Melting of the flux line lattice observed by specific heat experiments in YBa2Cu3O7-delta

High. resolution adiabatic specific heat experiments on YBa2Cu3O7-delta (0 less than or equal to delta less than or equal to 0.05) are performed in magnetic fields from 0 to 14 T (B parallel to c and B perpendicular to c). In a 0.3 gram, twinned crystal with strong pinning, a step is consistently observed cat the melting temperature T-m of the vortex solid up to a critical point that depends on delta. The field B-m and step temperature T-m obey the relation B-m = B-m0(delta).(1-T-m/T-c)(approximate to 4/3). The anisotropy of B-m and that of the upper critical field B-c2 are found to be equal. Alternatively, in, a 18 mg, twinned crystal of high purity with low pinning first-order-like specific heat peaks are observed on the melting line from 8 to 14 T. The entropy under these peaks is approximate to 0.5 k(B)/vortex/bilayer. These characteristic features are attributed to the melting of a vortex glass in the former case and that of a vortex lattice in the latter case.