Impurity-induced bound states in iron-based superconductors with $s$-wave $\text{cos}\text{ }{k}_{x}\ensuremath{\cdot}\text{cos}\text{ }{k}_{y}$ pairing symmetry

Impurity-induced bound states in iron-based superconductors with $s$ -wave $cos k_{x} \cdot cos k_{y}$ pairing symmetry

Wei-Feng Tsai, Yan-Yang Zhang, Chen Fang, and Jiangping Hu

Phys. Rev. B 80, 064513 – Published 20 August 2009

Abstract

Using both the self-consistent Bogoliubov-de Gennes formulation and non-self-consistent $T$ -matrix approach, we perform a comprehensive investigation of the in-gap bound states induced by a localized single impurity in iron-based superconductors. We focus on studying signatures associated with the unconventional sign-changed $s$ -wave pairing symmetry. For a nonmagnetic impurity, we find that there are two in-gap bounds, symmetric with respect to zero energy, only in the sign-changed $s$ -wave pairing state, not in the sign-unchanged $s$ -wave state, due to the existence of nontrivial Andreev bound states caused by the sign change. For a magnetic impurity, we find that due to the breakdown of the local time-reversal symmetry, there exist only bound-state solutions (with orbital degeneracy) carrying one of the electron-spin polarizations around the impurity. As increasing the scattering strength, the system undergoes a quantum phase transition (level crossing) from a spin-unpolarized ground state to a spin-polarized one. While the results for the magnetic impurity are qualitatively similar in both the sign-changed and sign-unchanged $s$ -wave superconducting (SC) states, the bound states in the first case are more robust and there is no $π$ phase shift of the SC gap near the impurity in the strong scattering regime.