Symmetry-Dependent Diverse Spin-Orbit Coupling Effects in Two-Dimensional Transition Metal-based Chalcogenide Materials

Chakraborty, Souvick (2025) Symmetry-Dependent Diverse Spin-Orbit Coupling Effects in Two-Dimensional Transition Metal-based Chalcogenide Materials. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Souvick Chakraborty (20RS042)) 20RS042.pdf - Submitted Version Restricted to Repository staff only Download (21MB)

Abstract

This thesis develops a unified theoretical framework for engineering spin-orbit coupling (SOC) phenomena in two-dimensional transition-metal materials to drive advancements in spintronics. Using density functional theory, we demonstrate how deliberate symmetry breaking through alloying, Janusification, external fields, and strain unlocks and tunes both conventional and exotic SOC effects, including Rashba, Dresselhaus, persistent spin texture (PST), and band splitting with vanishing spin polarization (BSVSP). Beginning with transition-metal dichalcogenide (TMD) monolayers, our work overcomes the inherent out-of-plane mirror symmetry that suppresses Rashba SOC by designing stable MSSe (M = Mo, W) alloys with a low C2v point group symmetry. These alloys exhibit robust, anisotropic Rashba splitting and complex spin textures that match the performance of Janus TMDs while offering superior structural stability. In case of the nonpolar TMD alloys, we show how introducing an external out-of-plane electric field further enhances tunability, driving the Rashba effect from linearisotropic to strongly nonlinear-anisotropic depending on the system symmetry. Additional strain and electric field modulation refine the SOC parameters of the alloys for precise control. Next, we study how the different SOC effects uniquely influence the spin Hall effect (SHE), a cornerstone of spin-charge interconversion. Our study reveals that both ternary and Janus quaternary alloys substantially boost spin Hall conductivity (SHC) via hybridization and SOCinduced band anticrossings and unconventional SOC-induced spin splitting near the Fermi level. Crucially, we identify BSVSP, where SOC-split bands generate SHC peaks without net spin polarization. Finally, to suppress the spin relaxation challenges encountered in all spin transport applications, we introduce the Janus gold chalcogenide Au2SSe, which hosts a symmetry-assisted PST at the X-point. This unidirectional spin alignment remains invariant under momentum scattering, effectively suppressing the Dyakonov-Perel spin relaxation and enabling long‐range spin transport. Strain tuning and the coexistence of Rashba, Dresselhaus, and Zeeman‐like effects in Au2SSe establish the Janus gold chalcogenides as versatile platforms for coherent spin manipulation. By bridging theoretical advances with experimental feasibility, this work lays out scalable, field-tunable design strategies for next-generation 2D spintronic devices.

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