0 30 55 93 v 1 2 6 M ay 2 00 3 Observation of vortex lattice melting in large untwinned YBa 2 Cu 3 O 7 − x single crystals

We present a study of the vortex lattice in untwinned YBa2Cu3O7−x crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements.