Non-orthogonal spin-momentum locking in strongly spin-orbit coupled materials

Orthogonal spin-momentum locking is a general feature of the Rashba spin-orbit coupled surfaces. Recently, experimental evidence for non-orthogonal spin-momentum locking (NOSML) has been reported on the topological surface of α-Sn. [1] Here, we discuss the non-magnetic impurity scattering as a candidate mechanism to this isotropic NOSML state. Our results are applicable to spin-orbit coupled systems generally and are not limited to topological states. The NOSML state is of interest in understanding and identifying spin-orbit dependent phenomena and their spectroscopic signatures in topological and non-topological materials.

[1] Scholz, M. R. et al., Topological surface state of α-Sn on InSb(001) as studied by photoemission, Phys. Rev. B 97, 075101 (2018).