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Direct Evidence for the Dirac-Cone Topological Surface States in Ternary Chalcogenide TlBiSe2

We have performed high-resolution angle-resolved photoemission spectroscopy on TlBiSe2, which is a member of the ternary chalcogenides theoretically proposed as candidates for a new class of three-dimensional topological insulators. By measuring the energy band dispersions over the entire surface Brillouin zone, we found a direct evidence for a non-trivial surface metallic state showing a X-shaped energy dispersion within the bulk band gap. The present result unambiguously establishes that TlBiSe2 is a strong topological insulator with a single Dirac cone at the Brillouin-zone center. The observed bulk band gap of 0.4 eV is the largest among known topological insulators, making TlBiSe2 the most promising material for studying room-temperature topological phenomena.