Specific heat $C\left(T\right)$ measurements were made on single crystals of the superconducting filled skutterudite series $\mathrm{Pr}{\left({\mathrm{Os}}_{1-x}{\mathrm{Ru}}_x\right)}_4{\mathrm{Sb}}_{12}$ down to $0.6\mathrm{K}$. Crystalline electric field fits in the normal state produced parameters which were in agreement with previous measurements. Bulk superconductivity was observed for all values of the $\mathrm{Ru}$ concentration $x$ with transition temperatures consistent with previous experiments, confirming a minimum in $T_c$ at $x=0.6$. The $C\left(T\right)$ data below $T_c$ appear to be more consistent with power-law behavior for $x=0$ $\left({\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}\right)$, and with exponential behavior for $0.05⩽x⩽0.2$. An enhanced electronic specific heat coefficient $\gamma$ was observed for $x⩽0.4$, further supporting $x\simeq 0.6$ as a critical concentration where the physical properties abruptly change. Significant enhancement of $\Delta C∕T_c$ above the weak-coupling value was observed only for $x=0$ and 0.05.

• Received 27 May 2004

DOI:https://doi.org/10.1103/PhysRevB.71.064508