ABSORPTION SPECTROSCOPY OF ELECTRONIC STATES NEAR FERMI LEVEL: TEMPERATURE CHANGES OF SUPERCONDUCTING YBa2Cu3O7−δ FILMS AT Tc

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Abstract
The temperature behavior of the absorption spectra of superconducting YBa 2Cu3O7−δ textured films is measured in a 1.5÷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>Tc undergoes critical changes under the superconducting transition and for Tc the spectrum ceases to depend on temperature. The spectra display an exciton maximum in the optical gap Eg=1.95 eV. The spectrum freezing at Tc 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.

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