Ryszard Buczko (26.02, 10:30)
Affiliation: Institute of Physics, Polish Academy of Sciences, Poland Title: Valley splitting of Dirac cones at topological/trivial semiconductor interface and at uneven (Pb,Sn)Se surface. Abstract:The family of IV-VI topological crystalline insulator (TCI) materials feature band inversions located away from the Brillouin zone center. The resulting possibility of both valley interactions and mirror symmetry protection is responsible for splitting of Dirac cones and generation of secondary Dirac points away from time reversal invariant momenta. Our study of bulk (Pb,Sn)Se (001) crystals overgrown with PbSe atomic layers has shown that such valley splitting is extremely sensitive to atomic-scale details of the TCI-trivial interface and the top layer, exhibiting non-monotonic changes as PbSe deposition proceeds [1]. This includes an apparent total collapse of the splitting for sub-monolayer coverage. The detailed theoretical analysis supported by realistic tight binding model calculations has led to the conclusion that in the case of a small number of PbSe layers the collapse of valley splitting depends mainly on the coverage of surface by a sufficiently dense array of terraces or steps. Similar calculations have been also performed for rough (001) surface of (Pb,Sn)Se [2]. Within the envelope function model it has been shown that valley mixing and the collapse of Dirac cone splitting depend crucially on the surface structure. When atomic terraces are wide enough the valley splitting is recovered and the odd-height steps define domain boundaries with additional 1D topological states reported by Sessi et al. [3]. The adjacent terraces turn out to be described by different values of the winding number topological invariant.