Energy gap in the optical spectrum of superconductors

Abstract
We study the superconducting gap Δ which is manifested in optical spectra for frequencies ω≫Δ/ħ; these frequencies correspond to the interband transitions. The peak in the difference spectrum of the superconducting and normal phases is predicted, with the peak width being connected to Δ. The effect is more pronounced for transitions from the heavy-effective-mass band to the light-effective-mass band. Experimental manifestations of the predicted effect are discussed.