ABSORPTION-SPECTROSCOPY OF ELECTRONIC STATES NEAR FERMI LEVEL - TEMPERATURE-CHANGES OF SUPERCONDUCTING YBA2CU3O7-DELTA FILMS AT TC

The temperature behavior of the absorption spectra of superconducting YBa2Cu3O7-delta textured films is measured in a 1.5 divided-by 3.0 eV energy region with varying temperatures from 25 to 300 K. The spectra are found to be sensitive to a superconducting transition: the temperature-dependent behaviour of absorption at T > T(c) undergoes critical changes under the superconducting transition and for T < T(c) the spectrum ceases to depend on temperature. The spectra display an exciton maximum in the optical gap E(g) = 1.95 eV. The spectrum freezing at T < T(c) can be explained assuming that the Fermi level is in temperature resonance with a narrow local level which is responsible for a delta-like singularity in the density of states.