A Collective Heavy Fermion State and Superconductivity in Pr₁₋ₓLaₓOs₄Sb₁₂: Specific Heat and Susceptibility Study

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Low temperature susceptibility and specific heat for single crystals of Pr₁₋ₓLaₓOs₄Sb₁₂ (0 ≤ x ≤ 1) are reported. La-doping leaves CEF energies of Pr essentially unchanged. The average Tc 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 Tc, on the other hand, is drastically reduced from about 1000 mJ/K²mol for x=0 to 200 mJ/K²mol for x=0.2. ΔC/Tc decreases further for alloys corresponding to x≥ 0.6 to the value of a conventional superconductor LaOs₄Sb₁₂. This behavior implies non-single impurity origin of the heavy fermion state in PrOs₄Sb₁₂. We argue that critical quadrupolar fluctuations are responsible for this heavy fermion state.