Vortex slush regime in the Josephson vortex phase diagram of 60-K ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystals

We have studied the Josephson vortex phase diagram of 60-K ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystals by measuring the in-plane resistivity. We find that the phase transition of the Josephson vortices in high magnetic fields above 60 kOe is a two-stage process which consists of first-order vortex liquid-to-slush and second-order vortex slush-to-glass transitions, while a single second-order vortex liquid-to-glass transition is observed in low magnetic fields below 50 kOe. The obtained results show that the vortex slush appears as the intermediate regime between the ordinary vortex liquid and glassy phases in the Josephson vortex system.