Flux pinning in single-crystal YBa2Cu3O7-x

The authors report a study in which the non-contact techniques of SQUID magnetometry and ac susceptometry have been used to measure critical current density and to observe thermally activated flux creep in single crystals of YBa2Cu3O7-x which show a variation in twin densities under optical microscopic examination. The pinning energy calculated from flux creep measurements in an untwinned crystal of orthorhombic YBa2Cu3O7-x achieved without chemical doping is at the lower end of the distribution of values measured for heavily twinned crystals suggesting that twin boundaries on [110] may play a minor though detectable role, at least at low temperatures, in vortex pinning in YBa2Cu3O7-x. Critical current density measurements at 4.2 K support this result, and recent flux lattice decoration experiments and measurements using ac susceptibility show unequivocal evidence for pinning by twin boundaries