NMR as a Probe of Band Inversion in Topologically Nontrivial Half-Heusler Compounds

Bi-209 nudear magnetic resonance (NMR) was studied in half-Heusler compounds YPdBi and YPtBi. The observed NMR spectra exhibit low intensity because of short nuclear transverse relaxation time (T-2) governed by sizable structural disorder. In both compounds, the frequency shift of the Bi-209 bulk nuclei is dominated by the chemical shielding (delta(cs)) contribution. Remarkably, it is positive for topologically trivial YPdBi yet strongly negative for topologically nontrivial YPtBi. At the same time, the spin-lattice relaxation rate (1/T-1) is distinctly larger in YPtBi due to stronger spin-orbit coupling and postulated inversion of the electronic bands near the Fermi level. The unique features of delta(cs) and 1/T-1 established for YPtBi can be considered as possibly universal fingerprints of topologically nontrivial state in similar materials.