ENHANCEMENT OF C-AXIS VORTEX CORRELATION BY TWIN BOUNDARIES AND COLUMNAR DEFECTS IN YBA2CU3O7-DELTA

We report on transport measurements using the flux transformer configu ration in a twinned single crystal of YBa2Cu3O7-delta with columnar de fects. The vortex liquid shows velocity correlation along the c axis ( the direction of the applied magnetic field and the columnar defects) at temperatures that correspond to an uncorrelated liquid in unirradia ted samples of the same thickness. These results and previous work abo ut the effect of twin boundaries on vortex velocity correlation show t hat phase coherence in the field direction is not an intrinsic propert y of vortex liquids but one induced by extended defects.