Momentum relaxation in a semiconductor proximity-coupled to a disordered s-wave superconductor: Effect of scattering on topological superconductivity

We study the proximity effect between a conventional semiconductor and a disordered s-wave superconductor. We calculate the effective momentum relaxation rate in the semiconductor due to processes involving electron tunneling into a disordered superconductor and scattering off impurities. The magnitude of the effective disorder scattering rate is important for understanding the stability of the topological superconducting state that emerges in the semiconductor, since disorder scattering has a detrimental effect and can drive the system into a nontopological state. We find that the effective impurity scattering rate involves higher-order tunneling processes and is suppressed due to the destructive quantum interference of quasiparticle and quasihole trajectories. We show that, despite the fact that both the proximity-induced gap and the effective impurity scattering rate depend on interface transparency, there is a large parameter regime where the topological superconducting phase is robust against disorder in the superconductor. Thus, we establish that the static disorder in the superconductor does not suppress the proximity-induced topological superconductivity in the semiconductor.