Abstract
The low energy crystalline electric field (CEF) excitations of f electrons can play a twofold role in heavy fermion and unconventional superconductors. They may act as a glue for the formation of Cooper pairs and simultaneously as a probe to investigate the symmetry of the order parameter. The former has been found in the skutterudite heavy fermion compound PrOs4Sb12, where a singlet–triplet CEF excitation Δ contributes to the pair formation and enhances Tc with respect to L aOs4Sb12. In substituted P r(Os1−xRux)4Sb12, a continuous increase of Δ leads to a crossing with the local Pr rattling phonon mode and vibronic mode formation caused by magnetoelastic interaction. Furthermore, the Tc enhancement turns into a reduction. It is proposed that this signifies a crossover from mainly pair-forming aspherical Coulomb to pair-breaking dipolar exchange scattering with increasing Ru content. In the Ce-based Fe pnictides CEF transitions show anomalous temperature dependence of the line width due to a pronounced feedback effect, which gives direct evidence for the unconventional 3d superconductivity.
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