Optically induced changes and long-term relaxations of resistivity and critical temperature in He + irradiated YBa 2Cu 3O x

Abstract

Thin films of YBa 2Cu 3O x with superconducting critical temperatures ( T c) 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 T c 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 YBa 2Cu 3O x 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 T c towards larger values that takes place even when the samples are kept in the dark. Our results indicate that T c 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 YBa 2Cu 3O x.