Two-dimensional atomic layer materials with a thickness of a few atomic layers can exhibit spin-polarized electronic bands due to strong spin-orbit coupling and broken inversion symmetry even without magnetic order. In the case of bilayer materials, however, the coexistence of opposite local inversion asymmetry and global inversion symmetry causes hidden spin states. In this paper, we demonstrate that forming a thallium (Tl) bilayer crystal on a Ag(111) substrate reveals the hidden spins, and also the presence of a unique phenomenon intrinsic to centrosymmetric bilayer crystals weakly bonded on a substrate: the splitting of electronic bands in both spin and real space. Furthermore, we discuss the advantages and technical considerations of the spin rotator apparatus, which has been recently introduced into spin- and angle-resolved photoelectron spectrometer, by using the obtained spin-resolved photoelectron spectra of the Tl bilayer crystal.
Toichi et al. (Thu,) studied this question.