Exploring the nontrivial band edge in the bulk of the topological insulators Bi2Se3 and Bi2Te3

Bi2Se3 and related compounds are prototype three-dimensional topological insulators with a single Dirac cone in the surface band structure. While the topological surface states can be characterized with surface-sensitive methods, the underlying bulk energy band inversion has not been investigated in detail. Here, a study is presented that combines density-functional theory and nuclear magnetic resonance to explore the nontrivial band edge of Bi2Se3 and Bi2Te3. It is found that the topological band inversion is not a discrete reversal of the order of the valence and conduction band at the I' point. Rather, the bands closest to the Fermi level become well mixed and spread evenly below and above the band gap, such that the characters of the valence- and conduction-band edges become indistinguishable. Beside those bands relevant for the band inversion, i.e., Bi and Se pz, also Bi px and py states are involved. As a part of this mixture of states, the band inversion shows no edges in k space.