Thermodynamic properties of superconducting and non-superconducting Pr₂Ba₄Cu₇O_{15 - δ} compounds with metallic double chains

To examine the thermodynamic properties of Pr₂Ba₄Cu₇O_15-δ 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 λ-like enhancement in the specific heat measurement appeared near the antiferromagnetic transition temperature _TN=17 K. In contrast, the reduced superconducting sample with Tcon=26.5 K exhibited no obvious superconducting anomaly in its specific heat data, but a Schottky-like broad maximum appeared at low temperatures. The Schottky-like anomaly was attributed to low-lying quasi-triplet splitting of ^Pr3+ ions under the crystal field effect.