Nuclear magnetic resonance as a probe of electronic states of Bi2Se3

Abstract

We present magnetotransport and 209 Bi nuclear magnetic resonance (NMR) data on a series of single crystals of Bi2Se3, Bi2Te2Se and CuxBi2Se3 with varying carrier concentrations. The Knight shift of the bulk nuclei is strongly correlated with the carrier concentration via a hyperfine coupling of 27 µeV, which may have important consequences for scattering of the protected surface states. Surprisingly we find that the NMR linewidths and the spin lattice relaxation rate appear to be dominated by the presence of localized spins, which may be related to the presence of Se vacancies.

Se77nuclear magnetic resonance of topological insulatorBi2Se3

“…Here, the coupling is dominated by 209 Bi-209 Bi interactions, but does not lead to exchange narrowing [30] since the local symmetry at the Bi site causes a sizable (∼140 to 170 kHz) quadrupole shift [14,19] of the 209 Bi nuclear levels, such that nuclear neighbors may not be able to participate in exchange if they are in different spin states. Furthermore, the quadrupole shift can vary from one nucleus to the next, due to strain caused by imperfections.…”

“…This is supported by our preliminary 209 Bi NMR results, which give 209 T 1 ∼ 10 ms in Bi 2 Se 3 (data not shown), in agreement with Refs. [14,19]. Doping with Cu shortens the T 1 of 77 Se by about a factor of 10, likely due to an increase in DOS.…”

“…For example, although the two inequivalent Se (nuclear spin I = 1/2) or Te (I = 1/2) sites should give rise to different NMR signals, they have not been found or discussed, while signals from surface states have been invoked [23,25]. Even where reported, special line shapes or spin echo behaviors are not understood, e.g., for 209 Bi NMR (I = 9/2) [14,19,20], pointing to unusual electronic properties. Therefore, understanding the NMR of TIs opens up the possibility of a more detailed comprehension of these materials, which will ultimately help in a NMR-based characterization of surface states.…”

“…Although NMR parameters such as shifts, linewidths, relaxation are sensitive to the electronic structure, e.g., through the local spin and orbital susceptibilities, it is unclear what NMR can contribute to the understanding of the topological surface states or to any special bulk properties of TIs (note, however, that NMR has been proposed [18] as a probe of the pairing symmetry in topological superconductors). So far, there have been few NMR studies of TIs: 209 Bi NMR of Bi 2 Se 3 single crystals and powders [14,[19][20][21], 125 Te NMR of Bi 2 Te 3 [22][23][24][25], and 77 Se NMR of Bi 2 Se 3 powder [22]. While all of these confirm bulk conductivity qualitatively from fast longitudinal nuclear relaxation (1/T 1 ), they leave many questions unanswered.…”

“…Each QL contains two equivalent "outer" Se atoms (Se out ), two equivalent Bi atoms, and another "inner" Se atom (Se in ) located at the center of inversion (the unit cell comprises 3 QL) [6]. In spite of its energy gap, the bulk of Bi 2 Se 3 is conducting due to self-doping with electrons from Se vacancies, with carrier concentration (n) ranging from 2×10 17 to 2×10 19 cm −3 [11][12][13][14], which can be increased, e.g., by intercalation of Cu [15][16][17].…”