PRODUCTION AND IDENTIFICATION OF FLUX-PINNING DEFECTS BY ELECTRON-IRRADIATION IN YBA2CU3O7-X SINGLE-CRYSTALS

An enhancement in J(c) of YBa2Cu3O7-x single crystals in a magnetic field is observed after irradiation with 1-MeV electrons. Typically, a factor-of-2 increase in J(c) is deduced from magnetic hysteresis loops at 10 K and 1 T with H parallel-to c. This enhancement is about 1/2 of that produced by proton and neutron irradiations under similar measurement conditions. In situ transmission-electron-microscopy studies found no visible defects induced by electron irradiation, which means that point defects or small clusters (of size < 2 nm) are responsible for the extra pinning. Annealing studies suggest that effective pinning centers for H parallel-to c do not include oxygen vacancies in the Cu-O chains. Based on calculations of cross sections for displacements on the different sublattices, and in conjunction with the results of a J(c) calculation by Kes, we suggest that the most likely pinning defect is the displacement of a Cu atom from the CuO2-plane sites.