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Abstract:
Ce ions in a metallic environment give rise to a variety of fascinating many body phenomena, for example intermediate valence (IV), valence transitions and the Kondo effect1. Despite a controversial debate among spectroscopists2, there is widespread confidence that the above phenomena arise from the close proximity of the local 4f level of Ce and the Fermi level. Based on the Anderson model, in the limit of large Coulomb energy, an IV phenomenon should occur when the Fermi level EF intersects the 4f level. In a “high-temperature picture” this can be attributed to the action of charge fluctuations (“valence fluctuations”), which occur at a rate of the order of the 4f level width ∆. In the Born approximation ∆≃πN(EF) <v kf 2 >, where N(EF) is the conduction band density of states at EF per spin and vkf the matrix element for covalent 4f-conduction electron mixing. On the other hand, a Kondo effect is expected when the 4f level, at E4f, is reasonably well separated from EF, i.e., ∆<ε=EF−E4f. Therefore, to a good approximation, the Ce ions in Kondo systems can be considered to be trivalent, i.e. valence fluctuations can be considered to be unimportant. According to Schrieffer and Wolff4 the coupling between Ce3+ ions and conduction electrons can then be treated as an effective antiferromagnetic exchange interaction. We note that in the limit of negligible mixing, i.e. ∆<<ε, Ce3+ would behave similarly to “normal” rare-earth ions like Gd3+, in that its coupling to the conduction electrons would be rather weak and, eventually, ferromagnetic, i.e. governed by the intra-atomic Heisenberg exchange integral. These “normal” rare earths are sometimes referred to as exhibiting a “stable magnetic moment”.
