Abstract
Heavy-fermion compounds exhibit a considerably more stable 4f configuration, although the 4f occupancy still deviates slightly from integral values. They derive their properties at low temperatures from dramatically sharper scattering resonances near EF. The narrow scattering resonances at the Fermi level are due to many-body effects. They lead, within the framework of Landau's Fermi-liquid theory, to strongly renormalized electronic quasiparticles with very heavy masses. The formation of heavy quasiparticles takes place at low temperatures. Although a Fermi-liquid description of the low-temperature phase of heavy-fermion compounds is possible in principle, a straightforward application of the formulae of Sommerfeld's theory for one component, noninteracting Fermi system can be quite misleading. The structure of quasiparticles results from the interplay of two kinds of electronic states, that is, well localized ionic f states and rather delocalized conduction-band states, both, in general, anisotropic. Thus, the number of Fermi-liquid parameters required to understand the experimental observations is not small as in simple metals. In addition, the interactions among quasiparticles are very important in heavy-fermion compounds. Energies due to the interactions between heavy quasiparticles can be as large as the effective band width kBT* as inferred, for example, from cooperative effects found in these systems. Superconducting and magnetically ordered phases are formed within heavy-quasiparticle liquids.
