Double superconducting transition in the filled skutterudite PrOs₄Sb₁₂ and sample characterizations

Through the characterization of many samples of the filled skutterudite compound PrOs₄Sb₁₂, we found that even though the double superconducting transition in the specific heat (T_c1∼1.89 K and T_c2∼1.72 K) appears in samples of good quality, there are various pieces of evidence that cast doubt on its intrinsic nature. First, three samples exhibit a single sharp transition of ∼15 mK width at T_c∼1.7 K. The quality of the samples exhibiting a single transition is clearly better; the width of the transition is smallest and the normalized specific heat jump ((C−C_normal)/C_normal)_Tc is larger than the sum of the two specific heat jumps when a double transition exists. Second, the ratio of the two specific heat jumps ΔC(T_c1)/ΔC(T_c2) exhibits a wide variation with samples not only among different batches but also even within a batch. Finally, this ratio was strongly reduced by polishing a sample down to ∼100 μm, which points to bulk inhomogeneities as an origin of the spurious transition. Our x-ray diffraction analysis points to Pr-vacancy problem in the samples, but it does not show that they are the origin of the double transition. We provide the superconducting phase diagram under magnetic field of a sample exhibiting a single transition and fit the curve H_c2(T) with a two-band model taking into account the appropriate values for the gap as deduced from thermal conductivity measurements.