Kondo linewidth

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. 

Fig. 47. (a) Lattice Kondo temperature T read off positions of broad peaks in both the thermal expansion, a T) ( ), and resistivity, p T) (A), as well as residual quasielastic linewidths (HWHM), r(T), extrapolated from 7 >50Kto 7 = 0K ( ). In order to match the a(T) and p(T) results, p(T) peak positions are scaled by a factor of 1.25. (b) Positions of extrema, indicating antiferromagnetic phase transitions, in a(T) (A, A, x), specific heat, C(T) ( , V, +), p(T) ( ) and DC susceptibility ( , O). Closed symbols refer to onset of local-moment ordering below open symbols... 

It has been pointed out early that the ESR of local moments could be used to observe the Kondo effect directly by studying the resonance absorption of the Kondo impurity itself Spencer and Doniach (1967) calculated the g-shift and Walker (1968) the relaxation rate due to the Kondo effect. Both contributions are rather small and experimentally hard to measure. This is partly due to the large residual linewidths typically found in Kondo alloys. 

A number of multi-impurity experiments on Kondo systems were reported in the literature. Most of the early work is cited in the review by S.E. Barnes (1981a). A tutorial discussion on the Kondo effect has been given by Taylor (1975). Of these multi-impurity experiments, here we discuss Gd-ESR experiments obtained in LaA doped with Kondo impurities. Gd-ESR has been utilized to study the effects of a Kondo impurity in the pseudobinary alloy Lai jcCexAl2 Gd (Davidov et al. 1972). Similar experiments were performed by Weissenberger (1981) in the intermetallic compound (La,Y,Ce)Al2 Gd. In this alloy Ce behaves like a Kondo impurity and the Kondo temperature can be varied from 0.4 to 100 K. The most important result of this investigation was the dependence of the residual linewidth A/7o on the state of the Ce impurity, e.g. on the single-ion Kondo temperature. 

In Kondo lattices the resonance absorption of the f ions is usually not observable and the systems have to be studied using a further ESR probe, mostly gadolinium. a-Ce is the most prominent mixed-valence system. However, it has been demonstrated that in this compound the 4f electrons do not contribute significantly to the linewidth and to the g-value of the ESR probe (Rettori et al. 1978). Hence, a-cerium was discussed in detail in sect. 4.1. Valence transitions generally evolve slowly as a function of temperature. Besides the well-defined transition in a-cerium another sharp, temperature-induced valence transition has been detected in Ybo.4Ino.6Cu2 (Felner and Nowik 1986). 

Fisk 1992). It is reasonable to believe that the hybridization of the f-electrons with the band states accounts for the unusual properties of these materials. ESR experiments on SmBs are discussed in detail in sect. 5.3.2. There it was shown that the opening of a gap indeed can be deduced from the temperature dependence of the linewidth. It is clearly apparent that ESR is an extremely useful technique to study the dynamical susceptibilities in Kondo insulators. 

Fig, 55. The QE linewidth of some Kondo-lattice and heavy-femion systems as a function of temperature, (CeCujSij and CeAh adapted from Horn et al. 1981b and CeAg adapted from Frick et al. 1983.)... 

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