Energy gap in optical spectrum of superconductors

Superconducting gap manifestations have been studied in optical spectra at frequencies which substantially exceed sureconducting gap value and correspond to the interband transition with the conduction band participation. The interband contribution into the imaginery part of the magnetic permeability has been calculated. In the difference spectrum of the imaginary part of the dielectric permittivity of a superconductor a normal metal characteristic structure is to be observed, the width of this structure being unambiguously related to the energy gap. Possible experimental manifestations of the predicted effect are discussed. Calculation results are compared with experimental data for YBa2Cu3O7-δ films. 12 refs., 3 figs