Double superconducting transition in the filled skutterudite PrOs4Sb12 and sample characterizations

TL;DR

This study thoroughly characterizes PrOs4Sb12 samples, examining the double superconducting transition, and suggests it may not be an intrinsic property but influenced by sample quality and defects.

Contribution

It provides evidence that the double superconducting transition in PrOs4Sb12 is likely extrinsic, influenced by sample preparation and defects, challenging previous assumptions of intrinsic behavior.

Findings

Double transition correlates with sample quality and defects.

Polishing reduces or eliminates the double transition.

Single transition samples show sharper, more robust superconducting features.

Abstract

A thorough characterization of many samples of the filled skutterudite compound PrOs4Sb12 is provided. We find that the double superconducting transition in the specific heat Tc1~1.89K and Tc2~1.72K tends to appear in samples with a large residual resistivity ratio, large specific heat jump at the superconducting transition and with the highest absolute value of the specific heat above Tc1. However, we present evidence which casts doubt on the intrinsic nature of the double superconducting transition. The ratio of the two specific heat jumps \Delta C(Tc1)/\Delta C(Tc2) shows a wide range of values on crystals from different batches but also within the same batch. This ratio was strongly reduced by polishing a sample down to 120um. Remarkably, three samples exhibit a single sharp transition of ~15mK in width at Tc~1.7K. The normalized specific heat jump (C-Cnormal)/Cnormal at Tc of two of them is higher than ~32% so larger than the sum of the two specific heat jumps when a double transition exists. As an evidence of better quality, the slope in the transition is at least two time steeper. We discuss the origins of the double transition; in particular we consider, based on X-ray diffraction results, a scenario involving Pr-vacancies. The superconducting phase diagram under magnetic field of a sample with a single transition is fitted with a two-band model taking into account the good values for the gap as deduced from thermal conductivity measurements.