Nonmagnetic Impurity Resonances as a Signature of Sign-Reversal Pairing in the FeAs-based Superconductors

TL;DR

This paper investigates how nonmagnetic impurities create in-gap resonances in FeAs-based superconductors, revealing signatures of sign-reversal pairing symmetry through local density of states analysis.

Contribution

It demonstrates that nonmagnetic impurity resonances can serve as a signature of sign-reversal pairing in FeAs superconductors, using a tight-binding model and T-matrix approach.

Findings

Two major in-gap resonances depend on impurity potential strength

Resonances originate from Andreev bound states between hole and electron Fermi surfaces

Resonance peaks are sensitive to the impurity scattering potential

Abstract

The low energy band structure of the FeAs based superconductors is fitted by a tight binding model with two Fe ions per unit cell and two degenerate orbitals per Fe ion. Based on this, superconductivity with extended s-wave pairing symmetry of the form $\cos k_x +\cos k_y$ is examined. The local density of states near an impurity is also investigated by using T-matrix approach. For the nonmagnetic scattering potential, we found that there exist two major resonances inside the gap. The height of the resonance peaks depends on the strength of the impurity potential. These in-gap resonances are originated in the Andreev's bound states due to the quasiparticle scattering between the hole Fermi surfaces around $\Gamma$ point with positive order parameter and the electron Fermi surfaces around $M$ point with negative order parameter.