f-Electron Behavior in Rare Earth Based Systems: Localization or Itinerancy?

Measurement of the two-dimensional angular correlation of the electronpositron annihilation radiation complemented with ab initio calculations can provide decisive information about the character of the f -electrons in rare earth compounds.We provide examples of f-electron localized and f-electron itinerant systems, respectively.(i) In the case of the antiferromagnetic heavy fermion and superconductor CeIn 3 the multisheet Fermi surface, reconstructed from our measurements in the paramagnetic phase, agrees closely with the predictions of band structure calculations regarding the Ce 4f electrons as fully localized.(ii) On the other hand, our studies of the antiferromagnet actinide based UGa3 in the paramagnetic phase, compared with calculations which include the effects due to the non-uniform positron density and the electron-positron correlations, produce a substantial evidence that an unconstrained 5f -electron itinerant description applies.