Spin-singlet superconductivity in the doped topological crystalline insulator Sn0.96In0.04Te

The In-doped topological crystalline insulator Sn1-x In-x Te is a candidate for a topological superconductor, where a pseudo-spin-triplet state has been proposed. To clarify the spin symmetry of Sn1-x In-x Te, we perform Te-125-nuclear magnetic resonance (NMR) measurements in polycrystalline samples with 0 <= x <= 0.15. The penetration depth calculated from the NMR line width is T independent below half the superconducting transition temperature (T-c) in polycrystalline Sn0.96In0.04Te, which indicates a fully opened superconducting gap. In this sample, the spin susceptibility measured by the spin Knight shift (K-s) at an external magnetic field of H-mu 0(0) = 0.0872 T decreases below T-c, and K-s(T = 0)/K-s(T = T-c) reaches 0.36 +/- 0.10, which is far below the limiting value 2/3 expected for a spin-triplet state for a cubic crystal structure. Our result indicates that polycrystalline Sn0.96In0.04Te is a spin-singlet superconductor.