Role of surface termination in realizing well-isolated topological surface states within the bulk band gap in TlBiSe_2 and TlBiTe_2

摘要

Electronic structures associated with the flat (polar) Se/Te- or Tl-terminated surfaces of TlBiSe_2 and TlBiTe_2 are predicted to harbor not only Dirac cone states, but also trivial dangling bond states near the Fermi energy. However, the latter, trivial states have never been observed in photoemission measurements. In order to address this discrepancy, we have carried out ab initio calculations for various surfaces of TlBiSe_2 and TlBiTe_2. A rough nonpolar surface with an equal number of Se/Te and Tl atoms in the surface atomic layer is found to destroy the trivial dangling bond states, leaving only the Dirac cone states in the bulk energy gap. The resulting energy dispersions of the Dirac states are in good accord with the corresponding experimental dispersions in TlBiSe_2 as well as TlBiTe_2. We also show that in the case of flat, Se terminated, high-index (221) and (112) surfaces of TlBiSe_2, the trivial surface states shift energetically below the Dirac node and become well separated from the Dirac cone states.