OBSERVATION OF 2ND-ORDER TRANSITIONS BELOW T-C IN THE SPECIFIC-HEAT OF YBA2CU3OX - CASE FOR THE MELTING OF A VORTEX-GLASS

High accuracy specific heat experiments in closely spaced fields from 0 to 14 T an reported for a 0.3 g crystal of YBa2Cu3Ox. Specific heat steps are consistently observed on a line B-m(T) that is well separate d from the upper critical field cross-over B-c2(T). Such second-order transitions are predicted to occur when a vortex glass melts. The B-m( T) line is studied for different oxygen concentrations, using the same sample. The anisotropy of the melting field B-m and that of the upper critical field B-c2 are found to be equal. B-m is nearly proportional to the mass anisotropy ratio (m(ab)/m(c))(1/2) < 1. The lines B-m(T) and B-c2(T) move in opposite directions when the anisotropy increases. The XY critical scaling properties of the specific heat step, the ana lytical shape of the transition line B-m(T) proportional to (1 -T/T-c) (approximate to 4/3) and the observation of critical points are consis tent with a second-order transition. The transition line B-m(T) coinci des with that of high purity YBa2Cu3Ox crystals with reduced pinning a nd for which first-order transitions have been reported. (C) 1998 Else vier Science B.V.