A Collective Heavy Fermion State and Superconductivity in Pr$_{1-x}$La$_x$Os$_4$Sb$_{12}$: Specific Heat and Susceptibility Study

TL;DR

This study investigates how La-doping affects the heavy fermion state and superconductivity in Pr$_{1-x}$La$_x$Os$_4$Sb$_{12}$, revealing that quadrupolar fluctuations likely drive the heavy fermion behavior.

Contribution

It demonstrates that the heavy fermion state in PrOs$_4$Sb$_{12}$ is non-single impurity in origin and is likely due to critical quadrupolar fluctuations, using specific heat and susceptibility measurements.

Findings

La-doping does not change CEF energies of Pr significantly.

The superconducting transition temperature $T_c$ varies weakly with La-doping.

The discontinuity in $C/T$ at $T_c$ decreases with La-doping, indicating a change in the heavy fermion state.

Abstract

Low temperature susceptibility and specific heat for single crystals of Pr$_{1-x}$La$_x$Os$_4$Sb$_{12}$ (0 $\leq x$ $\leq 1$) are reported. La-doping leaves CEF energies of Pr essentially unchanged. The average $T_c$ is only weakly affected by the La-substitution and varies approximately linearly between the end-compounds. The second superconducting transition disappears between $x$=0.05 and 0.1. The discontinuity in $C/T$ at $T_c$, on the other hand, is drastically reduced from about 1000 mJ/K$^2$mol for $x$=0 to 200 mJ/K$^2$mol for $x$=0.2. $\Delta C/T_c$ decreases further for alloys corresponding to $x\geq 0.6$ to the value of a conventional superconductor LaOs$_4$Sb$_{12}$. This behavior implies non-single impurity origin of the heavy fermion state in PrOs$_4$Sb$_{12}$. We argue that critical quadrupolar fluctuations are responsible for this heavy fermion state.