NMR Studies of the Candidate Topological Superconductor Sn$_{1-x}$In$_{x}$Te: Spin-Triplet Superconductivity Robust against Magnetic Impurities

In-doped SnTe is a low-carrier-density semiconductor with strong spin-orbit coupling, and has been proposed to be a topological superconductor. We report nuclear magnetic resonance (NMR) studies of both $^{119}$Sn and $^{125}$Te nuclei, performed on single crystals of Sn$_{1-x}$In$_{x}$Te, where $T_c = 1.8$ K for $x = 0.1$. Under an applied field of 0.33 T, the spin-lattice relaxation rate $1/^{119}T_1$ drops rapidly below 1.2 K, indicating bulk superconductivity. We observe absolutely no change in the Knight shift with temperature when $T < T_c$, which in NMR is normally an indicator of spin-triplet superconductivity. We find no coherence peak below $T_c$ in $1/^{119}T_1$, suggesting an unconventional order parameter but also the possible role of impurities. In the normal state we find that $1/^{119}T_1$ and $1/^{125}T_1$ have Fermi-liquid behavior at high fields, but at low fields show a large Curie-Weiss-type enhancement indicative of magnetic impurity effects. Thus the fact that $T_c$ in our samples is insensitive to the sample purity suggests that superconductivity in Sn$_{1-x}$In$_{x}$Te is robust against magnetic impurities, in contrast to the situation in conventional superconductors.