Electron heavy fermions

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. 

Heavy Fermion Systems (HFS) are particular metallic materials which contain so-called heavy electrons. The characteristic observation for these systems is that the electrons which are responsible for the electric transport possess a high effective mass. This phenomenon, however, arises at low temperatures of about 10 K. At high values, e.g. room temperature, HFS show a normal behavior. 

We first begin with a short survey of the basic experimental evidence in sect. 4.1. The basic physical properties of the unstable-moment systems will be surveyed. In order to discuss their elastic and lattice dynamical properties that are of central interest here an outline of current theoretical concepts to describe unstable-moment systems, especially heavy-fermion compounds is given first (sect. 4.2). Because in heavy-electron compounds is much smaller than the CEF splittings they exhibit magnetoelastic anomalies as described in sect. 2. However, in addition, Griineisen parameter coupling to the heavy-electron bands leads to elastic effects below T. For a few typical Ce compounds this is studied within a microscopic model calculation in sect. 4.3. A more complete discussion... 

However, later on Cox (1987) suggested that nuclear resonance is a more effective method to probe the heavy-fermion state than electron-spin resonance, simply because the probe nuclei usually is closer to the heavy-electron sites so that the strong range... 

Steglich et al. 1980). Therefore, the Kondo-lattice system is called a heavy-electron or heavy-Fermion system. 

Ott et al, (1985) treated UCuj as an example of a magnetically ordered heavy-fermion system. From p(T) measurement they found an additional phase transition in the vicinity of 1 K, which appears to be due to gaps in the excitation spectrum of the heavy quasiparticles (Ott 1987). In connection with this transition, the p(T) dependence of UCus passes through a minimum at about 1.6 K and increases below that temperature, down to 0.02 K, by a factor of seven. The high value of p observed at 0.02 K is unusual for a heavy-electron ground state and has to be studied in more detail. 

Second Quantized Description of a System of Noninteracting Spin Particles.—All the spin particles discovered thus far in nature have the property that particles and antiparticles are distinct from one another. In fact there operates in nature conservation laws (besides charge conservation) which prevent such a particle from turning into its antiparticle. These laws operate independently for light particles (leptons) and heavy particles (baryons). For the light fermions, i.e., the leptons neutrinos, muons, and electrons, the conservation law is that of leptons, requiring that the number of leptons minus the number of antileptons is conserved in any process. For the baryons (nucleons, A, E, and S hyperons) the conservation law is the... 

On the other hand, recent measurements on UBcis have shown this intermetallic to be a very special species of superconductor with a large density of electronic states at Ep ( 1000 mJ/mol K ). Such results will certainly motivate both theoreticians and experimentalists and the so called heavy-fermion superconductors family is already developing . ... 

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. 

---