Experimental evidence for a first-order vortex-lattice-melting transition in untwinned, single crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We report on current-voltage measurements in clean, untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ single crystals with picovolt voltage sensitivity and millikelvin temperature resolution in magnetic fields ranging up to 7 T. We find evidence for a melting transition in the vortex lattice which is hysteretic in both temperature and magnetic field. The measured thermal and magnetic hysteresis widths are related by the local slope of the phase boundary. This strongly supports the picture that, in the clean limit, the melting transition of the Abrikosov vortex lattice is a first-order phase transition.