Dilute Kondo systems

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. 

Sugawara observed the first resistance minimum phenomenon due to a lanthanide solute in the dilute alloy system YCe. Following this discovery of the Kondo effect in the YCe system, numerous dilute and concentrated lanthanide metallic systems have been investigated over the past decade. Of the thirteen lanthanide ions with partially-filled 4f shells, Kondo-like behavior has been observed for Ce, Pr, Sm, Eu, Tm and Yb. 

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. 

In this section, we discuss examples of dilute lanthanide systems which exhibit Kondo-like anomalies in both their normal and superconducting state physical properties. 

Although several concentrated alloys and compounds of Sm (notably the Sm monochalcogenides and SmB ) have been studied in connection with valence fluctuation phenomena, there is only one dilute alloy system reported to date containing Sm impurities which exhibits Kondo-like behavior. 

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... 

Using this local moment model, and using band theory or its variations, a number of workers have been able to formulate expressions which represent the measured magnetic data reasonably well, at least for the case where well-localized moments are developed on the solute atoms (II, 18). However, considerably more data has become available on other properties of dilute alloys, including data on resistivity and specific heat, neutron scattering, various magnetic resonance experiments, Moss-bauer measurements, Kondo effect, and the like. Measurements have been extended also to alloys of many other systems besides those involving the platinum metals. 

Contents H.Matsuda Atoms as Constituents of Matter. - T. Tsuneto System of Protons and Electrons. - T. Tsuneto Helium. - T. Tsuneto Superfluid Helium 3. - T.Matsubara Metals. - T.Matsubara Non-metals. - T.Matsubara Localized Electron Approximation. - T.Murao Magnetism. - T.Murao Magnetic Properties of Dilute Alloys - the Kondo Effect. - H.Matsuda Random Systems. - F. Yonezawa Coherent Potential Approximation (CPA).- References.- Subject Index. 

Knowing the excitation spectrum one can compute the thermodynamic properties. In the local-moment regime they exhibit low-temperature T 7 ) Kondo anomalies that are due to the resonance states. For example, the static magnetic susceptibilty x(T), the specific heat, various transport coefficients and also dynamical quantities (photoemission spectra, dynamical structure function for neutron scattering) have been calculated (Bickers et al. 1985, Cox et al. 1986). An excellent model system for comparison with experimental data are the dilute (La, Ce)Bg alloys because of a fourfold degenerate Fg ground state of cerium (Zirngiebl et al. 1984). 

Because of the wide range of magnetic character displayed by the lanthanide metallic systems reviewed herein, they provide a rich testing ground for the concepts and theories which have been advanced to account for Kondo-like anomalies in the physical properties of these systems. However, before reviewing the Kondo-like behavior of dilute and concentrated lanthanide metallic alloys and compounds in sections 3 and 4 of this article, respectively, we briefly discuss a number of models in the following section which have been proposed to describe the systems which exhibit Kondo-like behavior in their physical properties. The article is concluded in section 5. 

Kondo-like behavior in dilute lanthanide impurity systems... 

The evidence for the Kondo effect in dilute alloys containing Yb lacks the numerous cases of model behavior as have been found for dilute alloys containing Ce impurities, and (Ag, Au)Yb remains to date the only extensively-documented example of anomalous behavior for Yb impurities in a dilute alloy. Unfortunately, this system is also a classic example of the severe limitations that metallurgical effects can place on the clarity of the interpretation of even the most carefully performed experiments. The crux of the problem lies in the rather poorly understood levels of solubility of the lanthanides in Ag and Au. 

SmS, Ce metal and CeAb are three of the most thoroughly studied concentrated systems which exhibit Kondo-like anomalies. In analogy with the dilute (La, Th)Ce system, the magnetic state of the lanthanide ions in these concentrated materials has also been varied by applying external pressure or by alloying with other elements. 

Thermal conductivity of systems with heavy fermions, intermediate valency of lanthanide ions, dilute and concentrated Kondo lattices... 

In a number of theoretical works the behaviour of k T) and UJ) of systems with concentrated (CKL) and dilute (DKL) Kondo lattices have been investigated (Fisher 1971, Nakamura et al. 1987, Bhattacharjee and Coqblin 1988, Cox and Grewe 1988,... 

A comprehensive study of TESR in dilute Cu(Fe) alloys (5-lOppm Fe), was carried out by Ritter and Silsbee (1978) for temperatures 2Kfirst observation of TESR in a dilute magnetic alloy below its Kondo temperature. The authors analyzed the results in terms of coupled Bloch equations. The effective relaxation rate for the Cu(Fe) samples showed a clear minimum at approximately 2.5 K, with the rate increasing by about 40% to an apparent plateau at the lowest temperatures investigated. The effective g-value of the Cu(Fe) samples does not show any deviation of the observed g-value from that of pure Cu. The results indicate that the Cu(Fe) system is strongly bottlenecked. 

In the case of Cej Y Pd2Si2 (O x l) system the results of heat capacity, resistivity and susceptibility measurements (Besnus et al. 1987) show that a partial substitution of Ce by Y leads to a rapid decrease in the Neel temperature. Spontaneous magnetism and the Kondo effect coexist up to y = 0.4 (T = 3 K) where is estimated as 3 For higher Ce dilution the magnetic moment of the ground state doublet has become completely quenched and a genuine heavy fermion behaviour is evident, for example, y = 0.875 J/K Ce atom and = 10 K at Y = 0.5 and y = 0.58 J/K Ce atom and = 15 K at y = 0.75. 

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