Heavy-fermion materials

In addition, there are certain metals involving 4f- or 5f-states, formerly called Kondo metals and now described as heavy-fermion materials, which have a very large electronic specific heat and may be described as crystals in which every rare-earth metal is envisaged, coherently, in a Kondo-type spin flip. 

Heavy fermion materials correspond to the second case. In CeAl3 for example there is a 15 2 just below the Fermi level,... 

CeCu6 and CeRu2Si2 show neither superconductivity nor magnetic order down to 20 mK. Therefore, the behaviour of 1/7) in these compounds presents a fundamental property of heavy fermion material. As seen in Fig. 3,11,12 1/7) is temperature independent above the Kondo temperature, / , corresponding to a localized state of the 4/ electron. At higher temperatures 1/7) is expected to decrease due to the increase in the fluctuation rate of the localized 4f spin. Below 7k, 1/7) shows a 7) T= constant behaviour corresponding to the Fermi liquid state. 

Acknowledgements. The author wishes to thank his collaborators in the joint research referred to in this chapter. In particular he has benefit-ted from O.K. Andersen s expertise on his linear methods (LMTO, LAPW), from the extensive collaboration with M. Cardona on the electronic structures on semiconductors, not least the spin-splitting problems. R.C. Albers, M. Boring and G. Zwicknagl are thanked for their collaborations on the relativistic electronic structures of heavy-fermion materials, and A. Svane and L. Petit for several discussions and valuable information on their progress in the description of strongly correlated electron systems. 

UAuPt4, with a suspected preferential occupation of larger tetrahedra by Au atoms, is a non-magnetic heavy-fermion material with an extrapolated C/T value of 725 mJ/mol K2 (Ott et al. 1987). 

Uranium heavy-fermion materials whose magnetism has been studied with itSR... 

Other heavy-fermion materials that have been studied by tSR with respect to magnetism... 

In most cases the low-temperature specific heat (at temperatures much lower than the Debye temperature) can be described by Cp = yT+ T, where yT is the so-called linear term due to the excitations of the conduction electrons, and is the low-temperature approximation of the specific heat of the lattice. For normal conductors, y is of the order of l-10mJ/(molK ), while heavy fermions show y-values up to 1000mJ/(molK ). Details about cerium- and uranium-based heavy-fermion materials have been reviewed by Grewe and Steglich (1991), Sereni (1991), Loewenhaupt and Fischer (1993), Fournier and Gratz (1993), Wachter (1994), Nieuwenhuys (1995), Onuki and Hasegawa (1995) and Arko et al. (1999). 

The effect of chemical pressure on YbPtBi single crystals was studied by heat-capacity measurements on yttrium- and lutetium-doped samples (Lacerda et al. 1993). According to these measurements, the heavy-fermion state of this compound seems to be imchangeable by a relatively large amoimt of nonmagnetic doping (yttrium or lutetium). Furthermore, the heat capacity measurements reveal only a small pressure dependence when compared with other heavy-fermion materials. 

De Haas-van Alphen (dHvA)-type quantum oscillations as observed in the sound velocity and sound attenuation provide important information about the Fermi surface and the electron-phonon interaction (Roberts 1968, Fawcett et al. 1980). This technique has been successfully applied to intermetallic rare-earth compounds as discussed below. Recent progress in dHvA techniques for heavy-fermion materials (Taillefer et al. 1987, Reinders et al. 1986) should make similar MAQO experiments also possible. Compounds studied so far are LaAg, LaB5, LaAlj, RBj, CeSn3, CeB, CeCu and CePbj. 

The influence of high magnetic fields on elastic properties of heavy-fermion materials can lead to interesting effects. One might think of three possible sources for anomalous behaviour ... 

The discovery of superconductivity in CeCu2Si2 by Steglich et al. (1979) initiated the rapid development of heavy fermion physics. For nearly two decades, this material was the only Ce-based heavy-fermion superconductor at ambient pressure. Only recently, superconductivity at ambient pressure was found in the new class of heavy-fermion materials Ce MmIn3 2iH where M stands for the transition metal ion Co, Ir or Rh and m = I while n = 1 or n = 2 (Thompson et al., 2001). Typical examples are CeColus (Petrovic et al., 2001b) and Celrins (Petrovic et al., 2001a). 

Fig. 41. Schematic representation of the interactions that contribute to the magnetic relaxation in heavy-fermion compounds. Here a Gd ion is taken as ESR probe in a Ce-based heavy-fermion material.

In this chapter we do not describe the theory potentially applicable to single-crystal neutron data because it is the subject of Handbook chapters by Norman and Koelling and by Liu (Vol. 17, chs. 110 and 111, respectively). Scattering overlaps with and complements two other techniques sensitive to microscopic magnetic correlations. The techniques are nuclear resonance (NMR) and muon spin relaxation (p SR) spectroscopies whose applications to heavy-fermion materials are reviewed by Nakamura et al. (1988) (NMR) and Barth et al. (1988) and Schenck (1992) (p SR). Finally, there have by now been nearly countless general reviews of experiments and theory on heavy-fermion systems, many of which have appeared in this Handbook series (Vol. 10, chs. 63 and 70, Vol. 14, chs. 94, 96 and 97, Vol. 15, ch, 98, Vol. 16, chs. 105 and 106, Vol. 17, clis. 110 and 111 and this volume, chs. 130, 132 and 133). For a recent pedagogical introduction, the reader can consult the book by Hewson (1993). 

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