Persistent photoconductivity in oxygen-deficient and ion-irradiated YBa2Cu3Ox

The persistent photoconductivity (PPC) in oxygen-deficient YBa2Cu3Ox (YBCO) has many peculiar features. The illumination of YBCO with visible light or ultraviolet radiation leads to significant changes in the electronic properties: the free carrier concentration increases, the carrier mobility changes and, in metallic samples, the superconducting transition temperature Tc increases. Most intriguing is the persistence of the photo-induced effects up to surprisingly high temperatures. Below 250 K, PPC relaxation is practically unobservable and, at higher temperatures, the decay of the photo-induced changes takes hours or even days to complete. This temperature stability has stimulated the term photodoping for the photo-induced changes. Our studies have shown a pronounced dependence of the photodoping results on the temperature at which the experiment is performed. Hall effect studies demonstrated that the changes of the carrier concentration and of the mobility are completely different at high temperatures compared to lower temperatures. The photo-induced Tc increase was twice as large at high temperatures. Also, at high temperatures, our mobility data very clearly show the sign of a competition between two processes that contribute to the photodoping. We have identified one of the processes as photo-assisted oxygen ordering within the chain layer of YBCO. The investigation of the PPC effect was noticeably promoted by a new technique of growing YBCO thin films on so-called off-axis oriented substrates. From photodoping experiments on such films, the photo-induced changes of the in-plane ρab and of the out-of-plane ρc resistivities were derived. Both resistivity components decrease under illumination with light, but asynchronously, so the electronic anisotropy ρc/ρab gradually changes during the illumination. Our results are consistent with previously proposed models of the PPC based on the charge transfer from the copper-oxide planes to the copper-oxide chains. We obtained evidence that this process is accompanied by a re-arrangement of the oxygen atoms and/or oxygen vacancies in the chain layer. The latter process seems to be more pronounced at high temperatures, most likely because of the higher mobility of the oxygen atoms at those temperatures.