Vortex liquid disorder and the first order melting transition in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We present transport measurements in high quality untwinned crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ in fields up to 8 T. We find an extremely sharp resistive transition at the first order vortex freezing transition ${\mathit{T}}_{\mathit{m}}$, 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.