devices because of their unique magnetic and transport properties. Mainly, perovskites with 3d-4f transition metal-rare-earth perovskites have gained widespread attention due to the strong interplay between the lattice, electron spin, orbit, and crystal structure. The present work provides a glimpse of growth of such systems using pulsed laser deposition technique and their electronic/magnetic structure in the form of 2D superlattice structures. In particular, the current work deals with the superlattices of Pr0.7Ca0.3MnO3/SrTiO3]15 and [Pr0.5Ca0.5MnO3/SrTiO3]15 on (001) oriented SrTiO3 and LaAlO3 single crystal substrates. Elastic strain induced electronic reconstructions at the interface enhanced the interlayer ferromagnetic interactions in the case of x = 0.3 superlattices on SrTiO3 exhibiting the highest HK ∼ 9 kOe and K1 ∼ 8 × 105 erg/cc. Tunable spin–flopped transition (∼ 30 kOe), significant negative exchange-bias field (HEB ∼ 2.5 kOe), huge coercive field (HC ∼ 22 kOe) and large NM (ΔM ∼ 280 emu/mole) are the unique characteristic features of the Ce incorporated YCrO3 polycrystals. The H–T phase diagram, clearly distinguishes three prominent regions below the TN (∼ 150 K), viz (i) long-range canted AFM + weak FM phase ( ), (ii) Γ24 mixed phase and (iii) robust ) AFM + FM phases. Extensive magnetization measurements reveal the existence of orbital-ordering in Pr0.45-xYbxSr0.55MnO3 accompanied by antiferromagnetic (AFM) Néel temperature, at as low as 158 K below the high- (302 K) ferromagnetic (FM) phase. Irreversible metamagnetic transitions from the AFM-FM phase occurs for a specific composition Pr40 (x = 0.05) till T ≤ 220 K. The admixture of metastable states of AFM and FM is quite robust in the investigated system whereas AFM state is mediated by Yb3+ ions, while, the FM state arising by field driven thermo-magnetic kinetics. In the Gd0.9Ce0.1CrO3, overall magnetization M(T) undergoes a second transition at the low temperatures associated with spin-flip transition triggered by the critical field, HC = 200 Oe at (10 K). The system exhibits better magneto-entropy value -ΔSM = 42 J/Kg-K which is higher than the previously reported GdCrO3 values. The reduction in the Cr-O-Cr bond angle through the substitution of Ce3+ at the Gd3+ site deduce such betterment in the magnetic entropy value. The presented works find potential utility in the fields of magnetoelectronic, thermo-magnetic sensors and spintronic device applications.
Rare-earth elements based Perovskites: Bulk and Low Dimensional Superlattices
(2023) Das, Shaona
Perovskites have been a subject of extensive research due to their immense potential applications in magneto-electronics and photovoltaics, ever since the discovery of Calcium titanate (CaTiO3) in the Russian Ural Mountains by Gustav Rose in 1839. The mineral was named after the Russian mineralogist Lev Perovski. The present thesis delineates the investigation of thin films, including superlattices and bilayer, as well as polycrystalline perovskite oxides that comprise transition metal and rare earth elements. Bilayers comprising [La0.7Sr0.3MnO3(5 nm)/LaCoO3(15 nm)] were fabricated on SrTiO3 substrates using two different deposition sequences. Our findings indicate the presence of magnetic characteristics, specifically the pseudo antiferromagnetic (AFM) pinned character, in one bilayer, while the other exhibits solely ferromagnetic (FM) nature. Additionally, we observed that the lattice mismatch and lattice strain resulted in the suppression of the Curie temperature and other physical properties. The present study investigates the structural, morphological, electronic, optical, and magnetic properties of [La0.7Sr0.3MnO3/LaNiO3]10 superlattices that were deposited through pulsed laser deposition (PLD) on SrTiO3-(001), (011), and (111) substrates. The study reveals that the mixed valence Ni2+/3+ and Mn3+/4+ electronic states are dominant at the core level. Furthermore, the relative intensity ratio of the Mn ions is found to be higher in the superlattices grown on (111) oriented SrTiO3 compared to the other two orientations. The calculated hopping energies, obtained from the variable range hopping mechanism, are of significant magnitude (≥ 40 meV). A noteworthy observation was made regarding the decrease in Curie temperature from 67 K to 110 K, coupled with a marked increase in the effective exchange interaction. Polycrystalline samples of Dy1-xCexCrO3 were prepared, where x ranged from 0.1 to 0.5. Our findings indicate that the tolerance factor increases while the octahedral distortion factor decreases with increasing Ce doping. This suggests that greater stability is achieved in samples with higher levels of Ce doping. The study also documented an increase in the Néel temperature from 156 K to 162 K in the heavily doped samples exhibiting G-type AFM character with 𝛤4(𝐺𝑥,𝐴𝑦,𝐹𝑧) spin-configuration. In contrast, samples with x = 0.2-0.5 demonstrated a phase-transition across TPC (< 𝑇N1) with 𝛤2(𝐹𝑥,𝐶𝑦,𝐺𝑧), while samples with x = 0.1-0.3 underwent another magnetic phase transition TSR (< TPC) with 𝛤25 (𝐹𝑥,𝐶𝑦,𝐺𝑧; 𝐹𝑥𝑅,𝐶𝑦𝑅,𝐺𝑥𝑅,𝐴𝑦𝑅). An increase in the magnetic entropy change (Δ𝑆𝑀) was observed in the DyCrO3 system with 10% Ce substitution and improved refrigerant capacity (RCP) of approximately 360 J/kg. This was measured at a temperature of 5 K and a magnetic field strength of 40 kOe, suggesting potential advancements in magnetic refrigeration. Previous studies on DyCrO3 under the same conditions reported a Δ𝑆𝑀 of 256 J/kg. The Gd1-xSmxCrO3 samples were synthesized with varying Sm concentrations, specifically x = 0.1 (GSO1), 0.5 (GSO5), and 0.9 (GSO9). Notably, GSO5 demonstrated multiple magnetization switching behavior across all three ZFCW, FCC, and FCW protocols, rendering it a promising candidate for magnetic switching applications. This phenomenon has not been previously reported in any perovskite oxide materials. The coexistence of metastable magnetic phases Γ4 (𝐺𝑥 , 𝐴𝑦 , 𝐹𝑧) and Γ2 (𝐹𝑥 , 𝐶𝑦 , 𝐺𝑧) was observed in the GSO9 sample. This phenomenon resulted from the clustering of ferromagnetic islands within an antiferromagnetic matrix in the face-centered cubic case, which is referred to as a magnetic glass-like signature. The analysis of refined structural parameters obtained from X-ray diffraction indicates a fluctuation in the tilt angles, which can be attributed to the quasi-harmonic effect resulting from the exchange interaction between Gd3+ and Cr3+ ions. This effect causes a reduction in the stiffness of the A1g (3) mode as the temperature increases.
