VORTEX LIQUID DISORDER AND THE FIRST-ORDER MELTING TRANSITION IN YBA2CU3O7-DELTA

We present transport measurements in high quality untwinned crystals of YBa2Cu3O7-delta in fields up to 8 T. We find an extremely sharp resistive transition at the first order vortex freezing transition T-m, with width much narrower than the zero field transition width. The resistive transition as a function of magnetic held is equally sharp with freezing occurring within 10 Oe in a field of about 6 T. There is dramatic asymmetric hysteresis in the field dependence of the resistivity related to the unpinned nature of the vortex liquid state.