Tunable spin-orbit coupling and symmetry-protected edge states in graphene/WS$_2$

TL;DR

This paper demonstrates tunable Rashba spin-orbit coupling and symmetry-protected edge states in WS2-covered graphene, revealing potential for novel topological phenomena and spintronic applications.

Contribution

It provides experimental evidence of tunable Rashba SOC in graphene/WS2 and predicts valley-Zeeman SOC and protected edge states from first-principles calculations.

Findings

Weak anti-localization observed in WS2-covered graphene

Rashba SOC relaxation rate estimated at ~0.2 ps^{-1} and tunable by electric fields

Prediction of valley-Zeeman SOC and symmetry-protected edge states

Abstract

We demonstrate clear weak anti-localization (WAL) effect arising from induced Rashba spin-orbit coupling (SOC) in WS$_2$-covered single-layer and bilayer graphene devices. Contrary to the uncovered region of a shared single-layer graphene flake, WAL in WS$_2$-covered graphene occurs over a wide range of carrier densities on both electron and hole sides. At high carrier densities, we estimate the Rashba SOC relaxation rate to be $\sim 0.2 \rm{ps}^{-1}$ and show that it can be tuned by transverse electric fields. In addition to the Rashba SOC, we also predict the existence of a `valley-Zeeman' SOC from first-principles calculations. The interplay between these two SOC's can open a non-topological but interesting gap in graphene; in particular, zigzag boundaries host four sub-gap edge states protected by time-reversal and crystalline symmetries. The graphene/WS$_2$ system provides a possible platform for these novel edge states.