X-ray edges of superconducting metals

The existence of the energy gap in the excitation spectrum of superconducting electrons prevents the excitation of an infinite number of low-energy quasiparticles, i.e., prevents the infrared catastrophe. The ground states of the conduction electrons with and without the presence of an impurity potential are no longer orthogonal. Effects on x-ray edges and x-ray photoemission spectra of superconducting metals are discussed. It appears that all the edges are divergent in the limit of infinite hole lifetime and that the photoemission spectra become discrete, reflecting the fact that the quasiparticles in superconductors have finite minimum energy