SUPERHEATING OF THE ABRIKOSOV FLUX LATTICE

We report on thermal cycling of an YBa2Cu3O7 single crystal close to t he melting temperature of the flux lattice for H(parallel-to)ab. Trans port current is along the c axis and the temperature stability is bett er than 1 mK. The melting (freezing) transition of the flux lattice oc curs at different temperatures (DELTAT = 7 mK) between the cooling and the heating cycle. Hysteresis disappears for slightly increased curre nt density (0.1 A/cm2). Comparison of thermal cycles reveals that the Abrikosov lattice superheats, in contrast with the usual behavior in r eal crystals where surface premelting is important. This can be due to its imbedding in the real crystal and the consequent surface pinning.