Spin-singlet superconductivity in the doped topological crystalline insulator Sn_0.96In_0.04Te

The In-doped topological crystalline insulator Sn_1−xIn_xTe is a candidate for a topological superconductor, where a pseudo-spin-triplet state has been proposed. To clarify the spin symmetry of Sn_1−xIn_xTe, we perform ¹²⁵Te-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 Sn_0.96In_0.04Te, 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 µ₀H₀ = 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 Sn_0.96In_0.04Te is a spin-singlet superconductor.