Manifestations of impurity-induced s± ⇒ s++ transition: multiband model for dynamical response functions

We investigate the effects of disorder on the density of states, the single-particle response function and optical conductivity in multiband superconductors with s(+/-) symmetry of the order parameter, where s(+/-) -> s(++) transition may take place. In the vicinity of the transition, the superconductive gapless regime is realized. It manifests itself in anomalies in the above-mentioned properties. As a result, intrinsically phase-insensitive experimental methods such as angle-resolved photoemission spectroscopy, tunneling and terahertz spectroscopy may be used to reveal information about the underlying order parameter symmetry.