Opto-spintronics and Quantum Transport in Topological Quantum Materials

Nath, Sambhu G (2026) Opto-spintronics and Quantum Transport in Topological Quantum Materials. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Sambhu G Nath (20RS055)) 20RS055.pdf - Submitted Version Restricted to Repository staff only Download (73MB)

Abstract

The discovery of topological phases of matter has fundamentally reshaped our understanding of quantum materials by revealing electronic states protected by topology rather than symmetry breaking. Three-dimensional topological insulators and Dirac semimetals occupy a central position within this framework, as they host relativistic quasiparticles, strong spin-orbit coupling, and symmetry protected transport phenomena. These materials provide a fertile platform for exploring novel charge, spin, and light-matter interactions, with promising implications for optoelectronic and spintronic technologies. This thesis presents an extensive experimental investigation of transport, optoelectronic, and spin dynamic phenomena in topological insulators and Dirac semimetals, with a focus on understanding how topology, disorder, symmetry, and spin-orbit coupling collectively govern their physical responses. The thesis begins by establishing the theoretical foundations necessary to contextualize topological quantum materials. Key concepts such as Berry phase, Berry curvature, topological invariants, and their manifestations in quantum Hall systems are discussed, followed by an overview of two and threedimensional topological insulators and topological semimetals. This framework motivates the experimental studies that form the core of the thesis. A wide range of thin film growth, microfabrication, and characterization techniques are employed, including pulsed laser deposition, thermal evaporation, lithographic patterning, low temperature transport measurements, polarization resolved photocurrent microscopy, and ferromagnetic resonance spectroscopy. A central part of this work focuses on the evolution of electronic transport across a topological to trivial quantum phase transition in Inx(Bi₀.₃Sb₀.₇)₂−xTe₃ thin films. By systematically increasing the indium concentration, the effective spin-orbit coupling strength is reduced, driving a transition from a bandinverted topological insulator to a trivial insulating phase. Magnetotransport measurements reveal that, in the weakly disordered regime, the system exhibits diffusive transport characterized by weak antilocalization arising from strong spin-orbit coupling. The evolution of the weak antilocalization prefactor correlates directly with the suppression of band inversion, establishing a clear experimental signature of the topological phase transition. At higher indium concentrations, enhanced disorder leads to a breakdown of diffusive transport and the emergence of variable range hopping conduction. This crossover is accompanied by a striking reversal of low field magnetoconductance from negative to positive, displaying pronounced anisotropy and nontrivial temperature dependence. These observations are explained using an orbital magnetotransport framework that incorporates incoherent hopping and wavefunction shrinkage, providing a unified description of transport across both coherent and incoherent regimes. First principles electronic structure calculations further support the experimental findings by elucidating the microscopic role of indium derived states in suppressing band inversion. Beyond electronic transport, the thesis explores nonlinear optoelectronic responses in the centrosymmetric Dirac semimetal PdTe. In ideal Dirac systems, inversion and time-reversal symmetries prohibit second-order photocurrents; however, polarization and angle-resolved photocurrent measurements demonstrate that incident photons can dynamically break symmetry through momentum transfer. Using circularly and linearly polarized light, distinct contributions from circular photogalvanic effect, geometric shift current, surface photogalvanic effect, and photon drag mediated processes are identified. The helicity dependent photocurrent exhibits characteristic angular dependencies and reverses sign with the direction of incidence, indicating the involvement of spin-polarized surface states and momentum assisted circular shift currents. Linear polarization responses display symmetry properties consistent with surface and photon drag mechanisms. These results establish PdTe as a powerful platform for probing quantum geometric effects and hidden topological responses in centrosymmetric Dirac semimetals. The thesis further investigates spin-charge interconversion in PdTe through spin pumping experiments performed on PdTe/Ni₈₀Fe₂₀ heterostructures. Using broadband ferromagnetic resonance at low temperatures, pure spin currents are dynamically injected into the Dirac semimetal. The resulting transverse voltages exhibit symmetry characteristics consistent with conversion via the inverse spin Hall effect. A pronounced enhancement of the effective Gilbert damping is observed, indicating efficient transfer of spin angular momentum across the ferromagnet/Dirac semimetal interface. These findings demonstrate robust spin-orbit mediated spin-charge conversion in PdTe and highlight the potential of Dirac semimetals as active components in spintronic devices. Taken together, the results presented in this thesis provide a comprehensive experimental picture of how topology, spin-orbit coupling, disorder, and symmetry breaking govern charge, spin, and photoinduced transport in topological materials. By combining transport, optical, and spin-dynamic probes across multiple material platforms, this work advances the understanding of topological quantum materials and establishes experimental pathways for their integration into future optoelectronic and spintronic technologies.

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