3D X-Y scaling of the specific heat of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals

The specific heat of a single crystal sample of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ has been measured with magnetic fields up to 8 T applied parallel to the c axis of the crystal. The results provide strong evidence for the existence of a critical regime within which there is scaling behavior characteristic of the three-dimensional X-Y model with critical exponents consistent with those observed in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$.