Specific-heat jump at ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$

The superconducting free-energy surface of grain-aligned ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ was measured to determine whether these electron-carrier materials have thermodynamic variables similar to the other high-temperature superconductors. There is a wide area in the H-T plane near ${\mathit{T}}_{\mathit{c}}$, where the magnetization is reversible, just as was found for the Y, Bi, and Tl cuprates. The magnetic-field dependence of the specific heat derived from these data, ${\mathit{C}}_0$-${\mathit{C}}_{\mathit{H}}$, shows a peak in the fluctuation region near ${\mathit{T}}_{\mathit{c}}$ of about 1.2 mJ/${\mathrm{cm}}^3$ K for H=2 T and H∥c. At lower temperatures, below 16 K, the magnetically derived specific heat is nearly independent of field just as was found for the other high-${\mathit{T}}_{\mathit{c}}$ materials. A study of the transformation from the reversible to the irreversible regime using flux-creep and critical-current studies shows that the pinning potential to thermal energy ratio, ${\mathit{U}}_{\mathrm{eff}}$/kT, gradually rises from 2 to 20 at roughly the same value of reduced temperature, T/${\mathit{T}}_{\mathit{c}}$, as the Bi and Tl compounds.