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

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.

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Thermodynamic and transport properties of CeCu6

Recent advances on ferromagnetic superconductors, UGe 2 , URhGe and UCoGe are presented. The superconductivity (SC) peacefully coexists with the ferromagnetism (FM), forming the spin-triplet state of Cooper pairs. The striking new phenomena, such as SC reinforced by the magnetic field, are associated with Ising-type ferromagnetic fluctuations. A variety of ferromagnetic ordered moments between UGe 2 , URhGe and UCoGe affords to understand the relation between FM, tricriticality and SC.

Ferromagnetism and Superconductivity in Uranium Compounds

Even in the absence of both magnetic order and superconductivity, Ce‐based Kondo lattice systems show distinct anomalies in the low‐temperature specific heat and thermopower. These features can be understood, in the frame of a microscopic treatment of the asymmetric Anderson model, by taking into account the periodicity of the Ce sites. A substantial variation in the observed phenomena between CeAl3 and normal (n) state CeCu2Si2 on the one hand as well as CeRu2Si2 and CeCu6 on the other highlight the importance of realistic band structure calculations for the quasiparticles (‘‘heavy fermions’’) in these different compounds. The temperature dependence of the upper critical magnetic field Bc2(T) for the heavy‐fermion superconductor CeCu2Si2 (Tc≂0.6 K) shows a flat maximum at ∼0.2 Tc, which is ascribed to coherence‐derived structure in the n‐state quasiparticle density of states. A dc Josephson effect with a critical pair current of ordinary size is observed for the first time on a weak link between CeCu2Si2...

Heavy fermions in Kondo lattice compounds (invited)

1.1 Introduction 1.2 Localisation, valence and magnetism 1.3 From the Kondo impurity to the Kondo lattice 1.4 The Doniach model 1.5 Spin fluctuations and the non Fermi liquid properties 1.6 Quantum phase transition 1.7 Fermi surface 1.8 Comparison with He 1.9 Experiments : crystals, pressure, method 1.9.1 Materials, measurements 1.9.2 From transport measurement to heavy fermion properties

On the Heavy Fermion Road

Specific heat measurements on CeAl3 are described and compared to thermal‐expansion results. At very low temperatures, the Gruneisen parameter reaches giant negative values. Comparisons are made with the properties of 3He and the unstable‐valent 4f compounds (YbCuAl, CeBe13, CeSn3, and CePd3). It is suggested that the magnetic anisotropy may play a major role.

J. C. Lasjaunias

Papers

Low‐temperature properties of CeAl3