NMR Probe of Metallic States in Nanoscale Topological Insulators

Abstract

A 125Te NMR study of bismuth telluride nanoparticles as a function of particle size revealed that the spin-lattice relaxation is enhanced below 33 nm, accompanied by a transition of NMR spectra from the single to the bimodal regime. The satellite peak features a negative Knight shift and higher relaxivity, consistent with core polarization from p-band carriers. Whereas nanocrystals follow a Korringa law in the range 140-420 K, micrometer particles do so only below 200 K. The results reveal increased metallicity of these nanoscale topological insulators in the limit of higher surface-to-volume ratios.

Se77nuclear magnetic resonance of topological insulatorBi2Se3

“…[25], but ascribed to surface and bulk states. In view of our results, this interpretation and some related explanations [22,23] based on 125 Te NMR should be revised.…”

“…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.…”

“…While all of these confirm bulk conductivity qualitatively from fast longitudinal nuclear relaxation (1/T 1 ), they leave many questions unanswered. 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.…”

Site‐Specific Contributions to the Band Inversion in a Topological Crystalline Insulator

“…They can report on relativistic effects (such as SOC) related to heavy Z-elements in alloys [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Also, NMR can probe materials that are either p-type or ntype and also materials containing multiple fractions of different carrier types [12,13].…”

“…By observing the resonances of the different nuclei, site-specific information about the contributions of local orbitals to the band structure can be extracted. While NMR has been used for the study of TIs [8][9][10] e) NMR spectra and spin-lattice relaxation time (T 1 ) data were acquired using a Bruker DSX-300 spectrometer operating at frequencies of 62. …”