Double superconducting transition in the filled skutteruditePrOs4Sb12and sample characterizations

Abstract

Through the characterization of many samples of the filled skutterudite compound ${\text{PrOs}}_{4}{\text{Sb}}_{12}$, we found that even though the double superconducting transition in the specific heat (${T}_{c1}\ensuremath{\sim}1.89\text{ }\text{K}$ and ${T}_{c2}\ensuremath{\sim}1.72\text{ }\text{K}$) 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 $\ensuremath{\sim}15\text{ }\text{mK}$ width at ${T}_{c}\ensuremath{\sim}1.7\text{ }\text{K}$. 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 ${\phantom{(}(C\ensuremath{-}{C}_{\text{normal}})/{C}_{\text{normal}})}_{{T}_{c}}$ 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 $\frac{\ensuremath{\Delta}C({T}_{c1})}{\ensuremath{\Delta}C({T}_{c2})}$ 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 $\ensuremath{\sim}100\text{ }\ensuremath{\mu}\text{m}$, 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 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.