Evidence for a superconducting surface state in the half-heusler alloy LuPtBi

The half-Heusler alloy LuPtBi is predicted by band structure calculations to be a potential candidate for topological superconductivity. We present experimental evidence for a superconducting surface state at much higher temperatures than the bulk $T_c$ of $0.9$K (seen from transport). STM measurements of the $111$ surface show a well developed superconducting gap at temperatures below $2.4 K$, with an IV curve consistent with d-wave superconductivity. The ratio of $\frac{\Delta}{k_bTc}$ is about $12.0$ as opposed to the BCS value of $1.76$, implying that the true $T_c$ is about $6-7$ K. This result is supported SQUID magnetization measurements as well as theoretical predictions for enhanced surface superconductivity due to a Van-Hove singularity in a Bi-terminated [111] surface. We discuss implications of our measurements and possible future experiments.