Large Rashba unidirectional magnetoresistance in the Fe/Ge(111) interface states

The structure inversion asymmetry at surfaces and interfaces gives rise to the Rashba spin-orbit interaction (SOI), that breaks the spin degeneracy of surface or interface states. Hence, when an electric current runs through a surface or interface, this Rashba effect generates an effective magnetic field acting on the electron spin. This provides an additional tool to manipulate the spin state in materials such as Si and Ge possessing inversion symmetry in their bulk form. The existence of Rashba states could be demonstrated by photoemission spectroscopy at the interface between different metals and Ge(111) and by spin-charge conversion experiments at the Fe/Ge(111) interface even though it is made of two light elements. In this paper, we identify the fingerprint of the Rashba states at the Fe/Ge(111) interface by magnetotransport measurements in the form of a large unidirectional magnetoresistance of up to 0.1%. From its temperature dependence, we find that the Rashba energy splitting is larger than in pure Ge(111) subsurface states.