Kondo anomalies

Summing up it can be said that the Kondo anomaly should be observable if ... 

Knowing the excitation spectrum one can compute the thermodynamic properties. In the local-moment regime they exhibit low-temperature T 7 ) Kondo anomalies that are due to the resonance states. For example, the static magnetic susceptibilty x(T), the specific heat, various transport coefficients and also dynamical quantities (photoemission spectra, dynamical structure function for neutron scattering) have been calculated (Bickers et al. 1985, Cox et al. 1986). An excellent model system for comparison with experimental data are the dilute (La, Ce)Bg alloys because of a fourfold degenerate Fg ground state of cerium (Zirngiebl et al. 1984). 

Historically, the types of normal state Kondo anomalies described above were first studied in dilute alloys of 3d (Fe-group) transition metals dissolved in simple metal hosts such as the noble metals Cu, Ag and Au. However, in 1965,... 

Kondo anomalies are qualitatively consistent with the mixed valence picture, although at present there is no first principles theory based on the mixed... 

Lawrence et al. (1975) have studied the electronic properties of the Ce, Th system in some detail. While there are definite Kondo anomalies in this system, the main thrust of this work is concerned with the thermodynamic behavior near valence instabilities, particularly near the critical poinL A detailed discussion of the work is beyond the scope of this article and we refer the reader to the following references Parks and Lawrence (1976) and Varma (1976 and references therein). 

With the exception of Ce systems, lanthanide compounds and alloys made with Yb most frequently exhibit Kondo anomalies. Two of the most thoroughly studied Yb systems are YbA and YbAls. The phase diagram for the Yb-Al system has been reported by Palenzona (1972). YbA crystallizes in the cubic MgCu2 Laves phase while YbAL forms in the cubic CujAu structure. landelli and Palenzona (1972) measured the lattice constants of both compounds from 80-800 K. 

J. Kondo and K. Yamaji, Density correlation of classical 1-D electron Gas with reference to the 4kf Anomaly in TTF-TCNQ, J. Phys. Soc. Japan 43 424-H36 (1977). 

It is well known that dilute magnetic alloys which have a Kondo minimum in the resistivity, also exhibit a giant thermopower anomaly. This anomaly was first treated theoretically by Kondo (1965) and later by Suhl and Wong (1967), Fischer (1967) and Maki (1969). 

Kondo s treatnient omitted several important interaction terms but the latter authors were able to show that the thermopower depends in an interconnected manner on exchange coupling, potential scattering, and the range of the interaction (see Blatt et al., 1976). No theoretical work has as yet been done for Kondo lattices. However, the thermopower of CeAl2 which is a Kondo lattice does in fact show a giant thermopower anomaly similar to that found in dilute Kondo alloys. 

Fig. 26. High-temperature specific heat of three Kondo eompounds with an abnormal Schottky anomaly, after de Boer et al. (1985) and Felten (1987). The continuous curve is a Schottky contribution for a F-j-Fg thermal promotion.

Therefore in this review we shall focus on the electron-phonon coupling in metallic R compounds. They have shifted to the center of interest in the last ten years because of new phenomena such as valence fluctuations, Kondo lattice behavior and the formation of heavy-electron bands. The varying degree of hybridization between 4f states and conduction electron states leads to low lying electronic states whose character is very different from the localized states due to many-body effects. Consequently they lead to a rich variety of electronic and elastic or vibrational anomalies at low temperatures. 

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. 

---