Kondo system

The CeNi2Sb2 compound was classified as Kondo system by Kaczmarska et al. (1993) from magnetic properties and resistivity measurements. The resistivity shows a maximum at 2.5 K and a minimum at 28 K (fig. 39). As reported by Skolozdraet al. (1994), it has aCurie-Weiss... 

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. 

Finite size algorithms have been used extensively to study the edge states and systems with impurities, where substantial improvement of the accuracy is needed to characterize the various properties of a finite system. The DMRG method has been applied to diverse problems in magnetism study of spin chains with s > 1/2 [72], chains with dimerization and/or frustration [71, 73, 74], coupled spin chains [71, 75, 76], to list a few. The method has also been used to study models with itinerant fermions [77, 78], Kondo systems[79, 80, 81, 82], as well as coupled fermion chains [83, 84], including doping. Formulations for systems with a single impurity [85, 86] as well as randomly distributed impurities [87] and disorder [88] have also been reported. There has also been a study of the disordered bosonic Hubbard model in one dimension [89]. 

Some of the Kondo systems investigated in this section are not in fact dilute magnetic alloys. Instead they are Kondo lattices in which the magnetic moments lie on a sublattice of the RI compound (e.g. CeAy. The R-atom in all these compounds is Ce for which the d-f admixture interaction mentioned in section 3.1.2 is dominant. Such RI compounds are therefore Kondo systems in which eq. (30) for the resistivity holds. However, for temperatures T < Tr the magnetic moment is usually not totally compensated in a Kondo lattice since now there are too few conduction electrons to achieve full compensation on every magnetic lattice site. 

The Kondo system CeAlj has attracted continuous interest over the past decade due to its anomalously strong coupling between the elastic and magnetic (elec-... 

The electrical resistivities of the dense Kondo systems CeNiln, CePdln, and CePtln have been measured under hydrostatic pressures up to 19 kbar (Kurisu et al., 1990). The Kondo temperature of CeNiln and CePtln shifts linearly with pressure to higher temperatures at rates of 2.3 and 1.5 K/kbar, respectively. For CePdIn, the pressures were not high enough to reach the CePtln or CeNiln state. Measurements of the elastic properties of CePdln reveal that all elastic constants exhibit softening at low temperatures due to the crystal electric field effect and the antiferromagnetic ordering (Suzuki et al., 1990). 

The reader is cautioned to observe, however, that existing theories for the various physical properties of Kondo systems do not provide a self-consistent definition for the characteristic temperature as defined by the temperatures of the typically broad anomalies in the thermoelectric power, electrical resistivity, heat capacity and magnetic susceptibility. Thus the values of the characteristic temperature inferred from different measurements may differ by as much as an order of magnitude. 

Early work on dilute lanthanide systems was motivated by the vast experimental and theoretical effort that had been expended on understanding dilute magnetic 3d impurities in noble metal hosts. In 1965, Sugawara discovered a resistance minimum in the YCe system, providing the first evidence of Kondo behavior for a lanthanide solute. This led to the discovery of numerous lanthanide Kondo systems which exhibited anomalies in their physical properties qualitatively identical to those found in 3d Kondo systems. 

As mentioned in the Introduction (section 1), the electrical resistivity, specific heat, magnetic susceptibility and thermoelectric power anomalies of concentrated lanthanide Kondo systems are qualitatively similar to those found in the dilute lanthanide systems discussed previously. However the importance of the concentrated systems is that they provide a totally new view of Kondo-like phenomena. Lattice constant. X-ray photoemission (XPS) and Mossbauer isomer shift measurements indicate a strong correlation between systems that exhibit Kondo-like anomalies and systems in which the lanthanide ion has a mixed or intermediate valence. By mixed valence we mean that there are two 4f electron configurations accessible to each rare earth ion (e.g., Ce -Ce, Eu -Eu ", Yb -Yb ). Phenomenologically the traditional... 

It was remarked earlier that there is a strong correlation between systems which exhibit a mixed or fluctuating valence and systems which show Kondo-like anomalies. This connection was particularly clear in the Ce systems considered previously. A wide variety of experimental measurements has shown that in SmB6 and collapsed SmS, the Sm ions have a mixed valence (Sm -Sm ). We consider these systems not because they are classic Kondo systems (they are not), but because they provide considerable insight into the nature of the mixed valence state. These systems are also interesting with respect to the study of metal-insulator transitions, but this aspect will be discussed in ch. 20 by Jayaraman. 

The UCu4fiAl8 i system has been obtained in the form of amorphous thin film. From ac resistivity measurements it follows that compared to the results in the crystalline bulk alloys, the onset of magnetic order is suppressed at low Cu concentrations, while the onset of a coherent heavy-fermion state is suppressed at high x. The system reveals a single-ion Kondo behavior down to the lowest temperatures, but significant deviations were detected from the behavior of dipolar Kondo system (Lunkenheimer et al. 1994). 

The R ions form a periodic lattice, which leads for the 4f electrons together with the conduction electrons to the formation of quasi-particle bands, i.e. the electrons are in a coherent state. Since the magnetic moments either vanish (in the non-magnetic Kondo state) or form themselves a periodic magnetic structure (Kondo systems with magnetic order) there is no elastic scattering of the conduction electrons and therefore Pn,(0) = 0. This is different at high temperatures, where even in a periodic lattice one has disordered moments, which scatter elastically. The coefficient Ai can be calculated analytically. One finds A = j j + with the resistivity in the unitarity limit... 

Electron scattering by spin disorder in the paramagnetic region in Kondo systems (electrical conductivity, thermal conductivity)... 

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