Anisotropic Latent Heat of Vortex-Lattice Melting in UntwinnedYBa2Cu3O7−δ

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 ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystal. Latent heats and discontinuities in specific heat were observed for each configuration. The entropies of melting and the melting lines $H_{\mathrm{m}}\left(T\right)$ both scale with an anisotropy parameter $\gamma \approx 8$. The specific heat of the vortex fluid is considerably larger than that of the vortex solid (by up to $2\mathrm{mJ}/\mathrm{mole}{\mathrm{K}}^2$), which is not explained by simple arguments based on counting the numbers of thermodynamic degrees of freedom.