On the nature of two superconducting transitions in the specific heat of PrOs${}_4$Sb${}_{12}$: Effects of crystal grinding

Abstract

Specific heat, dc- and ac-magnetic susceptibility are reported for a large single crystal of PrOs${}_4$Sb${}_{12}$ and, after grinding, its powder. The room temperature effective paramagnetic moment of the crystal was consistent with the Pr${}^{3+}$ ionic configuration and full occupancy of the Pr-sublattice. The crystal showed two distinct anomalies in the specific heat and an overall discontinuity in $C/T$ of more than 1000 mJ/K${}^2$mol. The upper transition (at $T_{c1}$) was rounded, in an agreement with previous reports. The anomaly at $T_{c2}$ was very sharp, consistent with a good quality of the crystal. We observed a shoulder in $\ddot{I}‡$' and two peaks in $\ddot{I}‡$'' below $T_{c1}$. However, there were no signatures in $\ddot{I}‡$' of the lower temperature transition. PrOs${}_4$Sb${}_{12}$ is extremely sensitive to grinding, which suppresses the upper superconducting transition in both the specific heat and magnetic susceptibility. $\hat{I}”C/T_c$ was reduced to 140 mJ/K${}^2$ mol in the powdered sample. Existing data on ground, polished, and sliced crystals suggests the existence of a length scale of order 100 $\hat{I}¼$, characterizing the higher temperature superconducting phase.