Bound State and Order Parameter Mixing Effect by Nonmagnetic Impurity Scattering in Two-band Superconductors

TL;DR

This paper explores how nonmagnetic impurities affect two-band superconductors, revealing that strong interband scattering can restore two-gap features and induce impurity-bound states, extending previous models beyond the Born approximation.

Contribution

It advances understanding by analyzing interband scattering effects using the $t$-matrix approximation, showing the recovery of two-gap features and bound states in strong scattering regimes.

Findings

Two-gap structure reappears in the density of states at strong interband scattering.

Bound states form around impurities depending on the phase of scattering potential.

Interband scattering effects are weaker in the strong scattering limit, contrary to previous expectations.

Abstract

We investigate nonmagnetic impurity effects in two-band superconductors, focusing on the effects of interband scatterings. Within the Born approximation, it is known that interband scatterings mix order parameters in the two bands. In particular, only one averaged energy gap appears in the excitation spectrum in the dirty limit. [G. Gusman: J. Phys. Chem. Solids {\bf 28} (1967) 2327.] In this paper, we take into account the interband scattering within the $t$-matrix approximation beyond the Born approximation in the previous work. We show that, although the interband scattering is responsible for the mixing effect, this effect becomes weak when the interband scattering becomes very strong. In the strong interband scattering limit, a two-gap structure corresponding to two order parameters recovers in the superconducting density of states. We also show that a bound state appears around a nonmagnetic impurity depending on the phase of interband scattering potential.