Band structure and superconductivity in In-doped topological insulator (Pb_0.5Sn_0.5)_1-<i>x</i>In_<i>x</i>Te probed by NMR spectroscopy

The bulk band structure and superconductivity in the In-doped topological crystalline insulators (Pb_0.5Sn_0.5)_1-xIn_xTe, for x = 0, 0.1, 0.2, and 0.3, have been investigated by nuclear magnetic resonance (NMR) techniques. We found that the NMR frequency shifts of ¹¹⁷Sn, and ²⁰⁷Pb are dominated by the spins of free charge carriers, whereas the ¹²⁵Te frequency shift is determined by the magnetic orbital moments of the binding electrons. By analyzing these shifts, we conclude that In substitution for 0 ≤ x ≤ 0.3 provides not just hole carriers but also lifts the chemical potential from the valence bands to the conduction bands, in consistent with the results from the electric transport measurements. The superconductivity in the x = 0.3 sample is investigated by the nuclear spin-lattice relaxation rate (1/T₁) as a function of temperature, where a Hebel-Slichter coherence peak is observed near the critical temperature. This suggests that the superconducting gap in the bulk is fully opened and this feature may favor that (Pb_0.5Sn_0.5)_0.7In_0.3Te is a conventional superconductor rather than a chiral p-wave topological superconductor.