Thermodynamic Properties of Superconducting and Non-Superconducting Pr₂Ba₄Cu₇O_(15-δ) Compounds with Metallic Double Chains

To examine the thermodynamic properties of Pr$_{2}$Ba$_{4}$Cu$_{7}$O$_{15-\delta }$ compounds with metallic CuO double chains, we measured the specific heats of superconducting and non-superconducting polycrystalline samples at low temperatures (1.8-40 K) under various magnetic fields (up to 9 T). In the as-sintered non-superconducting sample, a $\lambda-$like enhancement in the specific heat measurement appeared near the antiferromagnetic transition temperature $T_{N}=17$ K. In contrast, the reduced superconducting sample with $T_{c,on}=26.5$ K exhibited no obvious superconducting anomaly in its specific data, but a Schottky-like broad maximum appeared at low temperatures. The Schottky-like anomaly was attributed to low-lying quasi-triplet splitting of Pr$^{3+}$ ions under the crystal field effect.