Investigations of heavy-fermion systems or metals with heavy electrons have developed into a new branch of metal physics. For recent theoretical and experimental reviews see, e.g., [13.1–5] and [13.6–8], respectively. In all cases those systems contain either Ce, Yb, U, or Np as one of their constituents, i.e., there are always 4for 5felectrons involved. Characteristic examples are: CeAl3, CeCu2Si2, CeRu2Si2, CeCu6, CeB6, YbAl3, YbCu2Si2, UBe1 3, UPt3, UCd11 U2Zn17, and NpBe1 3. Below a characteristic temperature T*, heavy-fermion systems show Fermi-liquid behavior with huge effective masses m* of the quasiparticles. In fact, m* may be several hundred times the free electron mass. Consequently, the low-temperature specific heat C = γT has an enormously large γ coefficient:γ is of order of 1 J mol−1 K−2, rather than 1 mJ mol−1 K−2 as in sodium metal. The Pauli paramagnetic susceptibility is similarly enhanced in this temperature range. As the temperature increases to values above T*, the quasiparticles lose their heavy masses. The specific heat levels off as indicated in Fig. 13.1, and the susceptibility changes from a Pauli to a Curie-like behavior. With increasing temperature the Ce and U ions behave more and more like ions with well-localized f electrons.
KeywordsFermi Surface Fermi Energy Orbital Degeneracy Crystalline Electric Field Quasiparticle Band
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