Direct measurement of the bulk spin structure of noncentrosymmetric BiTeCl

A strong spin-orbit interaction leads to a Rashba-type splitting of the bulk bands of BiTeCl, which results in toroidal Fermi surfaces with distinct spin structures depending on the chemical potential. Here, we present spin-and angle-resolved photoelectron spectroscopy measurements of the three-dimensional spin-orbit split state at the top of the valence band. Its polarization is systematically studied in dependence of the photon energy and compared to ab initio calculations of the initial state. The measured spin orientation is in plane to the (0001) surface and perpendicular to the momentum, independently of the photon energy.