HEAVY-FERMION BEHAVIOR IN LANTHANIDE AND ACTINIDE MATERIALS

The electronic properties of Ce and U intermetallics are strongly affected by f-ligand hybridization leading to delocalization of the f-states, and by Coulomb interactions responsible for electron-electron correlations. Heavy-fermion behaviour is distinguished by a large electron specific heat coefficient gamma pointing to a renormalization of the electron effective mass. In Ce compounds this appears in Kondo or Kondo lattice systems owing to a narrow resonance in the quasiparticle density of states at E(F). The Ce charge state is very close to 4f(1) and the 4f(1) states are located well below E(F). The U 5f electron wave functions tend to a more itinerant state, and the band picture is a more appropriate starting point for understanding U systems. The Ce and U materials exhibit different crossovers to magnetism. In Ce compounds, this is typically the transition from high gamma Kondo systems to low gamma local-moment antiferromagnets. The maximum gamma values can be observed in U materials close to a magnetic instability or in the antiferromagnetic ordering of small 5f moments. The 5f states seem to be present at E(F) without any doubt, and the enhancement of gamma is due probably to magnetic (spin) fluctuations. Itinerant 5f states can be concluded from a broad quasielastic response in inelastic neutron scattering and from electron spectroscopy experiments. The development towards the heavy-fermion state is compared for CeTX, CeT2,X(2), UTX and UT(2)X(2) compounds.