Evidence for a direct band gap in the topological insulator Bi sub(2) Se sub(3) from theory and experiment

Using angle-resolved photoelectron spectroscopy and ab initio GW calculations, we unambiguously show that the widely investigated three-dimensional topological insulator Bi sub(2) Se sub(3) has a direct band gap at the [Gamma] point. Experimentally, this is shown by a three-dimensional band mapping in large fractions of the Brillouin zone. Theoretically, we demonstrate that the valence-band maximum is located at the [Gamma] point only if many-body effects are included in the calculation. Otherwise, it is found in a high-symmetry mirror plane away from the zone center.