Kondo semimetal

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. 

A recent addition is CeRhAs (Yoshii et al. 1996), 3iidiich is closely related to CeRhSb. All these materials contain 4f elements that ate known candidates for intermediate valence behavior and at high temperatures one finds evidence for such an electronic state in the majority of cases. Published [iSR data exist mainly for the first two cases and also, to a more limited extend, for (10). The latter is, together with (3) and (6), a prime example of a typical Kondo insulator (Kasuya 1996), while (1) and (2) have always been classified as Kondo semiconductors instead. As better samples have become available, it has been realized that they are now better described as Kondo semimetals (Takabatake et al. 1996, 1998a). More details are provided below. 

Rare earth monopnictides RX possess the NaCl type crystal structure. The primitive cell contains one molecule of RX. Figure 45 shows the crystal structure and its Brillouin zone. Especially the CeX compounds, in which X stands for N, P, As, Sb and Bi, have attracted a particular interest because of various anomalous magnetic and transport properties (Kasuya et al, 1987). LaX is expected to be a semimetal with a small and equal number of electrons and holes. A similar semimetallic character is observed in CeX, except CeN. CeX is a Kondo-lattice compound, having anomalous magnetic properties at low temperatures. Even though the carrier number is small, the Kondo effect is strong and the... 

---