Magnetic field induced charge symmetry in the bulk of the topological insulators Bi₂Se₃ and Bi₂Te₃investigated with NMR

Experiments on 3-dimensional topological insulators focus on the characterization of the special, gapless surface states that emerge as a consequence of the bulk energy band inversion. The bulk states, however, are important, as well, as they provide a direct access to the topologically non-trivial band structure. For example, with nuclear magnetic resonance (NMR) we were able to measure the real-space fingerprint of the bulk energy band inversion, i.e. the redistribution of charges in the chemical structure, in the model topological insulator Bi₂Se₃ [1]. In this system, free carriers as induced by self-doping play a key role because they populate those bands subject to the band inversion. Our measurements further reveal that these conducting electrons possess special properties governed by a strong spin-orbit interaction [1-3]. We show on the basis of a comprehensive experimental evidence from ²⁰⁹Bi NMR in Bi₂Se₃ and Bi₂Te₃ how the local charge symmetry at the nuclear site apparently follows the external magnetic field. This behavior may constitute a so far undocumented rotational degree of freedom of strongly spin-orbit coupled conduction electrons with partially unquenched orbital angular momentum.