Anisotropic irreversibility of the Abrikosov and Josephson flux dynamics in YBa/sub 2-x/Sr/sub x/Cu/sub 3/O/sub 7-[delta]/ single crystals : Bose-glass and vortex-glass features

Abstract

Very detailed magnetic irreversibility data for fields applied along the c axis or the ab plane of a pure and untwinned YBa₂Cu₃O₇₋₈ single crystal and Sr-doped and heavily twinned YBa₂₋xSrxCu₃O₇₋₈ (x=0.25, 0.37, and 0.5) single crystals are reported. The irreversibility lines Tirr(H) of the pure single crystal show a considerable planar anisotropy but follow the same power-law regime, for both field orientations, arising within the conventional flux-creep theories in the whole field range. Very differently, however, the Tirr(H) lines of the doped superconductors exhibit besides large anisotropies, several different regimes. In fields lower than 8 kOe the Tirr(H) data of the doped samples display the de Almeida–Thouless (AT) and Gabay-Toulouse (GT)-like power-law behaviors, the signature of a frustrated superconductor. For higher-field values, and in particular for H||c, flux dynamics seems to be conventional. However, for H||ab and field values above 30 kOe, the flux dynamics displays sharp directional properties along the twinning planes (TP’s) for rotations about the c axis. This behavior is cusplike, comparable to that caused by columnar defects, which characterizes a Bose-flux-glass phase. We appoint the superconducting granularity and frustration as responsible for the AT and GT behaviors below 8 kOe and the strong anisotropic pinning for H parallel to the TP’s as the cause of the Bose-glass features. On the other hand, the isotropic pinning for large angular displacements or for any angle in fields below 30 kOe is the most probable cause of the vortex-glass features.