Photoinduced phase transitions in cuprate superconductors: ordering vs. doping effects*

Complex oxides posses an extremely rich phase diagram, in which the transitions between different ground states are triggered by small changes of the carrier density or by disorder. Interestingly, the interplay between these two mechanisms –and the resulting phase– can be externally manipulated by illumination. Despite the amount of work performed on cuprate superconductors, a clear picture of the mechanisms governing photo-induced effects has not emerged. In particular, whether doping or ordering effects are dominant has not been settled. Here we investigate that through light illumination effects in oxygen deficient YBa₂Cu₃O_7-x thin films in which different degrees of disorder are induced through i) changes in the growth conditions (“quenching”) and ii) ion irradiation. Generally, light induces an increase in the superconducting critical temperature and in the carrier density coupled to a decrease in the normal state resistivity. However differences in the response of quenched and irradiated samples have been observed, which suggests that light-induced ordering may play an important role in the photoinduced effects.