Superheating of the Abrikosov flux lattice

We report on thermal cycling of an ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ single crystal close to the melting temperature of the flux lattice for H‖‖ab. Transport current is along the c axis and the temperature stability is better than 1 mK. The melting (freezing) transition of the flux lattice occurs at different temperatures (ΔT=7 mK) between the cooling and the heating cycle. Hysteresis disappears for slightly increased current density (0.1 A/${\mathrm{cm}}^2$). Comparison of thermal cycles reveals that the Abrikosiv lattice superheats, in contrast with the usual behavior in real crystals where surface premelting is important. This can be due to its imbedding in the real crystal and the consequent surface pinning.