Engineering ap+ipsuperconductor: Comparison of topological insulator and Rashba spin-orbit-coupled materials

Abstract

We compare topological insulator materials and Rashba-coupled surfaces as candidates for engineering $p+\mathit{ip}$ superconductivity. Specifically, in each type of material we examine (1) the limitations to inducing superconductivity by proximity to an ordinary $s$-wave superconductor, and (2) the robustness of the resulting superconductivity against disorder. We find that topological insulators have strong advantages in both regards: There are no fundamental barriers to inducing superconductivity, and the induced superconductivity is immune to disorder. In contrast, for Rashba-coupled quantum wires or surface states, the achievable gap from induced superconductivity is limited unless the Rashba coupling is large. Furthermore, for small Rashba coupling the induced superconductivity is strongly susceptible to disorder. These features pose serious difficulties for realizing $p+\mathit{ip}$ superconductors in semiconductor materials due to their weak spin-orbit coupling and suggest the need to seek alternatives. Some candidate materials are discussed.