Evolution of crystalline electric field effects, superconductivity, and heavy fermion behavior in the specific heat of Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$

TL;DR

This study investigates how crystalline electric field effects, superconductivity, and heavy fermion behavior evolve in Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ through specific heat measurements, revealing critical concentration points and changes in superconducting behavior.

Contribution

It provides detailed specific heat data across the series, identifying a critical Ru concentration where physical properties change abruptly and clarifying the nature of the superconducting state.

Findings

Bulk superconductivity persists across all compositions.

Superconducting transition temperature $T_c$ has a minimum at $x=0.6$.

Behavior of specific heat below $T_c$ varies with Ru content, indicating different superconducting regimes.

Abstract

Specific heat $C(T)$ measurements were made on single crystals of the superconducting filled skutterudite series Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ down to 0.6 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 Ru concentration $x$ with transition temperatures consistent with previous experiments, confirming a minimum in $T_{c}$ at $x=0.6$. The $C(T)$ data below $T_{c}$ appear to be more consistent with power law behavior for $x=0$ (PrOs$_4$Sb$_{12}$), and with exponential behavior for $0.05 \leq x \leq 0.2$. An enhanced electronic specific heat coefficient $\gamma$ was observed for $x \leq 0.4$, 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 only observed for $x=0$ and $x=0.05$.