Optically induced changes and long-term relaxations of resistivity and critical temperature in He+ irradiated YBa2Cu3Ox

Thin films of YBa2Cu3Ox with superconducting critical temperatures (Tc) ranging from 58 K to 72 K were prepared by pulsed-laser deposition and subsequent irradiation with 75 keV He+ ions. Optical excitation of the films with a He-Ne laser resulted in a small resistivity reduction, when the experiment was carried out a short period of time after the ion irradiation. The value of Tc was essentially unchanged by the optical treatment. Further light excitation experiments did not show a significant effect on the resistivity of the samples. We conclude that ion-irradiated metallic YBa2Cu3Ox does not exhibit the persistent photoconductivity observed in oxygen-depleted samples with comparable critical temperatures. On the other hand we have observed a long-term relaxation of Tc towards larger values that takes place even when the samples are kept in the dark. Our results indicate that Tc may increase after ion irradiation to an asmptotic value about 20% over the initial value on a typical time scale of years. This observation may be important for possible applications of light-ion irradiated YBa2Cu3Ox.