Impact of topology on the impurity effects in extended $s$-wave superconductors with spin-orbit coupling

Abstract

We investigate the impact of topology on the existence of impurity subgap states in a time-reversal-invariant superconductor with an extended $s$-wave pairing and strong spin-orbit coupling. By simply tuning the chemical potential we access three distinct phases: topologically trivial $s$-wave, topologically non-trivial $s_\pm$-wave, and nodal superconducting phase. For a single potential impurity we find subgap impurity bound states in the topological phase, but notably no subgap states in the trivial phase. These subgap impurity states have always finite energies for any strength of the potential scattering and subsequently, the superconducting gap in the topological $s_\pm$-wave phase survives but is attenuated in the presence of finite disorder. By creating islands of potential impurities we smoothly connect the single impurity results to topological edge states of impurity island. On the other hand, magnetic impurities lead to the formation of Yu-Shiba-Rusinov states in both the trivial and topological phases, which even reach zero energy at certain scattering strengths. We thus propose that potential impurities can be a very valuable tool to detect time-reversal-invariant topological superconductivity.