VORTEX-GLASS MELTING IN SINGLE-CRYSTAL LA1.825SR0.075CUO4

The vortex phase diagram in single crystalline La1.85Sr0.075CuO4 has b een studied using an AC-susceptibility technique. A peak in the out-of -phase (chi '') component of the susceptibility indicates a transition from a pinned flux lattice (FLL) to an unpinned one. This peak is fre quency dependent for all the values of the magnetic field measured (0. 01 to 4 T), and this, as well as the general behavior found in the cup rates, has prompted us to interpret our data as evidence for a vortex- glass to liquid transition in the FLL. The activation energies obtaine d can be fitted to a theory developed by Vinokur et al. Measurements w ith the magnetic field at an angle with the Cu-O planes may also be un derstood qualitatively within this framework.