FLUX PINNING BY ORDERED OXYGEN-DEFICIENT PHASES IN NEARLY STOICHIOMETRIC YBA2CU3O7-DELTA SINGLE-CRYSTALS

We propose that the critical currents of nearly stoichiometric YBa2Cu3O7-delta single crystals (delta < 0.1) are determined by fluxon pinning at ordered oxygen-vacancy domains. Since these domains are also superconducting, albeit with lower T(c) and H(c) than the stoichiometric matrix, their pinning strength at a given temperature will increase as they are driven normal by an increasing applied field. Experimentally, the magnetization hysteresis curves display a secondary maximum, characteristic of pinning by a superconducting second phase. The ordered-vacancy domains are assumed to result from a spinodal decomposition. The density and size of the domains were calculated on this assumption. The bulk pinning force was computed by direct summation. In the limit of low temperatures, the calculated values of critical current density J(c) are in good agreement with measurements on high-quality single crystals.