Non-Magnetic Impurity induced in-gap bound states in two band s_(pm) superconductors

In this paper we study the effect of single non-magnetic impurity in two-band s-wave superconductors where the two s-wave order parameters have relative phase $\delta\neq0$ between them. We show that in-gap bound states are always induced by non-magnetic impurities when $\delta=\pi$ ($s_{\pm}$-wave superconductors). The bound state is a consequence of the topological nature of the corresponding Bogoliubov-de Gennes equation.