Calorimetric Transitions on the Melting Line of the Vortex System as a Function of Oxygen Deficiency in High-PurityYBa2Cu3Ox

We report adiabatic specific heat measurements for a high-purity ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ twinned single crystal. The order of the transition on the melting line of the vortex system $B_m\left(T\right)$ is investigated for three oxygen concentrations. For $x=7.00\mathrm{}$, no critical end point is observed. For $x=6.94\mathrm{}$, first-order transitions give way to second-order transitions above a tricritical point at a field $B_{\mathrm{cr}}=9.5\mathrm{T}$, and the slope of $B_m\left(T\right)$ changes. The latent heat, the slope of $B_m\left(T\right)$, and $B_{\mathrm{cr}}$ increase with $x$. Data for less pure crystals are included for comparison purposes.