d−WaveScaling Relations in the Mixed-State Specific Heat ofYBa2Cu3O7

The low temperature specific heat $C(T,B)$ of an ${\mathrm{YBa}}_2\mathrm{Cu}{}_3{\mathrm{O}}_{7.00}$ single crystal is measured from 1.2 to 10 K in magnetic fields up to 14 T. The anisotropic component $C_{\mathrm{aniso}}(T,B)\equiv C(T,B\parallel c)-C(T,B\perp c)$, a pure vortex quantity obtained directly from experiment, is found to obey a scaling relation predicted recently for superconductors with line nodes in the gap. Our experimental field and temperature range correspond to a crossover region, where the limit $C_{\mathrm{aniso}}(T,B)\propto {\mathrm{TB}}^{1/2}$ does not strictly apply. The variation of the entropy caused by the magnetic field at low $T$ is thermodynamically compatible with measurements near $T_c$.