Understanding bulk defects in topological insulators from nuclear-spin interactions

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. The bulk electronic properties are studied via ¹²⁵Te NMR in Bi₂Te₃, Sb₂Te ₃, Bi_0.5Sb_1.5Te₃, Bi ₂Te₂Se, and Bi₂Te₂S. A distribution of defects gives rise to asymmetry in the powder lineshapes. The Knight shift, line shape, and spin-lattice relaxation are investigated in terms of how they affect carrier density, spin-orbit coupling, and phase separation in the bulk. The present study confirms that the ordered ternary compound Bi ₂Te₂Se is the best topological insulator candidate material at the present time. These results, which are in good agreement with transport and angle-resolved photoemission spectroscopy studies, help establish the NMR probe as a valuable method to characterize the bulk properties of these materials. The role of bulk defects in topological insulators is investigated from the NMR standpoint. The NMR technique is validated against angle-resolved photoemission spectroscopy and transport measurements for the most commonly used binary and ternary topological insulators. Defect content is correlated with the NMR Knight shift and spin-lattice relaxation. NMR is a local probe which reports non-invasively on carrier concentration and dynamics.