Unrenormalized ultrasound attenuation in the heavy-fermion state

Quantitative studies of ultrasound absorption in the heavy-fermion state of ${\mathrm{UPt}}_3$ and ${\mathrm{UBe}}_{13}$ are reported. The magnitude of the absorption due to electrons in the normal state is not enhanced compared to that of ordinary metals, indicating a cancellation of the mass enhancement by a reduction of the electron-phonon coupling parameter. This implies that the mass enhancement is described by a different Landau Fermi-liquid parameter than in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. The $T^2$ variation of the normal-state sound velocity at lowest temperatures is consistent with the large electronic specific heat.