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Nonadiabatic effects of rattling phonons and 4f excitations in Pr(Os{1-x}Ru{x})4Sb12

In the skutterudite compounds the anharmonic 'rattling' oscillations of 4f-host ions in the surrounding Sb12 cages are found to have significant influence on the low temperature properties. Recently specific heat analysis of Pr(Os{1-x}Ru{x})4Sb12 has shown that the energy of crystalline electric field (CEF) singlet-triplet excitations increases strongly with Ru-concentration x and crosses the almost constant rattling mode frequency $ω_0$ at about x ~ 0.65. Due to magnetoelastic interactions this may entail prominent nonadiabatic effects in inelastic neutron scattering (INS) intensity and quadrupolar susceptibility. Furthermore the Ru- concentration dependence of the superconducting Tc, notably the minimum at intermediate x is explained as a crossover effect from pairforming aspherical Coulomb scattering to pairbreaking exchange scattering.