Search for New Physics at the future Lepton Coliders
(2024) Jahedi, Sahabub
The Standard Model (SM) of particle physics has successfully explained the fundamental forces of nature and was solidified with the discovery of the Higgs boson. However, various theoretical and experimental motivations drive us to explore beyond the SM (BSM). Statistical analysis plays a crucial role in this exploration, applied to different collider experiments to search for different BSM scenarios. Chapter 1 of the thesis introduces the SM particle spectrum, its limitations, and discusses approaches to address these limitations. It also outlines the outlook for past, present, and future colliders, focusing on lepton colliders. Chapter 2 delves into two statistical techniques: binned analysis and the optimal observable technique (OOT). Chapter 3 explores OOT in a BSM-dominated scenario, particularly in estimating Z boson couplings at e+e- collider and studying dark matter phenomenology. Chapter 4 shifts focus to scenarios where the SM dominates, investigating the determination of anomalous neutral triple gauge couplings (nTGCs) through diboson production and dimension-6 effective couplings through top-quark pair production at the e+e- collider. Chapter 5 examines experimental constraints on dimension-6 four-Fermi SMEFT operators and explores flavor probes through flavor-changing top-charm production at the muon collider. Chapter 6 studies optimal sensitivity of NP couplings in the presence of SM background using numerical techniques. Finally, Chapter 7 provides a summary of the thesis and suggests potential future directions
Macroscopic Quantum Phenomena in Hybrid Optomechanical Systems: A Theoretical Exploration
(2024) Kalita, Sampreet
Optomechanical systems serve as a versatile platform for the study of classical and quantum phenomena both in the mesoscopic and macroscopic regime. They are also useful to analyze higher-order nonlinear effects or control the transmission, storage and retrieval of optical signals. Moreover, by integrating such systems into solid-state platforms and coupling other degrees of freedom to the optical and mechanical modes, one can study a multitude of phenomena arising in hybrid systems. In this thesis, we theoretically explore a handful of such classical and quantum phenomena that emerge due to the radiation-pressure-induced optomechanical interaction in different configurations of hybrid open quantum systems. Specifically, we analyze (i) the behavior of quantum synchronization in opticallycoupled optomechanical systems, (ii) the transmission of a weak probe beam in an optomechanical analogue of annularly-trapped Bose-Einstein condensate placed inside a cavity, and (iii) the generation and the enhancement of entanglement and mechanical squeezing in modulated optomechanical setups. Our studies may find applications in optical sensing, quantum communication and quantum information processing with continuous variables.
Structural and magnetic properties of low dimensional Heusler alloys
(2024) Srivastava, Manisha
A simple template-less chemical route was developed to synthesize ternary Heusler alloy nanoparticles. viz., Co2FeGa, Fe2CoGa, and Fe2CoAl. After establishing a procedure to obtain phase pure and highly ordered nearstoichiometric Heusler alloy compounds in nanoparticle form, the variation of magnetic properties of these nanoparticles as a function of crystallite size was explored. Having mastered the procedure to obtain the nominal stoichiometry in the Heusler alloy compounds, Fe2-xCo1+xGa (0  x  1) nanoparticles were synthesized, and their composition dependent properties were determined. Finally, the synthesis of a quaternary Heusler alloy nanoparticle was demonstrated for the first time by preparing Fe2CoGa0.5Al0.5 nanoparticles. This showcased the prowess of the developed methodology to synthesize other intermediate off-stoichiometric quaternary Heusler alloy compositions. The experimental research carried out has been supported by theoretical studies. These include the prediction of magnetic properties, electronic density of states, and half metallic properties of these Heusler alloys and interpretation and validation of the experimental results with standard theoretical models. The results not only showcase the spirit of the methodology to obtain such a highly ordered, impurity free, single phase, single domain, soft ferromagnetic Heusler alloy nanoparticles with enhanced structural and magnetic properties but also bring out insights and information about these nanoparticle alloys not known so far. These studies also help us in understanding the behavior of these systems so that they can be effectively utilized in practical applications because of their enhanced magnetization, Curie temperature, and magnetic anisotropy coupled with low coercivity.
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

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