Transmission spectra of YBa2Cu3O7-δ crystal films: temperature changes of excitone lines at superconducting transition

Measurements of transmission spectra of YBa2Cu3O7-δ monocrystal films in the energy range hω = 1.5-3.0 eV were conducted in 300-25 K range. Two transmission lines (1.7 and 2.2 eV) were revealed. Their intensity changes critically in the point of space superconducting transition at Tc=82 K. At temperature below Tc, the transmission doesn't depend on temperature, but in the region of T>Tc, the transmission decreases with temperature growth. Data analysis shows, that temperature change of transmission is dictated by changes of metallization degree of CuO2 layers. This testifies to the exciton nature of 1.7 and 2.2 eV lines