Quasi-localized impurity state in doped topological crystalline insulator Sn_0.9In_0.1Te probed by ¹²⁵Te-NMR

The In-doped topological crystalline insulator Sn_1-xIn_x Te is a promising candidate for a topological superconductor, where it is theoretically suggested that In creates an impurity state responsible for superconductivity. We synthesized high purity Sn_1-xIn_x Te samples and performed ¹²⁵Te-nuclear magnetic resonance (NMR) measurements. The NMR spectra under a magnetic field of H₀ = 5 T show a broadening characteristic due to a localized impurity state. The spin-lattice relaxation rate (1=T₁) divided by temperature shows a Curie-Weiss like temperature-dependence under H₀ = 0.1 T but is temperature-independent under H₀ = 5 T. These results indicate the existence of quasi-localized impurity states due to In doping.