Nonmagnetic impurity effects in a three-dimensional topological superconductor; From - to -wave behaviors

Nagai, Yuki   ; Ota, Yukihiro; Machida, Masahiko  

Unconventional features in superconductivity are revealed by responses to impurity scattering. We study nonmagnetic impurity effects in a three-dimensional topological superconductor, focusing on an effective model (massive Dirac Hamiltonian with s-wave on-site pairing) of copper-doped bismuth-selenium compounds. Using a self-consistent T-matrix approach for impurity scattering, we examine in-gap states in density of states. We find that the results are well characterized by a single material variable, which measures relativistic effects in the Dirac Hamiltonian. In the nonrelativistic regime, an odd-parity superconducting state is fragile against nonmagnetic impurities. We show that this behavior is caused by a p-wave character involved in the topological superconducting state. In contrast, we show that in the relativistic regime the superconductivity is robust against nonmagnetic impurities, owing to an s-wave character. To summarize, the system has two aspects, p- and s-wave features, depending on the weight of relativistic effects.