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.
Categories: cond-mat.supr-con cond-mat.str-el