Abstract
Very detailed magnetoresistivity and magnetic irreversibility data for fields applied along the ab-plane of the heavily twinned YBa 2− x Sr x Cu 3O 7− δ ( x=0.1 and 0.37) single crystals are reported. While the resistive transition evidences a granular superconducting character, the magnetic irreversibility line ( T irr( H)) exhibits several different regimes and crossovers. For fields higher than 10 kOe and parallel to the twinning planes (TPs) the flux-dynamics conforms to the standards of the conventional flux-creep theory. However, under fields above 20 kOe, the flux-dynamics displays anisotropic properties in connection with the TPs, probably the signature of a Bose-flux-glass phase. In an intermediary field region the flux-dynamics is isotropic about the c-axis of the single crystal, characterizing a vortex-glass phase. In a low field region ( H<5 kOe) the irreversibility data follow the de Almeida–Thouless (AT) and Gabay–Toulouse (GT) like power law behaviors, usually observed in granular superconductors. We appoint frustration of the superconducting grain aggregate as responsible for the AT and GT like power law behaviors below 5 kOe, while the pinning of the TPs is responsible for the anisotropy in the highest fields that possibly is a signature of the Bose-flux-glass.
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