Unidirectional Rashba Spin Splitting in Single Layer WS$_{2(1-x)}$Se$_{2x}$ alloy

TL;DR

This study reveals a giant unidirectional Rashba spin splitting in a single-layer WS$_{1.4}$Se$_{0.6}$ alloy, linked to in-plane polarization and atomic order, with implications for advanced electronic and spintronic devices.

Contribution

First observation of unidirectional Rashba spin splitting in a 2D alloy, connecting electronic structure with atomic order and in-plane polarization.

Findings

Giant Rashba parameter of 2.8 eV detected

Anisotropic valence band structure observed

Link established between Rashba splitting and in-plane polarization

Abstract

Atomically thin two-dimensional (2D) layered semiconductors such as transition metal dichalcogenides (TMDs) have attracted considerable attention due to their tunable band gap, intriguing spin-valley physics, piezoelectric effects and potential device applications. Here we study the electronic properties of a single layer WS$_{1.4}$Se$_{0.6}$ alloys. The electronic structure of this alloy, explored using angle resolved photoemission spectroscopy, shows a clear valence band structure anisotropy characterized by two paraboloids shifted in one direction of the k-space by a constant in-plane vector. This band splitting is a signature of a unidirectional Rashba spin splitting with a related giant Rashba parameter of 2.8 0.7 eV . The combination of angle resolved photoemission spectroscopy with piezo force microscopy highlights the link between this giant unidirectional Rashba spin splitting and an in-plane polarization present in the alloy. These peculiar anisotropic properties of the WS$_{1.4}$Se$_{0.6}$ alloy can be related to local atomic orders induced during the growth process due the different size and electronegativity between S and Se atoms. This distorted crystal structure combined to the observed macroscopic tensile strain, as evidenced by photoluminescence, displays electric dipoles with a strong in-plane component, as shown by piezoelectric microscopy. The interplay between semiconducting properties, in-plane spontaneous polarization and giant out-of-plane Rashba spin-splitting in this two-dimensional material has potential for a wide range of applications in next-generation electronics, piezotronics and spintronics devices.