Vortex liquid disorder and the first order melting transition in YBa2Cu3O7- delta.

We present transport measurements in high quality untwinned crystals of YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] in fields up to 8 T. We find an extremely sharp resistive transition at the first order vortex freezing transition [ital T][sub [ital m]], with width much narrower than the zero field transition width. The resistive transition as a function of magnetic field 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.