Optical conductivity of a two-band superconductor

Within the two-band superconductor model, which is a generalization of the standard BCS model to the case of a complicated crystal structure, an expression has been obtained for the conductivity of the superconductor at an arbitrary frequency of the external electromagnetic field. This expression has been derived using the microscopic theory in the framework of the diagram technique for nonequilibrium processes. The σ(ω) dependencies calculated for T = 0 are compared with the results of single-band models with the s and d symmetries of the order parameter. It has been shown that the behavior of the optical conductivity as a function of the frequency depends strongly on the doping level.