We observed a metastable change in the superconducting properties of YBa2Cu3Oy (YBCO) and GdBa2Cu3Oz (GdBCO) induced by X-ray irradiation. Tc0, which is defined as the temperature below which the electric resistance is zero within the measurement accuracy, was increased by up to 1.9 K in YBCO and GdBCO. The irradiation effect was more pronounced for the YBCO sample with lower Tc0. These observations are consistent with those of light irradiation, and are attributable to hole doping in the CuO2 plane via the generation of electron-hole pairs by the X-ray and subsequent trapping of the electrons at oxygen vacancies. The relaxation of the irradiation effect was observed to be within 100 h from the viewpoint of conductivity and Raman spectra, and is explained as the detrapping of the electrons and recombination with holes, similarly to the case of light irradiation.
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