Concerning spectroscopic manifestations of quasi-independent electron subsystems in high-temperature superconductors

To investigate the energy spectrum near the Fermi level and temperature variations of its population, the absorption spectrum of YBa2Cu3O7-δ monocrystal films was measured in the temperature region 20 to 200 K. A linear increase of the absorption coefficient H with temperature was observed above the superconducting transition point Tc. Three peculiar points of the dependence H (T) were observed in the region T≤Tc. All the peculiar features of the H (T) are explained in the assumption that H (T)=f(Nh(T),Δ(T)), where Δ is the superconducting gap width and Nh is the number of holes in the oxygen band. Nh increases with temperature due to transfer of electrons from the CuO2 crystallographic plane to another subsystem. The peculiarity of the H (T) at Tc is related to arising of the superconducting gap which affects the oscillator strengths. Below Tc H (T) follows mainly Δ (T) up to a peculiar point Tfr where Nh is frozen. A lack of the thermodynamical equilibrium below Tfr results in a jump of the H (T) near Tfr. (author)