Understanding Bulk Defects in Topological Insulators from Nuclear-Spin Interactions

TL;DR

This study uses $^{125}$Te NMR to non-invasively analyze bulk defects and electronic properties in various topological insulators, confirming Bi$_2$Te$_2$Se as the most promising candidate.

Contribution

It demonstrates how NMR parameters can reveal defect distributions, carrier density, and phase separation in topological insulators, providing a new characterization approach.

Findings

Defect distributions cause asymmetry in NMR lineshapes.

NMR parameters correlate with carrier density and spin-orbit coupling.

Bi$_2$Te$_2$Se is identified as the best topological insulator candidate.

Abstract

Non-invasive local probes are needed to characterize bulk defects in binary and ternary chalcogenides. These defects contribute to the non-ideal behavior of topological insulators. We have studied bulk electronic properties via $^{125}$Te NMR in Bi$_2$Te$_3$, Sb$_2$Te$_3$, Bi$_{0.5}$Sb$_{1.5}$Te$_3$, Bi$_2$Te$_2$Se and Bi$_2$Te$_2$S. A distribution of defects gives rise to asymmetry in the powder lineshapes. We show how the Knight shift, line shape and spin-lattice relaxation report on carrier density, spin-orbit coupling and phase separation in the bulk. The present study confirms that the ordered ternary compound Bi$_2$Te$_2$Se is the best TI candidate material at the present time. Our results, which are in good agreement with transport and ARPES studies, help establish the NMR probe as a valuable method to characterize the bulk properties of these materials.