Classification of strongly correlated f-electron systems

Ce-based heavy-fermion (HF) metals behave as Kondo-lattice systems and can be classified with the aid of a single coupling parameter, \J\/W, where J < 0 is the local exchange integral and W the conduction-band width. Depending on its actual composition, the exemplary material CeCu2Si2 chooses one out of two ground states: HF superconductivity and a new magnetic HF phase ''A''. In a narrow composition range, these two phases are nearly degenerate and expell each other upon varying either the temperature or the external magnetic field. The one-parameter-scaling approach appears inapplicable to the U-based HF metals. For the exemplary material UPd2Al3, antiferromagnetic ordering between seemingly local 5f moments coexists, on a microscopic scale, with HF superconductivity. Whether this coexistence can be explained by assuming 5f states localized on a tetravalent U-ion with non-magnetic crystal-field ground state remains to be shown. We discuss arguments which invoke itinerant 5f states in the U-based HF metals to be distinguished from the localized 4f states in the Ce-based counterparts