SPECIFIC-HEAT PEAKS OBSERVED UP TO 16 T ON THE MELTING LINE OF THE VORTEX LATTICE IN YBA2CU3O7

Adiabatic specific heat experiments on YBa2Cu3O7.00 are performed in m agnetic fields from 0 to 16 T (B parallel to c and B perpendicular to c). The twinned, 99.995% pure 18 mg crystal is grown in an inert BaZrO 3 crucible, and oxidized in high oxygen pressure to suppress the magne tic ''fishtail'' effect. First-order-like specific heat peaks rising u p to 1.1% above the background are observed on the melting line of the vortex lattice from 6 to 16 T (B parallel to c). The entropy under th ese peaks is 0.6 +/- 0.1 k(B)/vortex/layer. The fields B-m and peak te mperatures T-m obey the relation B-m = B-0(1 - T-m/T-c)(4/3) with B-0 = 133 T. The order of the transition is investigated. Only faint anoma lies are observed for B parallel to ab. Thermodynamically incompatible specific heat steps and magnetization jumps are observed below 6 T (B parallel to c), suggesting irreversible glassy behaviour. No such fea tures are seen in an YBa2Cu3O6.92 crystal that is characterized by str ong pinning.