Spectroscopic studies of the persistent in-plane and out-of-plane photoconductivity in 60 K YBa₂Cu₃O_x

Investigations of the wavelength dependence of the so-called photodoping effect in 60 K YBa/sub 2/Cu/sub 3/O/sub x/ were performed using samples grown on "tilted" substrates, allowing to study the photo-induced changes of the in-plane and out-of-plane resistivities, rho /sub ab/ and rho /sub c/, respectively. The photo-excitation was carried out at photon energies from 1.16 to 2.05 eV and at several temperatures between 70 and 305 K. At 100 up to 305 K, the spectral efficiencies of the in-plane and out-of-plane photodoping showed a single maximum at about 1.8 eV. The anisotropy of the spectral efficiency at this maximum was significant at low temperatures, but decreased rapidly when the temperature approached room temperature, indicating an additional contribution to the out-of-plane spectral efficiency at high temperatures. At 70 K, we observed an additional maximum at 1.4 to 1.5 eV that is not predicted by the proposed models for the persistent photoconductivity in YBa/sub 2/Cu/sub 3/O/sub x/, which assume electronic excitations across the charge-transfer gap of 1.7 eV.