Heavy spin fluctuations

At this point we want to stress that the heavy fermion systems (22) are related to a very large spin enhanced susceptibility or localized magnetic moments, very narrow bands of elementary excitations at the Fermi level, and a new type of pairing, in the case where they become superconducting, at low temperatures even if they were spin fluctuators above the critical superconducting transition temperature Tg. 

An even broader response than found in U heavy-fermion compounds was observed in the spin-fluctuator UA12, where a significant intensity extends to energies above 100 meV. The residual line width was deduced to be equal to 25 meV (Loong et al. 1986). 

UPt3 belongs to the most exotic U-compounds because of the co-existence of superconductivity and pronounced spin-fluctuation effects at low temperatures. Due to very high y value ( = 400 mJ/mol K2) it can be classified as a heavy-fermion... 

In chapter 98, Julian Sereni adds significantly to an evaluation of systematic, experimental low-temperature studies of the ambivalent behaviors of cerium (ferromagnetism, antiferromagnetism, spin glass, superconductivity, valence fluctuations, heavy Fermion, Kondo and spin fluctuations) which depend upon its environment in materials. The systematic conclusions arrived at should provide new data against which the theory can be advanced. 

Closely related to the heavy fermions and spin fluctuators are the valence fluctuation/intermediate valence materials. The origin of this phenomenon starts with cerium and its a 7 transformation (see sections 3.3.4 and 3.7.2). Today it involves many cerium materials and also compounds of samarium, europium, thulium and ytterbium. Because of the breadth of the subject matter and space limitations in this chapter we refer the reader to the following reviews Jayaraman (1979), Lawrence et d. (1981), de Chatel (1982), Coqblin (1982), Nowik (1983), Brandt and Moshchalkov (1984), Varma (1985) and Stassis (1986). 

Today the lanthanide eontraction is still one of the most important tools available to the scientist in applying systematics to the behavior of lanthanide materials. Deviations from the lanthanide contraction established for a given compound series gives a measure of anomalous valences for cerium, samarium, europiun, thulium and ytterbium (see section 3.2) which are important in evaluating the nature of these elements in valence fluctuation, heavy fermion, and spin fluctuation behaviors (see section 4.4.4). 

In the limit of weak hybridization of the 4f and conduction electrons, the charge fluctuations are strongly suppressed and there remain only spin fluctuations. In this Kondo limit the f-electron level width is small compared to the (negative) f-electron energy Sf as well as small compared to the Coulomb energy U. At low temperatures these systems exhibit an unusually high electronic specific heat coefficient y or a large effective mass m and are therefore called heavy-fermion systems. 

The main experimental information that usually can be deduced from ESR experiments in metals are the g-shift and the linewidth broadening. Both quantities can be measured as a function of an external parameter, like temperature or pressure. The g-shift corresponds to the Knight-shift in NMR experiments and yields information about the static susceptibility. This review nicely documents how the experimental results on the g-shifts in metals provide direct and detailed information on the band structure. The ESR linewidth in metals is determined by the spin-lattice relaxation time and has to be compared to l/T) as deduced from NMR results. The linewidth is determined by the density of states, but in addition yields detailed information on the low-frequency spectrum of spin fluctuations. This is of high relevance in the field of high-Tc superconductors and heavy-fermion compounds. 

---