Surface versus bulk contributions to the giant Rashba splitting in the ferroelectric {\alpha}-GeTe(111) semiconductor

TL;DR

This paper demonstrates that ferroelectric GeTe(111) exhibits complex Rashba spin-splitting in surface and bulk states, with tunable spin textures linked to ferroelectric polarization, enabling potential all-electric spin control.

Contribution

It reveals the coexistence and hybridization of surface, bulk, and resonant states in GeTe(111), showing their complex spin textures and the influence of ferroelectricity on Rashba splitting.

Findings

GeTe(111) displays complex spin textures in surface and bulk Rashba bands.

Surface-bulk resonant states exhibit hybridized wavefunctions with unconventional spin topologies.

The spin helicity is linked to ferroelectric polarization, enabling electric control of spin properties.

Abstract

In systems with broken inversion symmetry spin-orbit coupling (SOC) yields a Rashba-type spin splitting of electronic states, manifested in a k-dependent splitting of the bands. While most research had previously focused on 2D electron systems, recently a three-dimensional (3D) form of such Rashba-effect was found in a series of bismuth tellurohalides. Whereas these materials exhibit a very large spin-splitting, they lack an important property concerning functionalization, namely the possibility to switch or tune the spin texture. This limitation can be overcome in a new class of functional materials displaying Rashba-splitting coupled to ferroelectricity: the ferroelectric Rashba semiconductors (FERS). Using spin- and angle-resolved photoemission spectroscopy (SARPES) we show that GeTe(111) forms a prime member of this class, displaying a complex spin-texture for the Rashba-split surface and bulk bands arising from the intrinsic inversion symmetry breaking caused by the ferroelectric polarization of the bulk (FE). Apart from pure surface and bulk states we find surface-bulk resonant states (SBR) whose wavefunctions entangle the spinors from the bulk and surface contributions. At the Fermi level their hybridization results in unconventional spin topologies with cochiral helicities and concomitant gap opening. The GeTe(111) surface and SBR states make the semiconductor surface conducting. At the same time our SARPES data confirm that GeTe is a narrow-gap semiconductor, suggesting that GeTe(111) electronic states are endowed with spin properties that are theoretically challenging to anticipate. As the helicity of the spins in Rashba bands is connected to the direction of the FE polarization, this work paves the way to all-electric non-volatile control of spin-transport properties in semiconductors.