OPTICAL SPECTROSCOPY OF SUPERCONDUCTORS M ODIFICATION OF ELECTRONIC-STRUCTURE AND ELECTRONIC PROCESSES IN YBA2CU3O7-DELTA ON UNDERDOPED OVERDOPED REGIME CHANGE

Temperature dependence of absorption coefficient kappa is measured on single-crystal films of YBa2Cu3O7-delta in visible spectrum. Limiting cases of oxygen doping are examined. In the overdoped regime (delta al most-equal-to 0.05) as well as in the underdoped one (delta almost-equ al-to 0.5) the dependence kappa(T) has a peculiarity at the point T(c) . Above T(c) kappa varies substantially with temperature in both cases . dkappa/dT being negative in the former case and positive in the latt er. It is shown that the temperature behavior of absorption is related to the number of holes in the CuO2 plane, n(h). In the underdoped reg ime n(h) increases with temperature due to the electron outflow from t he CuO2 plane to other lattice fragments while in the overdoped regime n(h) decreases with temperature due to the inflow of electrons. The n oticeable scale of the phenomenon is explained by a high ratio of the ionic bond energy to the Fermi energy of holes, and the change in sign is caused by variation in the physical-chemical structure when changi ng from the underdoped regime to the overdoped one.