Understanding Bulk Defects in Topological Insulators from Nuclear-Spin Interactions

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.

24 pages, 6 figures

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