Valence fluctuation phenomena occur in rare-earth compounds in which the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration (valence) and/or of the magnetic moment. The authors review the experimental results observed in the subset of such systems for which the 4f ions form a lattice with identical valence on each site. The discussion includes key thermodynamic experiments, such as susceptibility and lattice constant, and spectroscopic experiments such as XPS and neutron scattering. This is followed by a review of existing theoretical work concerning both the ground states and the isomorphic phase transitions which occur in such compounds; the emphasis is on those aspects which make valence fluctuation phenomena such a challenging many-body problem.

https://iopscience.iop.org/article/10.1088/0034-4885/44/1/001/pdf

Valence fluctuation phenomena

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

Publisher Summary Heavy-fermion compounds exhibit a considerably more stable 4f configuration, although the 4f occupancy still deviates slightly from integral values. They derive their properties at low temperatures from dramatically sharper scattering resonances near E F . The narrow scattering resonances at the Fermi level are due to many-body effects. They lead, within the framework of Landau's Fermi-liquid theory, to strongly renormalized electronic quasiparticles with very heavy masses. The formation of heavy quasiparticles takes place at low temperatures. Although a Fermi-liquid description of the low-temperature phase of heavy-fermion compounds is possible in principle, a straightforward application of the formulae of Sommerfeld's theory for one component, noninteracting Fermi system can be quite misleading. The structure of quasiparticles results from the interplay of two kinds of electronic states, that is, well localized ionic f states and rather delocalized conduction-band states, both, in general, anisotropic. Thus, the number of Fermi-liquid parameters required to understand the experimental observations is not small as in simple metals. In addition, the interactions among quasiparticles are very important in heavy-fermion compounds. Energies due to the interactions between heavy quasiparticles can be as large as the effective band width k B T * as inferred, for example, from cooperative effects found in these systems. Superconducting and magnetically ordered phases are formed within heavy-quasiparticle liquids.

Chapter 97 Heavy fermions

We present results of low-temperature calorimetric and resistive measurements on the isostructural heavy-fermion compounds and . `Non-Fermi-liquid' effects are established which suggest the nearness of an antiferromagnetic quantum critical point (QCP) in both systems. The observed deviations from the properties of a Landau Fermi liquid (FL) may be related to anomalous energy dependences of both the quasiparticle mass and the quasiparticle - quasiparticle scattering cross section. For , a moderately heavy FL can be recovered by application of moderate values of either magnetic field or hydrostatic pressure. For p = 1.7 GPa a novel, non-superconducting, phase transition has been discovered at .

https://iopscience.iop.org/article/10.1088/0953-8984/8/48/016/pdf

Quantum critical phenomena in undoped heavy-fermion metals

We report measurements of the electrical resistivityρ, the thermal conductivityk and the thermoelectric powerS between 1.5K and 300K on the anomalous CeCu2Si2 compound and on LaCu2Si2 as reference compound. For LaCu2Si2 the temperature dependences ofρ andS are in accord with those found in otherd band metals. For CeCu2Si2 the observed resistivity (ρ≃220 µΩ cm at 200K) leads to a very short electronic mean free path which is of the order of the Ce-Ce spacing. Correspondingly,k is almost identical with the phonon contributionk

 p
 . Below 20K, resistivity and thermoelectric power strongly suggest Fermi liquid behavior with a “degeneracy temperature” between 20K and 40K. Above 200K, bothρ andS decrease proportionally to −ln(T/1 K).

Transport properties of LaCu2Si2 and CeCu2Si2 between 1.5K and 300K

— We discuss measurements of the susceptibility, specific heat, and transport properties (electrical and thermal resistivity, thermoelectric power) performed on polycrystalline UA12 samples. Despite the high disorder of the samples, we observed anomalies in the various quantities, which are believed to be intrinsic to UA12. Most of these anomalies can be interpreted with the existence of a narrow 5f band at the Fermi level and of paramagnon excitations within this 5f band. The intermetallic compound UA12 is the only known (atomically ordered) metal for which the exis tence of ferromagnetic spin fluctuations (with Tst — 25 K) was inferred from such different quantities as specific heat , c, susceptibility, x> and resistivi ty , p , which were measured above = 1 K [1]. In this article, we present new results concerning t ransport properties (thermal resistivity, TR, and thermoelectr ic power , TEP) together with resul ts of the quantities mentioned above (c, x, p)W e comment on the \\ and c measurements first. In bo th cases , the tempera ture regime was extended to well below 1 K. Details of these exper iments will be published e lsewhere . He re , we only want t o state that the analysis of x and c (between 0.3 and 2.2 K) confirms the exis tence of paramagnons ; however Tst being 4-7 K , i.e. considerably smaller than Fig. 1. — Electrical (left) and thermal (right) resistivities of UAU vs. temperature. Ts( = 25 K published by Brodsky [1]. A smaller value (— 10 K) was recently also found by Fournier [2]. Besides the disagreement in the Ts£ values we have also found disagreement in the y coefficients of the low temperature specific heat , our y values being 10-15 % smaller than Brodsky ' s [1]. Fu ture exper iments must show to what extent the metallurgical quality of the samples is responsible for this discrepancy. In figures 1 and 2 w e present the results of the t ransport measurements . A more detailed discussion will be published elsewhere. The salient features of the figures are : 1) A giant increase of p from its (large) residual value p n = 26 ixfl.cm to 190 (xfi.cm at 300 K, which cannot b e explained by electron-phonon scattering [3]. This has been attr ibuted to the unusual scattering process of the conduction electrons from the paramagnon excitations of the 5f band [4]. 2) Very large values of the Lorentz ratio

W. Franz

Papers

Transport properties of LaCu2Si2 and CeCu2Si2 between 1.5K and 300K

Transport properties, susceptibility and specific heat of UAl2

Transport anomalies in CeCu2Si2