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Abstract:
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.
