Kondo semiconductors

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. 

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. 

Kiefl, R.F., J.H. Brewer, S.R. Kreitzman, G.M. Luke, T.M. Riseman, T.L. Estle, M. Celio, and E.J. Ansaldo (1989b). Proceedings of the 15th International Conference on Defects in Semiconductors, edited by G. Ferenzi (Trans. Tech. Aedermannsdorf, Switzerland). Kondo, J. (1984) Physica 125B, 279. 

The model of a degenerate gas of spin polarons suggests that if the direct or RKKY interaction between moments is weak and EF too great to allow ferromagnetism then the moments might all resonate between their various orientations. This would mean that it is possible in principle to have a heavily doped magnetic semiconductor or rare-earth metal in which there is no magnetic order, even at absolute zero. This possibility is discussed further in Section 8 in connection with the Kondo effect. 

Many papers have been published on the theory of die Kondo effect, including some exact solutions. We recommend the 260 page review by Tsvelich and Weigmann (1983). Our aim in giving a simple non-mathematical account is to point out the similarity between the enhancement of the effective mass that occurs in crystalline metallic systems near to the conditions for a Mott transition (Chapter 4), and also to address the possible effects of free spins in doped semiconductors near the transition (Chapter 5). 

Shoji, I., Kondo T. A. and Ito R., Second-order nonlinear susceptibilities of various dielectric and semiconductor materials. Opt.Quantum Electr. (2002) 34 797-833. 

The largest peak in Fig. 4-3, labelled 2. can be easily identified with the fairly parallel bands separated by between 4 and 6 eV over most of the region shown in Fig. 4-4. (A careful study of this was made recently by Kondo and Moritani, 1977.) These bands arise from the Jones Zone, which will be discussed in detail in the treatment of tetrahedral semiconductors with pseudopotentials in Chapter 18. The energy at which this peak occurs was used earlier as a basis for obtaining experimental values for the covalent energy Fj (Harrison and Ciraci, 1974). 

Studies in magnetism are but one application of p,SR. Other fields where pSR has given important information is the diffusion of light interstitials, especially with regard to quantum diffusion in metals, semiconductors and insulators (Kehr et al. 1982, Kondo 1986, Kadono 1990, Prokof ev 1994, Storchak et al. 1996, Karlsson 1996). Other very active fields are applications to chemistry with emphasis on chemical kinetics especially in connection with radical formation (Brewer et al. 1975, Walker 1983, Fischer 1984, S.F.J. Cox and Symons 1986, Roduner 1990, 1999, Fleming and Senba 1992) and also the study of hydrogen states in semiconductors (Chow et al. 1995). pSR in life sciences is discussed by Nagamine (1999) in an article on Exotic applications of muons. 

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. 

Prusseit W, Corsepius S, Zwerger M, Berberich P, Kinder H, Eibl O, Jaekel C, Breuer U, Kurz H (1992) Epitaxial YBajCUjO. 5 films on silicon using YSZ/Y O buffer layers. Physica C 201 249-256 Randhaw H (1991) Review of plasma-assisted deposition processes. Thin Solid Films 196 329-349 Saji K, Kondo H, Takahashi H, Futata H, Angata K, Suzuki T (1993) Development of a thin-film oxygen sensor for combustion control of gas appliances. Sens Actuators B 13(14) 695-696 Sze SM (ed) (1994) Semiconductor sensors. WUey, New York Tiemann M (2008) Repeated templating. Chem Mater 20 961-971... 

CeRu2 is a mixed-valence system with a valency close to 4. The ESR experiments on this system are reviewed in sect. 12, which deals with local moment spin resonance in conventional superconductors. SmBs is an intermediate valence system, but belongs to a new class of small gap semiconductors. These materials exhibit Kondo-lattice behavior at elevated temperatures but evolve into semiconducting materials with small gaps of the order of a few kelvins or a few tens of kelvins as the temperature is lowered (Aeppli and... 

Differences in the surface conductivity with surface termination of diamond can be applied to the nanolithographic modification of diamond surfaces by use of atomic force microscopy (AFM) techniques [50-52]. Modification can be carried out by applying an electrical bias to the sample surface via a conductive cantilever tip, e.g., Au coated Si (Fig. 8.8). Surface modification using such an AFM technique is relatively general, and has been achieved for semiconductor materials such as Si [53], GaAs [54] and metals such as Ti [55]. Recently, Tachiki et al. and Kondo et al. have applied this technique to single-crystal homoepitaxial diamond thin films, undoped and boron-doped, respectively. In this section, we discuss the properties of diamond surfaces modified via AFM techniques and possible applications. 

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