Contributions to photodoping in oxygen-deficient YBa2Cu3Ox films

We report our studies on the superconducting and normal-state properties of metallic YBa2Cu3Ox thin films (Tc,mid∼ 52 K) exposed to long-term white-light illumination (photodoping). It was observed that the effects of photoexcitation strongly depended on the temperature at which the photodoping was performed. At low temperatures, both the Hall mobility and the Hall number were photoenhanced, whereas, at temperatures slightly below room temperature, the Hall mobility initially showed an abrupt increase followed by a long-term decrease, and the Hall number increased even stronger than at low temperatures. The enhancement of the film's superconducting transition temperature Tc, caused by photodoping, exhibited the same temperature dependence as the enhancement of the Hall number, being largest (ΔTc ∼ 2.6 K) at high temperatures. From the asynchronous behavior of the Hall quantities, we conclude that both the photoassisted oxygen ordering and charge transfer mechanisms contribute to photodoping. The relative contributions of both mechanisms and, thus, the electronic properties of the photoexcited state are strongly temperature dependent. Studies of the relaxation of the photoexcited state at 290 K showed an unexpectedly short relaxation time of the Hall mobility after termination of the illumination. The relaxation saturated somewhat below the initial, undoped value, similarly to the decrease of the Hall mobility,observed upon long illumination. These latter findings give evidence for a competition between the oxygen ordering and thermal disordering processes during and after the photoexcitation in the high-temperature range. (orig.)