OBSERVATION OF THE VORTEX MELTING LINE IN YBA2CU3O7 BY SPECIFIC-HEAT MEASUREMENTS IN VERY HIGH MAGNETIC-FIELD - H-LESS-THAN-OR-EQUAL-TO-23 TESLA

We have performed specific heat (C) measurements using a very sensitiv e adiabatic calorimeter on a twinned, high-purity single crystal (18mg ) of YBa2Cu3O7 (referred thereafter as YBCO). In the same crystal, for 5T less than or equal to B less than or equal to 16T, very well defin ed peaks in C have been reported at the vortex lattice melting transit ion occuring at a characteristic temperature T-m[1]. We tracked this l ine B(T-m) up to 21.5 Tesla and, finally, no anomaly was detectable in 23 Tesla, suggesting the existence of an ending critical point for a characteristic magnetic field B, 21.5T less than or equal to B* less than or equal to 23T. In these very strong magnetic fields, the shape of the anomaly in C associated with the freezing of the vortex-liquid state becomes strongly dependent on the experimental conditions.