Evidence for a direct band gap in the topological insulator Bi2Se3 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 Bi2Se3 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 Brillouin center 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. 
I. A. Nechaev, R. C. Hatch, M. Bianchi, D. Guan, C. Friedrich, I.   Aguilera, J. L. Mi, B. B. Iversen, S. Bl\"ugel, Ph. Hofm...
cond-mat.mtrl-sci