MECHANISMS OF PHOTODOPING IN OXYGEN-DEFICIENT YBA2CU3OX FILMS STUDIEDBY IN-SITU TRANSPORT MEASUREMENTS

In situ studies of the superconducting and normal-state transport prop el-ties in partially oxygen-depleted, metallic YBa2Cu3Ox (T-c.mid appr oximate to 52 K) thin films exposed to long-term white-light illuminat ion (photodoping) an reported. We observed that the effects of photoex citation strongly depended on the temperature at which the photodoping was performed. The Hall number increased during the illumination in t he entire tested temperature range from 70 to 290 K, with the stronges t increase near room temperature, whereas, the Hall mobility increased steadily only upon low-temperature illumination. At temperatures abov e 250 K, it showed an abrupt initial increase followed by a long-term decrease. At high temperatures, the Hall quantities react on the impac t of light excitation independently from each other, which strongly su ggests 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 pho toinduced enhancement of the superconducting transition temperature T- c exhibited essentially the same temperature dependence as the enhance ment of the Hall number, being largest (Delta T-c approximate to 2.6 K ) for the illumination performed at high temperatures. Thus, the T-c e nhancement results from the change of the density more likely than of the mobility of the charge carriers.