Direct observation of vortex lattice melting in Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// single crystals

Scanning Hall probe microscopy has been used to investigate the melting of the vortex solid in Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// single crystals. After waiting several minutes for fluctuations to die away, a flux solid with good six-fold order is observed below the melting line. Incommensurability effects lead to pronounced rotations of the solid as the field is varied. The peak-to-valley corrugation of the vortex lattice drops abruptly and discontinuously to zero at the melting line, consistent with a first order transition. Fits of a pancake vortex model to the data yield a Lindemann parameter C/sub L//spl sim/0.19 in fair agreement with other estimates. There is also a clear indication of softening of the solid at very low fields, though we find no direct evidence for re-entrant melting.