Kondo behaviour

The disappearance of antiferromagnetic order in U(Pt xPdx)3 for x a 0.1 coincides with the appearance of Kondo behaviour in the resistivity. This is illustrated in fig. 9, where for the concentration range 0.00 resistivity curves are displayed in the temperature range below 50 K.[l 1] For x=0, i.e. pure UPt3, the data in the lowest temperature interval can be represented by ... 

Figure 9. Resistivity of U(Pt1.xPdx)3 polycrystalline samples as a function of temperature the data have been normalised to 1 at 300 K note the change from Fermi-liquid to Kondo behaviour for x-values between 0.00 and 0.15 figure from ref.[l 1],...

MgAgAs structure (see below) and non-magnetic ground state with Kondo behaviour (Buschow et al. 1985, Palstra et al. 1987)... 

K and O2 = 0.5 K for the Curie-Weiss 9 of the two temperature regimes. Some data at low temperature are given in Figure 2. All data show that Fe Au is clearly a system in which even at very low impurity concentrations the interaction effects are important at low temperatures and low external fields. Thus, it is difficult to extract parameters for the intrinsic Kondo behaviour of the isolated impurities from the experimental data on this system. The data show that the Kondo temperature for Fe Au is most likely in the order of 10 K and not 0.5 K the lower temperature seems to be connected with some kind of magnetic order. 

At further increase of x, the long-range antiferromagnetic order is lost and a Kondo-type of behaviour is found in the resistivity curves of compounds withx > 0.10. 

The reason for this behaviour is the presence of Shockley surface states [176] on the noble metal surfaces. On these surfaces, the Fermi energy is placed in a band gap for electrons propagating normal to the surface. This leads to exponentially decaying solutions both into the bulk and into the vacuum, and creates a two-dimensional electron gas at the surface. The gas can often be treated with very simple quantum mechanical models [177, 178], and much research has been done, especially with regards to Kondo physics [179, 180, 181]. There has also been attempts to do ab initio calculations of quantum corrals [182, 183], with in general excellent results. 

CeCu6 and CeRu2Si2 show neither superconductivity nor magnetic order down to 20 mK. Therefore, the behaviour of 1/7) in these compounds presents a fundamental property of heavy fermion material. As seen in Fig. 3,11,12 1/7) is temperature independent above the Kondo temperature, / , corresponding to a localized state of the 4/ electron. At higher temperatures 1/7) is expected to decrease due to the increase in the fluctuation rate of the localized 4f spin. Below 7k, 1/7) shows a 7) T= constant behaviour corresponding to the Fermi liquid state. 

The behaviour pertinent to the opposite limit of the well localized f states is found in most rare-earth compounds. The 4f states are situated more than 5 eV below EF in most of them. The strength of the interaction of f and conduction-band electrons is considerable (= 0.1 eV), but contributes only indirectly to the magnetic coupling of f-moments via polarization of conduction electrons (RKKY), the 4f-moment magnitude remaining preserved. For f states closer to EF, as is the case of y-Ce or some Ce compounds (a situation comparable To some actinide compounds), the interactions between the f- and conduction-band states becomes stronger. The Kondo Hamiltonian can be written as... 

Unlike the Kondo systems, a qualitatively different behaviour was observed in the Ce and Yb mixed-valence materials. A stronger coupling between the 4f moments and the conduction-band states is manifest in the much faster relaxation rates and the values of residual IT are about one order of magnitude higher. The striking contrast is underlined by the nearly temperature independent values of T. 

At this point it should be mentioned that many more pseudobinary RI compounds are known to exhibit a Kondo-like behaviour ((LaCe)Al2, (LaCe)Al3), (LaCe)3Al, (LaCe)3ln, (LaCe)Iu3, (YCe)Al2, (LaPr)Su3, (, Sm)Su3, (AgAu)Yb, etc.). Of the thirteen lanthanide ions with partially filled 4f shells, Kondo-like behaviour has been observed in RI compounds with Ce, Pr, Sm, Eu, Tm, and Yb (Van Daal and Buschow, 1969a, b Buschow and Van Daal, 1970 Rao et al., 1971 Bakanowski and Mihalisin, 1977). 

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