Temperature effect on the quasiparticle spectrum of an impurity-doped superconductor with two separate electron groups

The two-gap BCS theory with incoherent single-particle scattering is applied to study the temperature dependence of the quasiparticle density of states in a superconductor with two separate electron groups. It is shown that in such a material nonmagnetic impurities can affect superconductivity in a nontrivial way. For considerable intergroup scattering rates comparable with clean-limit gap magnitudes we find a crossover from two-feature densities of states at low temperatures to standard BCS-type curves with a single maximum, as T increases. We point out that some unexpected temperature-dependent features observed for magnesium diboride, a two-band superconductor, may not represent its intrinsic characteristics.