Anisotropic latent heat of vortex-lattice melting in untwinned YBa2Cu3O7-delta

We report a distinct thermal signature of the first-order vortex-lattice melting for the external magnetic field H both parallel and perpendicular to the c axis of an untwinned YBa2Cu3O7-delta single crystal. Latent heats and discontinuities in specific heat were observed for each configuration. The entropies of melting and the melting lines H-m(T) both scale with an anisotropy parameter gamma approximate to 8. The specific heat of the vortex fluid is considerably larger than that of the vortex solid (by up to 2 mJ/mole K-2), which is not explained by simple arguments based on counting the numbers of thermodynamic degrees of freedom.