Impurity bound-state as a probe of order-parameter symmetry in iron-pnictide superconductors: T-matrix approach

Abstract

The impurity induced in-gap bound state in multi-orbital superconductors is discussed based on Bogoliubov-de Gennes equation and T-matrix approximation. It is shown that multiple bound states appear depending on the strength of the impurity potential in the case of unconventional s-wave (or s+−-wave) superconductivity. This result can be used to distinguish between the conventional s-wave and unconventional s-wave states. T-matrix approximation using a simplified model supports the numerical results obtained in the Bogoliubov-de Gennes equation. It is shown that the T-matrix can be block diagonalized when we use orbital representation. As a result, each orbital can make the impurity bound state independently, leading to the multiple structure of the bound states. From the analysis of the T-matrix approximation, we understand the reasons why the bound states always appear pair-wise at the positions of ±ω and why there is a large asymmetry of the two peaks. We also show that the particle-hole asymmetry of the density of states gives rise to the asymmetry with respect to the strength of the impurity potential. The connection between the obtained results and the iron-pnictide superconductors is also provided.