TEMPERATURE-VARIATIONS OF THE ENERGY-SPECTRUM OF HIGH-TEMPERATURE SUPERCONDUCTORS NEAR THE FERMI-LEVEL SEEN BY OPTICAL-ABSORPTION

The temperature dependence of optical absorption spectra in the visible range has been measured between 60 and 180 K on superconducting, single-crystalline films of YBa2Cu3O7-delta and Bi2Sr2CaCu2O8. The analysis of the experimental results was performed under the assumption of a high ratio between the total valence band width and the width of the narrow hole band. The linear increase of the absorption above T(c) is attributed to a variation of the number density of holes, which is caused by a temperature dependent redistribution of electrons between the conducting CuO2 plane and a second electron subsystem that acts as a charge carrier reservoir. The occurrence of superconductivity at T(c) is reflected in a peculiar behaviour of the optical absorption at T less-than-or-equal-to T(c). It is shown that optical spectroscopy in the visible range provides valuable information about temperature dependent changes of the electronic structure of HTSC near the Fermi level.