Vortex liquid entanglement in twinned YBa2Cu3O7/Y2BaCuO5 composite superconductors

The angular dependence of the in-plane resistivity rho(T,H, theta) of melt textured YBa2Cu3O7/Y2BaCuO5 composites has been measured in a large range o f magnetic fields and temperatures and from them, the intrinsic anisotropy of the superconducting state has been verified following the anisotropic Gi nzburg-Landau approach. The influence of correlated defects like twin bound aries and quenched disorder generated by Y2BaCuO5 precipitates on the pinni ng behavior of these composites in the liquid vortex state is analyzed, and the corresponding phase diagram is determined and compared to that of twin ned single crystals. We show that the irreversibility line displays an upwa rds shift due to twin boundary pinning enabling to define a ''quenched'' Bo se glass transition. A new region in the vortex liquid state is identified where twin boundary pinning defines a partially entangled liquid vortex sta te characterized by a short-range c-axis vortex coherence. The transition t o the entangled liquid phase is experimentally determined. The relevance of this depinning line and its unique position with respect to twinned single crystals is discussed. [S0163-1829(99)01741-5].