Heavy-fermion behavior

Preliminary results on low temperature transport properties in YbPdSb system with MgAgAs type structure are presented by Aliev et al. (1988). Resistivity shows a maximum at T = 50-100 K and the Fermi-liquid decrease of q go + AT2 at helium temperatures. The high value of the A coefficient 5 p 2 cm/K2 indicates the possibility of the heavy fermion behavior with y about 300MOO mJ/molK2. The Seebeck coefficient S is positive and shows a maximum Smax 22 pV/K at T 200 K. 

The diversity in structure and bonding possible for phosphides is effectively demonstrated by the monophosphides. Monophosphides MP of the group 1 and 2 elements (El, E2) are polyphosphides with i(P ) chains and P2" dumbbells, respectively. Ell and E12 monophosphides are not known. The E3 and E13 monophosphides are the so-called normal compounds with 3x = (M) (see Section 2). With El3, they form the zinc blende structure with tetrahedral heteroatomic bonds. Ternary derivatives such as MgGeP2 and CuSi2P3 have a random distribution of the M atoms, whereas CdGeP2, crystallizes in the ordered chalcopyrite type with a TO[GeP4/2] tetrahedral net (see Section 6.4). The E3 monophosphides form the NaCl structure. CeP is remarkable because of its physical properties (metal-semiconductor transition heavy-fermion behavior). The E14 monophosphides show the break usually observed when passing the Zintl border. Binary lead phosphides are not known SiP and GeP... 

From a theoretical point of view, the 3d electrons and the 4f electrons independently show some important characteristics, such as the Kondo effect and heavy-fermion behavior. The properties of the electrons have been studied by many researchers. A theoretical analysis of the electrons, however, in a physical sense, is still insufficient for explaining the magnetic properties of lanthanide-transition-metal magnets. The difficulties mainly come from the complexity of the mixture of the 3d and 4f electrons and of the interactions among them in these materials. 

A special situation occms for some of the cerimn and ytterbium based compoimds. Here, the 4/ (Yb +) configmation is the hole analogue of 4/ (Ce +). The degree of 4/ state contribution at the Fermi energy has a drastic influence on the physical properties of these materials, leading to valence instabihties or heavy fermion behavior. 

Short overviews on the unusual low-temperature properties of equiatomic CcTX compounds were given by Adroja and Malik (1991) and Fujita et al. (1992). These microreviews focus on valence fluctuation and heavy fermion behavior. The CeTIn indides are discussed together with the CeTGe germanides and CeTSn stannides. Those and most recent data on the indides are summarized here in the following paragraphs. 

Characteristic temperatures associated with heavy fermion behaviors were investigated by both macroscopic and microscopic measurements. On the viewpoint of the magnetic properties, the temperature dependence of the magnetic susceptibility suggests the presence of the characteristic temperature for heavy fermion behaviors. This is a maximum of the magnetic susceptibility at a certain temperature, called [20,22,30]. It has been believed that this behavior is correlated... 

The rare earth borides, carbides, and nitrides aU have compounds that manifest superconductivity, fundamental magnetic transitions, and other interesting phenomena such as strongly correlated heavy fermion behavior. Some of the rare earth transition metal borocarbides such as RTr2B2C have shown fascinating properties where magnetism and superconductivity phenomena coexist. 

The metallic Ce compounds exhibit a variety of properties which are related to their electronic states, including Kondo resistivity minima, intermediate valency, superconductivity, various types of magnetic ordering, and heavy fermion behavior (along with some of the properties previously mentioned). Some of these Ce compounds exhibit properties like those of Ce, including y-a phase transitions (related to variations in composition if not to variations in temperature or pressure). Others resemble either y-Ce or o Ce. 

Here Martin and Allen (1979) have shown that for an even count of f and d electrons, and with no other electrons in the conduction band, the appUcation of the Luttinger theorem (Luttinger 1960) leads to being in the gap of an intermediate-valent system. For an odd electron count the Fermi level lies in one of the density of states peaks resulting in intermediate-valent and heavy-fermion behavior (Martin 1982). This theory has been greatly stimulated by the establishment of an insulating behavior of SmBg at low temperatures, which has taken 12 years of research (Nickerson et al. 1971) and 3 conferences on intermediate valence (1977, 1981, 1982). 

Several models have been proposed in the past, usually explaining only a specific feature of the heavy fermions, for example the interplay between magnetism and heavy-fermion behavior (de Visser et al. 1991) or the effect of doping and disorder (Rasul 1991) or the transition from impurity to the lattice limit (Schonenberg and Keiter 1992, Wang Xu et al. 1991). 

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