Hybrid magnetic conducting system

Bis(dithiolene)-based systems have proved to be useful components of multifunctional conducting/magnetic hybrids, a challenging class of materials which represents one of the main targets in current materials science for their potential applications in... 

The low-temperature specific heat of the Ce compounds is dominated by magnetic contributions, which in most cases are related by a certain degree of hybridization between the 4f state and the conduction band. The magnetic properties of a system can obviously be studied by other techniques such as magnetization and electrical resistivity, or by more sophisticated techniques like neutron scattering. In this section we shall briefly discuss some relationships between the specific heat and the techniques mentioned. 

The distinction between Kondo metals, etc. and HF systems is fuzzy at best. As pointed out, the Kondo interaction is, among others, a basic ingredient of HF behavior. In a Kondo-lattice material one observes the effects of the Kondo interaction, for example on the magnetic properties, but very heavy quasiparticles are not formed and in consequence, the Sommerfeld constant is only slightly enhanced, That at least is the basis for a distinction we shall adopt. The hybridization between 4f and conduction electrons can lead to a hybridization gap in the density of states at the Fermi surface. The exact mechanism of gap formation is still under debate and also may vary from compound to compound. If a gap is present, one leaves the realm of Kondo metals and has, depending on the form of the gap (e.g., whether it is open in all crystallographic directions) and on its width, either a Kondo semimetal, semiconductor or insulator. The latter are certainly the most challenging class of Kondo compounds to understand. 

Near the Ce or Yb end of the R series, the 4f level thus approaches the Fermi level in energy and the 4f electrons hybridize more strongly with the conduction electrons with the kinetic energy E. This f-hybridized coupling constant is denoted by V. A theoretical treatment for such a system is called the periodic Anderson model (Anderson 1961). The parameters E, V, Ef and U predominantly control the dynamics of tiie system. These values depend actually on the crystal structure. The relation between flie magnetic ordering temperature and the distance between the Ce (or U) atoms is known as a Hill plot (Hill 1970). 

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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