Nonadiabatic effects of rattling phonons and 4f excitations in Pr(Os1-xRux)4Sb12

In the skutterudite compounds the anharmonic "rattling" oscillations of 4f-guest ions in the surrounding Sb-12 host cages are found to have significant influence on the low-temperature properties. Recently specific-heat analysis of Pr(Os1-xRux)4Sb(12) has shown that the energy of crystalline electric field singlet-triplet excitations increases strongly with Ru concentration x and crosses the almost constant rattling mode frequency omega(0) at about x similar or equal to 0.65. Due to magnetoelastic interactions this may entail prominent nonadiabatic effects in inelastic neutron-scattering intensity and quadrupolar susceptibility. Furthermore the Ru-concentration dependence of the superconducting T-c, notably the minimum at intermediate x is explained as a crossover effect from pair-forming aspherical Coulomb scattering to pair-breaking exchange scattering.