Kondo temperature dependence

The pressure-dependent electrical resistivity of the heavy-fermion compound YbNi2B2C (see also Section 4.12) could be explained by competing contributions from crystal-electric-field splitting and Kondo effect (Oomi et al., 2006). The pressure-dependent room-temperature thermoelectric power of YNi2B2C exhibits a peak around 2 GPa, which was explained by changes in the Fermi-surface topology (Meenakshi et al., 1998). A possible correlation with a small peak in the temperature-dependent thermopower around 200 K (Fisher et al., 1995 Section 3.4.3) needs further investigation. 

The resistivity v.v temperature curves of the U(Pti xPdx)3 system develop from a normal quadratic temperature dependence in pure UPt3 to a Kondo-type of temperature dependence for x > 0.10 ... 

The main equation for the d-electron GF in PAM coincides with the equation for the Hubbard model if the hopping matrix elements t, ) in the Hubbard model are replaced by the effective ones Athat are V2 and depend on frequency. By iteration of this equation with respect to Aij(u>) one can construct a perturbation theory near the atomic limit. A singular term in the expansions, describing the interaction of d-electrons with spin fluctuations, was found. This term leads to a resonance peak near the Fermi-level with a width of the order of the Kondo temperature. The dynamical spin susceptibility in the paramagnetic phase in the hydrodynamic limit was also calculated. 

An alternative approach to accounting for the maxima in the temperature dependence of p is based on the Kondo-lattice model (Lavagna et al. 1982). The periodic array of independent Kondo impurities, described by the single-ion Kondo temperature TK, provides a proper description at elevated temperatures, while a coherent state yielding a drop of the resistivity is attained when the system is cooled to below another characteristic temperature coh- Although this approach is suitable particularly for Ce compounds where the Kondo regime was identified inequiv-ocally, the coherence effects are probably significant also in narrow-band actinide materials, as indicated by an extreme sensitivity of the lower-temperature decrease of the resistivity to the presence of impurities. 

In summary, the new data suggest that the features of the pSR spectra of Ybj arise mainly from molecular dynamics (probably within the B12 clusters). This is supported by recent Yb NMR measurements (Ikushima et al. 2000) where a minimimi of l/T was seen around 15 K which ties in with the temperature dependence of ZF-pSR spectral shape. The NMR results on the Yb sites differs from those at the B sites requiring an additional relaxation process for the B ions. There is no compelling evidence that Yb carries a moment. The exact magnetic properties of YhBi2 in its Kondo state remain an open question. 

The pressiue dependence of the electrical resistivity of YbCuAl was investigated (Alami-Yadri et al. 1998, 1999a,b) up to 8 GPa. The resistivity at 300 K decreases with increasing pressure. At 8 GPa a dependence occurs at low temperature (Fermi-liquid behavior), and the Kondo temperature decreases with increasing pressure. The experimental setup for these measurements was presented by Jaccard et al. (1998). Furthermore, point-contact spectroscopy was used to measure the interconfigurational excitation energies and conduction-electron lifetime width of valence-fluctuating YbCuAl (Bussian et al. 1982). 

However, in the case of a dilute alloy with unstable-moment inpurities (e.g., Ce " ) many-body effects lead to the existence of narrow Kondo resonance states above the Fermi level as discussed in detail in sect. 4. This leads to a strongly energy-dependent scattering rate for conduction electrons E E) = 1/t( ) that is directly proportional to the density of many-body resonance states (see fig. 46). The energy scale for the t E) dependence is now the Kondo temperature which can be comparable with T. Therefore, in this case one should expect that MAQO and dHvA amplitudes may deviate from the LK formula. Experimentally this effect has been quite elusive. One of the rare cases where it was actually observed is the dilute alloy Laj Ce Bg (x = 0.10), see Thalmeier et al. (1987). Figure 41 shows the T-dependence of MAQO amplitudes for a very small extremal area with F = 6.5T or... 

Fig. 1. Temperature dependence of 4f-derived specific heat per mole of cerium as CJ T plotted against T for Ce,La,.,Cuj with x = l (O), 0.8 ( ) and 0.5 (x) (Onuki and Komatsubara 1987). Inset shows low-T data (Steglich et al. 1985). Dashed curve indicates result for an 5 = J Kondo impurity with r = 4.2K (Andrei et al. 1983).

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