CONTRIBUTIONS TO PHOTODOPING IN OXYGEN-DEFICIENT YBA2CU3OX FILMS

We report our studies on the superconducting and normal-state properti es of metallic YBa2Cu3Ox, thin films (T-c,T-mid approximate to 52 K) e xposed to long-term white-light illumination (photodoping). It was obs erved that the effects of photoexcitation strongly depended on the tem perature at which the photodoping was performed. At low temperatures, both tile Hall mobility and the Hall number were photoenhanced, wherea s; 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 T -c, caused by photodoping, exhibited the same temperature dependence a s the enhancement of the Hall number, being largest (Delta T-c approxi mate to 2.6 K) at high temperatures. From the asynchronous behavior of tile 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 p roperties of tile photoexcited state are strongly temperature dependen t. Studies of the relaxation of the photoexcited state at 290 K showed an unexpectedly short relaxation time of the Hall mobility after term ination of the illumination. The relaxation saturated somewhat below t he initial, undoped value, similarly to the decrease of the Hall mobil ity, observed upon long illumination. These latter findings give evide nce for a competition between the oxygen ordering and thermal disorder ing processes during and after the photoexcitation in the high-tempera ture range.