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 (Tc1 {approx}1.89 K and Tc2 {approx} 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 {approx}15 mK width at Tc{approx}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-Cnormal)/Cnormal)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 Delta C(Tc1)/Delta C(Tc2) 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 {approx}100 {micro}m, 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 amore » sample exhibiting a single transition and fit the curve Hc2(T) with a two-band model taking into account the appropriate values for the gap as deduced from thermal conductivity measurements.« less