Spectroscopic studies of the persistent in-plane and out-of-plane photoconductivity in 60 K YBa2Cu3Ox

Investigations of the wavelength dependence of the so-called photodoping ef fect in 60 K YBa2Cu3Ox, were performed using samples grown on "tilted" subs trates, allowing to study the photo-induced changes of the in-plane and out -of-plane resistivities, rho(ab) and rho(c) respectively. The photo-excitat ion 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 efficie ncies 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 tempera ture approached room temperature, indicating an additional contribution to the out-of-plane spectral efficiency at high temperatures. At 70 K, we obse rved an additional maximum at 1.4 to 1.5 eV that is not predicted by the pr oposed models for the persistent photoconductivity in YBa2Cu3Ox,, which ass ume electronic excitations across the charge-transfer gap of 1.7 eV.