Kondo compound

Symposia on Valence Instabilities, Kondo Effect, and Mixed Valence and Kondo Compounds, J. Appl Phys., 55, 1904, 1930, 1963 (1984). 

Chenevas-Paule et al. (1977) have recorded the resistivity versus temperature of the two phases in the range 1-300 K. From their results they estimate the Kondo temperature to lie between 20 and 50 K. They conclude that (M. SmS) is not a Kondo compound similar to CeAls and since the resistivity does not increase with temperature they believe in the presence of Sm. ... 

Fig. 27. High-temperature specific heat of two Kondo compounds (CePdsB,) and a normal antifer-romagnet (CePdjSio.s), after Nieva et al. (1987). The continuous curve is a Schottky contribution for a F -Fg thermal promotion.

The distinction between Kondo metals, etc. and HF systems is fuzzy at best. As pointed out, the Kondo interaction is, among others, a basic ingredient of HF behavior. In a Kondo-lattice material one observes the effects of the Kondo interaction, for example on the magnetic properties, but very heavy quasiparticles are not formed and in consequence, the Sommerfeld constant is only slightly enhanced, That at least is the basis for a distinction we shall adopt. The hybridization between 4f and conduction electrons can lead to a hybridization gap in the density of states at the Fermi surface. The exact mechanism of gap formation is still under debate and also may vary from compound to compound. If a gap is present, one leaves the realm of Kondo metals and has, depending on the form of the gap (e.g., whether it is open in all crystallographic directions) and on its width, either a Kondo semimetal, semiconductor or insulator. The latter are certainly the most challenging class of Kondo compounds to understand. 

High-resolution photoemission studies were performed for YbPdAl, YbPtAl, and YbAuAl (Reinert et al. 1999). For these intermediate-valent and Kondo compounds a clear correlation of the linewidths, the binding energies and the spectral-weight distributions of the 4f spectra with the characteristic temperature Tmax of the maximum in the magnetic susceptibility is observed. 

A more comprehensive presentation of the many experimental and theoretical aspects of IV and Kondo compounds as well as a compilation of the original references can be found in a number of conference proceedings and review articles as given, e.g., by Wachter and Boppart (1982), Miiller-Hartmann et al. (1984), Kasuya (1985) and Gupta and Malik (1987). 

Kondo-like resistivities can also be observed in concentrated Kondo compounds. Typical examples are CeAlj (Buschow et al. 1971), CeAl2 (Buschow et al. 1969), CeCu2Si2, (Steglich et al. 1985) and CeSnj (Gschneidner et al. 1985). A characteristic property is the low-temperature specific heat or its y-value, y = CIT, which has very large values of the order of 1 J Mol K (see table 5). This large value for y, expressed in electron density of states or effective masses m Im (of the order of 200) have led to the name of heavy-fermion compounds (Stewart 1984). 

These compoimds show interesting electronic properties depending on m. First, we consider the Kondo compound with m = 1 that has the CsCl-type structure CeZn, and then we show the case of m = 6 and a HF Kondo compound CeCue. 

The values of Bq and Bq for typical Ce Kondo compoimds are summarized in Table 1. It is seen that the values of Bq are strongly dependent on the nature of the compound. However, the values of Bq show a systematic behavior. The Bq is between 2 and 3 for the Ce Kondo compounds showing pressure-induced continuous valence transition (Kagayama and Oomi, 1996). In this sense, CeAu2Si2 is classified as a Kondo compound showing pressure-induced valence transition, which may occur very slowly. 

TABLE 1 So and Bq for several Ce Kondo compounds (Kagayama and Oomi, 1998 Ohashi et al., 2003a Ohmura et at, 2009)... 

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