Spin mapping of surface and bulk Rashba states in ferroelectric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-GeTe(111) films

Abstract

In regular semiconductors each electronic state is occupied by spin-up and spin-down electrons. It is well known that inversion symmetry breaking at a surface may lift this degeneracy by spin-orbit coupling (the Rashba effect). As already proposed by Rashba, bulk inversion symmetry breaking may cause a similar degeneracy lifting of bulk states. Ferroelectric materials provide such an inversion symmetry breaking. Thin ferroelectric $\ensuremath{\alpha}$-GeTe(111) films grown on Si(111) show a stable ferroelectric polarization with a polarization vector perpendicular to the surface that can be switched by an electric field into a metastable state with opposite polarization. Spin-resolved time-of-flight momentum microscopy reveals a Rashba-type spin splitting of the valence band caused by the inner electric field. Due to orbital polarization, the outer Rashba branch has a larger polarization than the inner branch. The corresponding net spin polarization provides new functionalities in spintronic devices, e.g. it may strongly affect the conversion process of spin into current via the spin-galvanic effect.