In-gap bound states induced by a single nonmagnetic impurity in sign-preserving s-wave superconductors with incipient bands

TL;DR

This study reveals that incipient bands in multiband superconductors can influence in-gap bound states caused by nonmagnetic impurities, challenging previous assumptions about their relation to pairing symmetry.

Contribution

It demonstrates that incipient bands affect in-gap bound states even with sign-preserving s-wave pairing, impacting interpretations of experimental data.

Findings

Incipient bands influence in-gap bound states despite large energy separation.

Impurity-induced bound states are not definitive evidence of sign-changing pairing.

Implications for determining pairing symmetry in FeSe-based superconductors.

Abstract

We have investigated the in-gap bound states (IGBS) induced by a single nonmagnetic impurity in multiband superconductors with incipient bands. Contrary to the naive expectation, we found that even if the superconducting (SC) order parameter is sign-preserving s-wave on the Fermi surfaces, the incipient bands may still affect the appearance and locations of the IGBS, although the gap between the incipient bands and the Fermi level is much larger than the SC gap. Therefore in scanning tunneling microscopy experiments, the IGBS induced by a single nonmagnetic impurity are not the definitive evidences for the sign-changing order parameter on the Fermi surfaces. Our findings have special implications for the experimental determination of the pairing symmetry in the FeSe-based superconductors.