Melting of the Abrikosov flux lattice in anisotropic superconductors

It has been proposed that the Abrikosov flux lattice in high-${\mathit{T}}_{\mathit{c}}$ superconductors is melted over a significant fraction of the phase diagram. We provide a thermodynamic argument which establishes that the angular dependence of the melting temperature is controlled by the superconducting mass anisotropy. Using a low-frequency torsional-oscillator technique, this relationship has been tested in untwinned single-crystal ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The results offer decisive support for the melting proposal.