Resonant impurity scattering in the $\pm$s-gap state of the Fe-based superconductors

TL;DR

This paper investigates impurity effects in $ ext{Fe}$-pnictide superconductors, revealing that strong impurity scattering creates bound states that alter the density of states to resemble that of d-wave superconductors, explaining experimental observations.

Contribution

It introduces a generalized $ ext{T}$-matrix approach for two-band superconductors and shows how impurity scattering modifies the DOS in $ ext{Fe}$-pnictides, resolving conflicting experimental results.

Findings

Impurity scattering in the unitary limit creates off-centered bound states.

The DOS transitions from a full gap to a V-shaped form.

Results explain the observed V-shaped DOS and power-law relaxation rates.

Abstract

We study the impurity scattering on the $\pm$s-wave superconductor, with realistic parameters for the Fe-pnictide superconductors. Using $\mathcal{T}$-matrix method, generalized for the two bands, we found that impurity scattering of the unitary limit forms off-centered bound states inside of the superconducting gap, which modifies, surprisingly, the density of states (DOS) of a fully opened gap to a V-shaped one as in the case of a d-wave superconductor. This behavior provides coherent explanations to the several conflicting experimental issues of the Fe-pnictide superconductors: the V-shaped DOS but with an isotropic gap observed in the photoemission and tunneling experiments; the power law behavior of the nuclear spin-lattice relaxation rate ($1/T_1 \approx T^{\alpha}$ ; $\alpha \approx 3$), down to very low temperatures.