Single dirac cone topological surface state and unusual thermoelectric property of compounds from a new topological insulator family

Angle resolved photoemission spectroscopy study on TlBiTe₂ and TlBiSe₂ from a thallium-based ternary chalcogenides family revealed a single surface Dirac cone at the center of the Brillouin zone for both compounds. For TlBiSe₂, the large bulk gap (∼200meV) makes it a topological insulator with better mechanical properties than the previous binary 3D topological insualtor family. For TlBiTe₂, the observed negative bulk gap indicates it as a semimetal, instead of a narrow-gap semiconductor as conventionally believed; this semimetality naturally explains its mysteriously small thermoelectric figure of merit comparing to other compounds in the family. Finally, the unique band structures of TlBiTe₂ also suggest it as a candidate for topological superconductors.