Kondo insulator

This review will include both types of studies, but will not discuss in any detail optically pumped NMR of semiconductors, which has been well-reviewed [5, 11, 12,14], or other unconventional techniques for detection of NMR signals. Physics-related NMR studies of more complicated semiconductor behavior such as Kondo insulators or semiconductors and other unusual semiconducting phases, and semiconducting phases of high-Tc superconductors, while very important in physics, will be neglected here. I have deemed it of some value to provide rather extensive citation of the older as well as of the more recent literature, since many of the key concepts and approaches relevant to current studies (e.g., of nanoparticle semiconductors) can be found in the older, often lesser-known, literature. My overall aim is to provide a necessarily individual perspective on experimental and theoretical approaches to the study of semiconductors by NMR techniques that will prove useful to chemists and other scientists. 

In reviews on Kondo insulators, Fisk et al. (1995, 1996) list eleven compounds as possible candidates ... 

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. 

The theory of intermediate valence (IV) forms the starting point for the treatment of HF materials. IV represents the limiting situation of a very strong coupling parameter g in strongly correlated electron materials, as can be seen from fig. 105. Kondo insulators in particular possess IV behavior at high temperatures as an essential ingredient. They form a borderline case. We have discussed them earlier as a separate class of materials. 

Q. Si, S. Paschen, Quantum phase transitions in heavy fermion metals and Kondo insulators. Phys. Status Solid B 250, 425 (2013)... 

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. 

Fisk 1992). It is reasonable to believe that the hybridization of the f-electrons with the band states accounts for the unusual properties of these materials. ESR experiments on SmBs are discussed in detail in sect. 5.3.2. There it was shown that the opening of a gap indeed can be deduced from the temperature dependence of the linewidth. It is clearly apparent that ESR is an extremely useful technique to study the dynamical susceptibilities in Kondo insulators. 

The second half of this review deals with newly developed branches of solid state physics. Here it is evident that ESR made major contributions to the physics of electronically highly correlated systems, like heavy-fermion systems and intermediate-valence compounds. In the latter compounds it still has to be proven that the low density of states, which have been detected experimentally, results from the so-called Kondo whole and is a characteristic feature of IVCs. Certainly a further interesting area of ESR will be the study of Kondo insulators. In KI a hybridization gap, as a consequence of the interaction of the band states with the f-electron system, develops at low temperatures. In KI a non-magnetic impurity will reveal an effective spin i and, hence, one expects that non-magnetic impurities become ESR active below a characteristic temperature. So far we are not aware of any ESR experiments of that type in Kondo insulators. 

Observation and characterization of the spin gap which appears upon the onset of semiconducting behavior in the Kondo insulator CeNiSn. 

If our proposal for the electronic structure of these materials is right, the terminology Kondo insulator (Fisk 1992) is not correct. The normal Kondo effect in a metal is a scattering of conduction electrons by the occupied 4f states and it manifests itself in... 

Fig. 35. Predictions of a slave-boson treatment of a Kondo insulator for the specific heat Cp (solid line) and temperature derivative of the f-occupation number dUf/dT (solid circles). As opposed to the Anderson impurity model, where dnf/dT peaks at a temperature which is 2-3 times larger than the temperature where Cp is maximum, for this Anderson Lattice calculation both quantities peak at roughly the same temperature. From Riseborough (1992).

Ce3Bi4Pt3 [245]) charge gap development in the Kondo insulators involves a redistribution of spectral weight from low to high frequencies, and influences electronic states over an energy scale that is much higher than the characteristic temperature at which gap development occurs. In the case of the Kondo insulators, the dramatic spectral weight redistribution is nicely accounted for in calculations of the periodic Anderson model [72]. 

Since its initial discovery as a prototypical Kondo insulator (Hundley et al., 1990 Severing et al., 1991), Ce3Pt3Bi4 has elicited substantial attention. Unlike most other f-electron intermetallics, this compound is not metallic instead, its resistivity shows activated behaviour corresponding to a narrow band gap on the order of 5 meV. The gap is believed to arise from the hybridization of f electrons with the conduction band. Considerable effort has gone into further characterizing this compound (which is generally formulated as Ce3Bi4Pt3 in the physics literature), but this is beyond the scope of the present review. 

Dit] Ditusa, J.F., Friemelt, K., Bucher, E., Aeppli, G., Ramirez, A.P., Heavy Fermion Metal-Kondo Insulator Transition in FeSii tAU > Phys. Rev. B, 58(16), 10288-10301 (1998) (Crys. Stracture, Experimental)... 

---