Valence-fluctuation regime

There is a big difference in f-electron character between the Kondo regime and the valence-fluctuation regime. One may be tempted to think that the 4f electrons in a Kondo lattice compound with a large value of Tk are itinerant. This seems to be true, as shown later in detail for CeSns and CeNi or CeRu2Si2. 

As will be explained in sect. 4, the 4f electrons in cerium compounds such as CeSna and CeNi, which belong to the valence-fluctuation regime, are itinerant in the ground state and contribute directly to the formation of the Fermi surfaces. Their Fermi surfaces can be well described by band theory. The enhancement factor in CeSn3 and CeNi is not large, less than 5, as in the 3d electrons of the iron-series transition metals. 

As pointed out in the theoretical sect. 4.2 the two regimes are characterized by two quite different fluctuation temperatures. In the single-ion Kondo case or the Kondo lattice case the Kondo temperature is typically 7 < 10 K. This implies by virtue of 1 — /ij [eq. (113)] almost integral valence, and anomalies of the physical quantities only at low temperature 7< 7. On the other hand, the mixed-valent regime (n, non-integer) has a fluctuation temperature typically larger than 7 > 100 K. Therefore, anomalies can be observed even at room temperature as discussed below. 

Fig. 6. The weight Pf fsp) of the valence spectrum at the Fermi energy as a function of the degeneracy N[. The exact result (full curve, eq. (114)) is compared with the (l/N,) result (dashed curve, eq. (115)) for the mixed valence ( f = 0.8) and spin-fluctuation ( f = 0.99) regimes.

---