Production and identification of flux-pinning defects by electron irradiation in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ single crystals

An enhancement in ${\mathit{J}}_{\mathit{c}}$ of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ single crystals in a magnetic field is observed after irradiation with 1-MeV electrons. Typically, a factor-of-2 increase in ${\mathit{J}}_{\mathit{c}}$ is deduced from magnetic hysteresis loops at 10 K and 1 T with H∥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∥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 ${\mathit{J}}_{\mathit{c}}$ calculation by Kes, we suggest that the most likely pinning defect is the displacement of a Cu atom from the ${\mathrm{CuO}}_2$-plane sites.