Electron Spectroscopy Using Metastable Atoms as Probes for Solid Surfaces.

This review considers the experimental and theoretical studies of concentrated Kondo systems (CKS), Kondo lattices, substitutional solid solutions and their transition from Kondo impurity to Kondo lattice, and ‘intermediate valence compounds’ which are, in fact, high T K CKS (T K is the Kondo temperature). The anomalous low temperature properties of CKS are related to the formation of the narrow (∼k B T K) high-amplitude Abrikosov-Suhl resonance E R in the vicinity of the Fermi level E F. This resonance is situated exactly at E F in low T K CKS with T K ΔCF (ΔCF is the crystal field splitting). In low T K ‘j=1/2’ CKS the condition E R=E F leads to an increase of the density of states at E F, which is large enough to induce heavy fermion superconductivity in CeCu2Si2, UBe13. We demonstrate that the transition from low T K (E R=E F) to high T K CKS (E R≠E F) might be what was formerly considered as a ‘Kondo-lattice-intermediate valence state’ transition...

Concentrated Kondo systems

In recent years the class of materials known as intermediate valent, Kondolattice or heavy-fermion systems has aroused much interest. The unusual properties of these materials arise from the behaviour of their 4f-(or 5f-)electrons. The strong electron-electron interactions within the 4f shell cause the f-bands of these rare earth or actinide metal containing species to show very high masses. Thus the densities of states are very high, leading to correspondingly high specific heats and magnetic susceptibilities. A mean field theory is described, based on the Anderson lattice model, in which these properties are straightforwardly explained. Its relationship to the 1/N expansion, and the corrections to mean field, are outlined.

Mean-field theory of intermediate valence/heavy fermion systems

An attempt is made to explain the occurrence of superconductivity in Kondo lattice systems with special reference to CeCu2Si2. Starting point is the Fermi liquid approach. It is generalized from a Kondo impurity to the Kondo lattice by means of the Korringa-Kohn-Rostocker method. From it a hybridization model is derived and discussed in detail. Two electron-phonon mechanisms are investigated which appear in Kondo lattices. One results from the additional phase shifts caused by the Kondo ions while the other is responsible for the so-called Kondo volume collapse. It is shown that the latter is sufficiently strong in order to explain why CeCu2Si2 is a superconductor while LaCu2Si2 is not. An estimate for the superconducting transition temperatureT
c produces the right order of magnitude.

On the theory of superconductivity in Kondo lattice systems

Measurements of the specific heat, magnetization, and transport properties (resistivity, thermoelectric power, and thermal conductivity) of single-crystal CeCu6 are reported with special emphasis on the effect of magnetic field. The relative field variations of the differential susceptibility, the coefficients of the linear temperature term of the specific heat, and theT
2 resistivity term are presented. They are directly related to the field curvature of the magnetization, indicating the itinerant nature of thef electrons. The Wilson ratio reaches a maximum atH=H*, which may correspond to a crossover from a low-field phase in which antiferromagnetic correlations dominate, to a highly polarized phase. Intersite coupling seems to play an important role in heavy-fermion compounds. Another interesting result is the occurrence of residual positive magnetoresistivity, which also appears to be a general feature of heavy-fermion compounds. The properties of CeCu6 are compared to those of otherf instability compounds and to present theories.

/pdf/thermodynamic-and-transport-properties-of-cecu6-521nd5fqgi.pdf

A. Yoshimori

Papers

Theory of dense Kondo system

Heavy electrons in alloys

Beam Kondo effect? — Possible anomalous penning deexcitation spectrum

Correlation effects on charge exchange in the time-dependent Newns-Anderson model

Theory of chemisorption on surfaces of ferromagnetic metals