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. 

An important feature of the heavy electron systems which have been discussed by Ott in the previous paper of this Symposium is that, even if the specific heat parameter and the paramagnetic susceptibility are both very large compared to the normal free electron metals, their ratio is not very different from what is found for normal metals (a review by Lee et al. of the theories, and experimental facts, of heavy electron systems, can be found in (17)). 

Heavy electron systems with a "normal low temperature ground state CeAl, UAI2. 

With these few examples we intended to demonstrate how changes in the chemical composition may affect the low-temperature properties of materials that are likely to show heavy-electron behaviour, without severely affecting other parameters like the lattice constant or even changing the crystal structure. In the next section we concentrate on changes of the ground state of heavy-electron systems that are induced by small variations of the chemical composition, without affecting too much the heavy-electron state itself. 

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... 

The so far highest y-value of 8000mJ/K was observed for YbPtBi (Fisk et al. 1991, Thompson et al. 1993). This bismuthide can be classified as a very heavy-electron system. The heavy-mass state in YbPtBi is unconventional in that it develops from Bloch states in an electron subsystem with a low carrier concentration (Hundley et al. 1997, Fisk et al. 1991). 

Striking behavior has been observed for CeB6, SmBe, and YbBe. CeBe was first pointed out by Kasuya and others to be a heavy electron system. s jg so-called Kondo lattice system with the Kondo effect from the localized f-electrons and conduction electrons playing a large role in the determination of the physical properties. Complex phases have been reported and quadrupole and octupole coupling are also indicated to be effective. SmBe exhibits valence fluctuation and is a Kondo insulator with a gap temperature of A = 27 K. YbBe has been reported to exhibit valence fluctuation also. 

Kondo and RKKY interactions may become poorly defined and strongly stress dependent. Thus, plausibly the pressure-dependent behavior described above reflects contributions not found in metallic heavy-electron systems. 

The electronic specific heat coefficient, y, is proportional to the DOSs at the Fermi level. In general, in pure metals, it is of the order of a few mj/ mol K. Simply thinking, the electronic DOSs at the Fermi level is proportional to the effective mass of conduction electrons. The most conspicuous and noticeable systems with respect to their electronic specific heat coefficients are heavy electron systems that include some actinide and cerium compounds to be mentioned later. For example, in the case of a typical heavy electron system CeCug, the electronic specific heat coefficient reaches 1.5 J/mol K, which is more than 1000 times larger than that of a normal metal (Satoh et al., 1989). 

Tp is defined as the temperature when electrons condense into the FL state. If Tp decreases from about 10,000 K, Fermi energy in the normal metal, to the order of 10 K in the Kondo metal, y will grow around 1000 times in value. The effect of pressure will be remarkable as we understand a heavy electron system, which has extremely narrow bandwidth. 

Coupling of 4f or 5f electron shells with delocalized itinerant electrons, which can result in valence instabilities (mixed-valence systems, fluctuating valence systems, heavy-electron systems). 

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