Experimental Realization of a Three-Dimensional Topological Insulator Phase in Ternary Chalcogenide TlBiSe2

We report the first observation of a topological surface state on the (111) surface of the ternary chalcogenide TlBiSe2 by angle-resolved photoemission spectroscopy. By tuning the synchrotron radiation energy we reveal that it features an almost ideal Dirac cone with the Dirac point well isolated from bulk continuum states. This suggests that TlBiSe2 is a promising material for realizing quantum topological transport.