Magneto-transport and Magneto-optical studies of Topological Insulator Thin Films and Devices

Manna, Subhadip (2024) Magneto-transport and Magneto-optical studies of Topological Insulator Thin Films and Devices. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Subhadip Manna (16RS021)) 16RS021.pdf - Submitted Version Restricted to Repository staff only Download (35MB)

Abstract

Magneto-transport and magneto-photoresponse of topological insulator thin films have been studied. Topological insulators are a remarkable class of materials that have attracted considerable attention in condensed matter physics due to their unique electronic properties. These materials feature an insulating bulk interior while exhibiting conductive states on their surfaces or edges. This distinctive behavior stems from the topological characteristics of their electronic band structures, which are safeguarded by time-reversal symmetry. At beginning, we present the growth of highly c-axis oriented thin films of the quaternary topological insulator (TI) BiSbTe1.5Se1.5 (BSTS) using the pulsed laser deposition (PLD) technique. Our findings demonstrate the ease with which the composition of these thin films can be adjusted by altering the deposition conditions, a flexibility not attainable in bulk single crystal growth methods. Variations in growth parameters such as substrate temperature, background argon pressure, and target to substrate distance significantly influence the structural, morphological, and lattice dynamics of the PLD-grown BSTS thin films. Films produced under optimized deposition parameters exhibit granular, stoichiometric, highly c-axis oriented structures and display all four Raman modes characteristic of the R3m space group. The crystal structure and lattice dynamics of these thin films are analysed through X-ray diffraction and Raman spectroscopy, respectively. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) are employed to investigate morphology and composition, respectively, of the BSTS thin films. Temperature-dependent measurements of sheet resistance confirm the strongly bulk insulating nature and predominant surface metallic transport at low temperatures. Due to the prevalence of surface transport in these thin films, magnetoresistance data exhibit enhanced weak antilocalization phenomena with a large phase coherence length. Our study offers a straightforward and adaptable approach for fabricating high-quality, bulk insulating thin films of various compositions. Next we proceed to measure the photocurrent in topological insulators.The notable presence of strong spin-orbit coupling in these systems results in a coupling of surface-carrier spin with its linear momentum. This phenomenon, known as spin-momentum locking, gives rise to helical surface states in momentum space. Here, the spin of a surface electron aligns perpendicular to its linear momentum, leading it to trace a helical path around the Dirac cone in momentum space. Consequently, electrons with a specific spin orientation flow unimpeded in a particular direction, in the absence of magnetic impurities or external magnetic fields. At equilibrium, although there is no net charge current, a pure spin current exists. However, the system can be perturbed out of equilibrium through electrical or optical excitations. Optical excitations, in particular, induce spin-polarized charge currents by generating a spin imbalance through spin-selective transitions, especially when circularly polarized light is employed for excitation. Circularly polarized light, carrying finite photon angular momentum, selectively excites carriers with specific spins, adhering to the conservation of angular momentum. The helicity of the spin polarization can be adjusted by altering the polarization of light, transitioning from left circular polarization (LCP) to right circular polarization (RCP).The helicity-dependent photocurrent (HDPC) arising from the Dirac-like surface state exhibits distinct behaviour for two different phases of topological insulators and trivial insulators. In our study, we present the polarization-dependent photocurrent observed on a topological insulator (Bi₀.₃Sb₀.₇)₂Te₃ (BST) and an indium-doped Bi₀.₃Sb₀.₇)₂Te₃ sample, which represents a trivial insulator. By varying the angle of incidence of the excitation beam, measurements unveil the microscopic origins of various components of the helical photocurrent, including the circular photogalvanic effect, as well as the linear and circular photon drag effects. Through systematic investigation, we demonstrate that the circular photogalvanic effect (CPGE) and a specific type of circular photon drag effect (CPDE) serve as key parameters enabling the differentiation between the topological and trivial phases of BST and IBST, respectively. Magnetotransport measurements provide further support for our findings. Additionally, performing photocurrent measurements at room temperature and in ambient conditions renders this probing tool unique and highly practical compared to others. After that we have studied longitudinal and Hall photothermal voltage responses to the angular dependence of the planar magnetic field scan in epitaxial thin films of the Topological Insulator (TI) Sb₂Te₃, grown using pulsed laser deposition (PLD). Unlike prior research that utilized polarized light to investigate the TI, our study introduces advancements based on unpolarized light-induced local heating. Precise positioning of a focused laser spot enables the exploration of the thermal gradient acting in various directions. The interaction between the applied thermal gradient and the magnetic field on the bulk band orbitals allows us to differentiate between the ordinary Nernst effect stemming from the out-of-plane thermal gradient and an extraordinary magneto-thermal contribution from the planar thermal gradient. These findings underscore the potential of PLD-grown epitaxial topological insulator thin films for optoelectronic devices, such as sensors, switches, and actuators, which can be tailored through position-dependent, non-invasive local heating and variations in the applied magnetic field direction.

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