Nature 382, 791 - 793 (29 August 1996); doi:10.1038/382791a0

Calorimetric measurement of the latent heat of vortex-lattice melting in untwinned YBa₂Cu₃O_7–

A. Schilling^*, R. A. Fisher^*, N. E. Phillips^*, U. Welp^†, D. Dasgupta^†, W. K. Kwok^† & G. W. Crabtree^†

^*Lawrence Berkeley National Laboratory and Department of Chemistry, University of California, BG4 Giauque, Berkeley, California 94720–1460, USA
^†Materials Science Division & Science and Technology Center for Superconductivity, Argonne National Laboratory, Argonne, Illinois 60439, USA

THE magnetic vortex lattice of copper oxide superconductors in the mixed (field-penetrated) state 'liquefies'^1,2 on increasing the temperature T or the external magnetic field H, giving rise to an ohmic resistivity well below the fluctuation-dominated crossover to the normal state at the upper critical field H _c2(T). Theoretical work suggests that in clean materials this melting is a first-order phase transition³; features in the resistivity^4–6 and magnetization^7–10, as well as results from muon spin rotation¹¹ and neutron-diffraction work¹², have been cited to support this hypothesis. A calorimetric measurement of a latent heat provides the most definitive proof of the occurrence of a first-order transition, but such measurements require very high sensitivity. Here we report calorimetric measurements on an untwinned single crystal of YBa₂Cu₃O_7– that have sufficient precision to clearly resolve the latent heat. The value obtained, 0.45k _B T per vortex per superconducting layer (where k _B is the Boltzmann constant), is consistent with that inferred from magnetization data using the Clapeyron equation. This result is compelling evidence for a first-order transition at a well defined phase boundary H _m(T).

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