SUPPRESSION OF THE FIRST-ORDER VORTEX MELTING TRANSITION BY INTRINSICPINNING IN YBA2CU3O7-DELTA

The first order vortex melting transition in single crystal YBa2Cu3O7- delta is investigated for magnetic field direction close to H parallel to ab. For exact field alignment H parallel to ab, the ''kink'' in p( T) associated with the first order melting transition is replaced with a continuous resistive transition in the presence of intrinsic pinnin g, indicative of a second order transition with dynamic scaling expone nt s=1.4. In addition, we demonstrate the abatement of the ''kink'' in the temperature dependence of the magnetoresistivity, associated with the first order melting transition for H parallel to c in the presenc e of only six twin boundaries.