Observation of Single Dirac Cone Topological Surface State in Compounds TlBiTe2 and TlBiSe2 from a New Topological Insulator Family

Angle resolved photoemission spectroscopy (ARPES) studies were performed on two compounds (TlBiTe${}_2$ and TlBiSe${}_2$) from a recently proposed three dimensional topological insulator family in Thallium-based III-V-VI${}_2$ ternary chalcogenides. For both materials, we show that the electronic band structures are in broad agreement with the $ab$ $initio$ calculations; by surveying over the entire surface Brillouin zone (BZ), we demonstrate that there is a single Dirac cone reside at the center of BZ, indicating its topological non-triviality. For TlBiSe${}_2$, the observed Dirac point resides at the top of the bulk valance band, making it a large gap ($\ge$200$meV$) topological insulator; while for TlBiTe${}_2$, we found there exist a negative indirect gap between the bulk conduction band at $M$ point and the bulk valance band near $\mathrm{\Gamma }$, making it a semi-metal at proper doping. Interestingly, the unique band structures of TlBiTe${}_2$ we observed further suggest TlBiTe${}_2$ may be a candidate for topological superconductors.