Transport Signatures of Spin-Orbit Coupling in Graphene-Based Materials

Citation

Tu, Min-Feng (2019) Transport Signatures of Spin-Orbit Coupling in Graphene-Based Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/KCPE-1F36. https://resolver.caltech.edu/CaltechTHESIS:08162018-100820958

Abstract

Topological materials have been a fastest growing research topic in the recent decade. Out of the numerous new phases proposed and/or discovered, "topological insulators" (TIs) are one of the most promising materials that could lead to further advances in high-performance electronics and to applications in quantum computing. Similar to the ordinary semiconductors, TIs have a bulk gap; yet they host robust edge/surface states which are protected from non-magnetic disorder and interactions while the gap remains open. This feature is a manifestation of the non-trivial topology of TIs, the crucial feature that distinguishes them from ordinary semiconductors. Although the search for more topological materials continues, discovered TI currently are limited by practical difficulties that prevent industrialization.

In this thesis, we study graphene, which is the first proposed TI candidate in the history, and its derivatives. With the intrinsic spin-orbital coupling (SOC) on graphene, one can open a topologically nontrivial band gap at the Dirac cones, although the SOC of the carbon atoms is exceedingly small for topological insulation to be observed in experiments. Many proposals exist to enhance the SOC on graphene by doping with adatoms, changing the functionality of the surface, placing graphene on top of other strong SOC materials, etc. However, few proposed TI signatures have been found experimentally. Furthermore, measuring these intrinsic SOCs through magnetoconductance is challenging due to their relatively weak signatures in transport. This work addresses the challenges in transport measurements from both analytical and numerical approaches on various graphene-based materials. Graphene’s Dirac band structure and open geometry underlie its exciting prospects for engineering new physics via impurity-induced spin-orbit coupling. As a tantalizing example, previous theory works predicted a robust quantum-spin-Hall phase in graphene covered with dilute heavy adatoms such as In, Tl, and Os, although experiments to date have not detected the required enhancement of spin-orbit coupling. Motivated by these experiments, we explore the consequences of adatom-generated spin-orbit couplings on magneto-transport in graphene. We attack the problem using diagrammatic techniques and the Landauer-Buttiker transport simulation informed by microscopics, and study various coverages, chemical potentials, and disorder types. We find that the induced spin-orbit couplings can contribute to magneto-conductance differently from conventional intrinsic and Rasbha spin-orbit couplings. Our results provide a possible rationale for the absence of spin-orbit signatures in recent experiments, and also highlight a roadmap for their discovery in future work.

In addition to the adatom-dedoped graphene, we also study graphene placing on top of strong SOC substrate, WS₂, by jointing theory, numerics, and experiment. We demonstrate, in experiment, a clear weak anti-localization (WAL) effect arising from induced Rashba spin–orbit coupling (SOC) in WS₂-covered single-layer and bilayer graphene devices. Contrary to the uncovered region of a shared single layer graphene flake, WAL in WS₂-covered graphene occurs over a wide range of carrier densities on both the electron and hole sides.

Thesis Files