Enhancement of c-axis vortex correlation by twin boundaries and columnar defects

We report on transport measurements using the flux transformer configuration in a twinned single crystal of (Formula presented)(Formula presented)(Formula presented) with columnar defects. 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 unirradiated samples of the same thickness. These results and previous work about the effect of twin boundaries on vortex velocity correlation show that phase coherence in the field direction is not an intrinsic property of vortex liquids but one induced by extended defects. © 1997 The American Physical Society.