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In situ studies of the superconducting and normal-state transport properties in partially oxygen-depleted, metallic YBa 2 Cu 3 O x (T c,mid Ϸ52 K) thin films exposed to long-term white-light illumination ͑photodoping͒ are reported. We observed that the effects of photoexcitation strongly depended on the temperature at which the photodoping was performed. The Hall number increased during the illumination in the entire tested temperature range from 70 to 290 K, with the strongest increase near room temperature, whereas, the Hall mobility increased steadily only upon low-temperature illumination. At temperatures above 250 K, it showed an abrupt initial increase followed by a long-term decrease. At high temperatures, the Hall quantities react on the impact of light excitation independently from each other, which strongly suggests that both the photoassisted oxygen ordering and charge-transfer mechanisms contribute to photodoping, the former acting mainly on the mobility, while the latter on the density of charge carriers. The photoinduced enhancement of the superconducting transition temperature T c exhibited essentially the same temperature dependence as the enhancement of the Hall number, being largest (⌬T c Ϸ2.6 K) for the illumination performed at high temperatures. Thus, the T c enhancement results from the change of the density more likely than of the mobility of the charge carriers. ͓S0163-1829͑98͒05714-2͔

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High quality thin films of oxygen-depleted YBa2Cu3Ox (x≈6.6) were prepared by pulsed-laser deposition on SrTiO3, substrates that were cut with tilt angles of 10° and 20° with respect to the [001] direction, resulting in a steplike growth of the layers. The resistance showed a semiconducting behavior along the projection of the c axis to the film surface, but a metallic behavior in the perpendicular direction, indicating that the former is dominated by the c-axis resistivity and the latter by the ab-plane resistivity of YBa2Cu3Ox. Long-term illumination of the samples with a 100 W halogen lamp resulted in a significant conductivity enhancement in both directions. The photoinduced change of the out-of-plane resistance vs temperature characteristics is comparable to the effect of large hydrostatic pressure, introducing structural changes similar to that of photoexcitation.