Persistent photoconductivity in oxygen-deficient and ion-irradiatedYBa2Cu3Ox

Abstract

The persistent photoconductivity (PPC) in oxygen-deficientYBa2Cu3Ox (YBCO) has many peculiar features. The illumination of YBCO with visiblelight or ultraviolet radiation leads to significant changes in the electronicproperties: the free carrier concentration increases, the carrier mobilitychanges and, in metallic samples, the superconducting transition temperatureTc increases. Most intriguing is the persistence of the photo-induced effects up to surprisinglyhigh temperatures. Below 250 K, PPC relaxation is practically unobservable and, at highertemperatures, 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-inducedchanges. Our studies have shown a pronounced dependence of the photodoping results onthe temperature at which the experiment is performed. Hall effect studies demonstratedthat the changes of the carrier concentration and of the mobility are completelydifferent at high temperatures compared to lower temperatures. The photo-inducedTc increase was twice as large at high temperatures. Also, at high temperatures, our mobilitydata very clearly show the sign of a competition between two processes that contribute tothe photodoping. We have identified one of the processes as photo-assisted oxygen orderingwithin the chain layer of YBCO. The investigation of the PPC effect was noticeably promotedby 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-planeρab and of theout-of-plane ρc resistivities were derived. Both resistivity components decrease underillumination with light, but asynchronously, so the electronic anisotropyρc/ρab gradually changes during the illumination. Our results are consistent with previouslyproposed models of the PPC based on the charge transfer from the copper-oxide planes tothe copper-oxide chains. We obtained evidence that this process is accompanied by are-arrangement of the oxygen atoms and/or oxygen vacancies in the chain layer.The latter process seems to be more pronounced at high temperatures, mostlikely because of the higher mobility of the oxygen atoms at those temperatures.Very recent results from the photo-excitation of ion-irradiated samples indicate that theobserved photo-induced changes have a different origin than the PPC in oxygen-deficientYBCO.