# Spin-singlet superconductivity in doped topological crystalline insulator Sn_{0.96}In_{0.04}Te

## TL;DR

This study uses NMR measurements to demonstrate that doped topological crystalline insulator Sn_{0.96}In_{0.04}Te exhibits spin-singlet superconductivity with a fully-opened gap, challenging previous triplet pairing proposals.

## Contribution

The paper provides experimental evidence clarifying the spin symmetry of In-doped SnTe, showing it is a spin-singlet superconductor, which is a novel insight for this material.

## Key findings

- Penetration depth is T-independent below half T_c, indicating a fully-opened gap.
- Spin Knight shift decreases below T_c, confirming spin-singlet pairing.
- K_s(T=0)/K_s(T_c) ratio is 0.36, far below the 2/3 expected for triplet state.

## Abstract

The In-doped topological crystalline insulator Sn_{1-x}In_xTe is a candidate for a topological superconductor, where pseudo-spin-triplet state has been proposed. To clarify the spin symmetry of Sn_{1-x}In_xTe, we perform ^{125}Te-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_{0.96}In_{0.04}Te, 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_0 = 0.0872 T decreases below T_c, and K_s(T=0)/K_s(T=T_c) reaches to 0.36 \pm 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_{0.96}In_{0.04}Te is a spin-singlet superconductor.