CALORIMETRIC MEASUREMENT OF THE LATENT-HEAT OF VORTEX-LATTICE MELTINGIN UNTWINNED YBA2CU3O7-DELTA

THE magnetic vortex lattice of copper oxide superconductors in the mix ed (field-penetrated) state 'liquefies'(1,2) on increasing the tempera ture T or the external magnetic field H, giving rise to an ohmic resis tivity well below the fluctuation-dominated crossover to the normal st ate at the upper critical field H-c2(T), Theoretical work suggests tha t in clean materials this melting is a first-order phase transition(3) ; features in the resistivity(4-6) and magnetization(7-10), as well as results from muon spin rotation(11) and neutron-diffraction work(12), have been cited to support this hypothesis, A calorimetric measuremen t of a latent heat provides the most definitive proof of the occurrenc e of a first-order transition, but such measurements require very high sensitivity, Here we report calorimetric measurements on an untwinned single crystal of YBa2Cu3O7-delta that have sufficient precision to c learly resolve the latent heat. The value obtained, similar to 0.45 k( B)T per vortex per superconducting layer (where k(B) is the Boltzmann constant), is consistent with that inferred from magnetization data us ing the Clapeyron equation, This result is compelling evidence for a f irst-order transition at a well defined phase boundary H-m(T).