Strongly anisotropic spin-orbit splitting in a two-dimensional electron gas

TL;DR

This study investigates the anisotropic spin-orbit splitting in a two-dimensional electron gas on Bi$_2$Te$_2$Se, revealing complex behavior explained by an advanced $ extbf{k} extbf{·} extbf{p}$ model that considers band structure and Rashba effects.

Contribution

It introduces a $ extbf{k} extbf{·} extbf{p}$ model accounting for anisotropic Rashba splitting and band structure effects to explain observed spin-orbit splitting anisotropy.

Findings

Large spin-orbit splitting (~0.06 Å$^{-1}$) along $ar{ extGamma}ar{M}$ direction.

Anisotropic splitting inconsistent with simple third-order Rashba Hamiltonian.

Negative isotropic third order Rashba contribution reduces splitting at high $k$.

Abstract

Near-surface two-dimensional electron gases on the topological insulator Bi$_2$Te$_2$Se are induced by electron doping and studied by angle-resolved photoemission spectroscopy. A pronounced spin-orbit splitting is observed for these states. The $k$-dependent splitting is strongly anisotropic to a degree where a large splitting ($\approx 0.06$ \AA$^{-1}$) can be found in the $\bar{\Gamma}\bar{M}$ direction while the states are hardly split along $\bar{\Gamma}\bar{K}$. The direction of the anisotropy is found to be qualitatively inconsistent with results expected for a third-order anisotropic Rashba Hamiltonian. However, a $\mathbf{k} \cdot \mathbf{p}$ model that includes the possibility of band structure anisotropy as well as both isotropic and anisotropic third order Rashba splitting can explain the results. The isotropic third order contribution to the Rashba Hamiltonian is found to be negative, reducing the energy splitting at high $k$. The interplay of band structure, higher order Rashba effect and tuneable doping offers the opportunity to engineer not only the size of the spin-orbit splitting but also its direction.