Specific heat ofGdBa2Cu3O7−δin the normal and superconducting states

The specific heat $c$, resistance $R$, and magnetic susceptibility $\chi$ of the high-temperature superconductor $\mathrm{Gd}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ have been measured from 1.5 to 180 K and compared with those of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ and ${\mathrm{La}}_{1.85}$ ${\mathrm{Ba}}_{0.15}$Cu${\mathrm{O}}_4$. For the Gd compound, an antiferromagnetic transition is seen in the specific-heat data at 2.24 K, and a Curie-Weiss dependence of $\chi$ is also seen, with $7.98{\mu }_B$ per Gd ion and $\frac{1}{\chi }$ extrapolating to zero at -5.89 K. The entropy associated with this peak in the specific heat is consistent with an assignment of $J=\frac{7}{2}$ to the total angular-momentum quantum number of the Gd ion, indicating that crystal-field effects are small. The specific heat of the Gd compound is larger than that of the Y compound at these temperatures; this is attributed to the larger mass of the Gd ion. A peak in $\frac{c}{T^3}$ near 20 K in the Gd compound indicates a sharply defined peak in the phonon density of states near 10 meV. The superconducting transition temperature $T_c$ of $\mathrm{Gd}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ indicated by the specific-heat data is 93.8 K, and the jump in the specific heat at $T_c$ is 301 mJ/K g-atom, in agreement with previous observations for $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. Evidence is seen for a temperature-dependent phonon density of states in $\mathrm{Gd}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$, and it is pointed out that such a temperature dependence is also seen in previously published data for ${\mathrm{La}}_{1.85}$ ${\mathrm{Ba}}_{0.15}$Cu${\mathrm{O}}_4$ and ${\mathrm{La}}_{1.8}$ ${\mathrm{Sr}}_{0.2}$Cu