Superconductivity, heavy-electron systems

The first example of a heavy electron system CeAl3, is a good material to study the heavy fermion systems because it presents an extreme case of these properties (it does not become superconducting at low temperatures). Then it should be useful to study the basic properties of these materials. 

In this short review we have given examples of chemically induced changes in the behaviour of heavy-electron materials. First we noticed that the mere occurrence of the heavy-electron state seems almost unpredictable if only the chemical composition and the crystal structure of a compound are given. Then we pointed out that the heavy-electron state itself is quite sensitive to small alterations of the chemical composition in the sense that it can be destroyed or created by minor variations of the constituents of respective materials. Finally we emphasized that possible ground states of heavy-electron systems, like superconductivity or magnetic order, may be... 

Stassis and co-workers (Stassis et al. 1985, 1986) have measured the neutron paramagnetic form factors of three superconducting heavy-fermion systems, CeCu2Si2, UPtj and UBei3, both above and below their critical temperatures. The material CeCu2Si2 has 1 K. The form factor x(G) at 4.2 K is well fitted by the theoretical atomic form factor for Ce state, i.e. one electron in the 4f sheU, as shown in fig. 10. The form factor remains the same at 300 K where the susceptibility is Curie-Weiss, and at 0.1 K where the material is superconducting. This indicates that the same set of electrons are polarized by the magnetic field at all three temperatures. The resistivity maximum of this material appears at about 20 K and the Curie temperature is... 

However, in the preceding two decades, there have been many experimental discoveries, beside high-Tc superconductivity, evidencing that we do not have yet the proper theoretical skills and tools to deal well with strongly correlated electron systems. For instance, heavy-fermions, fractional quantum Hall effect, ladder materials, and very specially high-Tc superconductivity seem not accessible from the weak coupling limit. 

Not only a surprising superconducting state below 0.85 K, but also an exotic p(T) behaviour (shown in fig. 3.34) was observed by Ott et al. (1983). As T decreases below room temperature, p(T) increases (despite a number of anomalies) monotonically towards a sharp maximum at 2.5 K, where an enormous value of 200 p.S2 cm was found. The superconducting state, originally attributed to a spurious phase (Bucher et al. 1975), was confirmed to be a bulk effect by the low-field x(T) and low-temperature C(T) measurements. From these measurements it was concluded that superconductivity originates from the subsystem of heavy quasi-particles. The existence of a strongly correlated electronic system follows from the C/T... 

We have ended this review with heavy-fermion systems. In fact, in terms of their hybridization, they lie intermediate between the materials discussed in sect. 4, and those discussed in sect. 5. What is extraordinaiy about the heavy-fermion compounds, and has been responsible for the great interest in them, has been the formation of a correlated state at low temperature. That the superconductivity appearing in some of these systems is intimately connected, if not driven by, the magnetic interactions is beyond doubt, although a complete theory is still lacking. The heavy-fermion state involves both the localized-like f electrons and the conduction-electron states. For example, extremely laige masses are seen in the de Haas-van Alphen experiments (Reinders et al. 1986, Taillefer and Lonzarich 1988), but these do not account for all the susceptibility a large... 

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