Mechanisms of photodoping in oxygen-deficientYBa2Cu3Oxfilms studied byin situtransport measurements

In situ studies of the superconducting and normal-state transport properties in partially oxygen-depleted, metallic ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ ${(T}_{c,\mathrm{mid}}\approx 52\mathrm{K})$ thin films exposed to long-term white-light illumination (photodoping) 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 $\left(\Delta T_c\approx 2.6\mathrm{K}\right)$ 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.