Atomic dynamics of low-lying rare-earth guest modes in heavy fermion filled skutterudites ROs ₄Sb ₁₂ (R = light rare-earth)

The atomic dynamics of the guest modes in heavy fermion filled skutterudites ROs ₄Sb ₁₂ were investigated by inelastic x-ray scattering. Comparison of the phonon dispersion relations for the filled skutterudite SmOs ₄Sb ₁₂ with those for the unfilled skutterudite IrSb ₃ indicates that the guest modes in filled skutterudites are observed as additional modes in the unfilled skutterudites. The dispersion relations for IrSb ₃ and SmOs ₄Sb ₁₂ agree reasonably well with the results obtained by ab initio calculations for IrSb ₃ and LaOs ₄Sb ₁₂, respectively. The rare-earth dependence of the inelastic x-ray scattering spectrum in the series of ROs ₄Sb ₁₂ compounds is difficult to interpret in terms of changes in the atomic mass and electron number of the rare-earth atom inserted into the Sb cage. The combination of inelastic x-ray scattering and x-ray absorption spectroscopy reveals a correlation between the guest mode energy and the guest free space in these compounds. The rare-earth dependence of the inelastic x-ray scattering spectrum indicates the mediation of heavy fermion behavior by the atomic motion of the guest in some ROs ₄Sb ₁₂ compounds and the electron-phonon coupling between the guest modes with 4f-electrons and conduction electrons in the series of ROs ₄Sb ₁₂ compounds.