ANOMALOUS FLUX-PINNING AND FLUX-CREEP NEAR THE CRITICAL-STATE

Magnetization measurements are reported on very high-quality single cr ystals of YBa2Cu3O7-delta. We show that the vortex-entry and the vorte x-exit branches of the hysteresis cycle exhibit very different pinning and creep behaviors at low temperature, near the critical state, Besi des, this asymmetric effect depends strongly on the angle theta of the applied field with the c-axis, It is the highest at theta and T close to zero and increases with the magnitude of the applied field. We sho w that the observed flux behavior cannot be understood in the framewor k of usual pinning models. However, we present strong evidence that it is a twin-boundary effect, which is suppressed by inhomogeneities in less perfect YBCO materials.