Kondo lattice

PrOs4Pi2 is a metallic with a relatively temperature independent resistivity for temperatures between 50 and 300 K. Below 50 K there is a rapid drop in the resistivity similar to that observed in Pr metal or Kondo lattice Ce compounds such as CeFe4Sbi2 (fig. 9). There is also a small kink in the resistivity at 7 K of unknown origin. The magnetic susceptibility... 

Keywords Anderson model, Kondo lattice, magnetic susceptibility, strong correlation. 

The Kondo-lattice Hamiltonian conserves total spin and being an interacting model is nontrivial to solve. However, as with the conjugated systems, it is possible to solve finite Kondo chains efficiently by employing the VB method. The VB... 

The transfer of spectral weight from low frequencies to high frequencies that accompanies the formation of pseudogap matter is the inverse of process in heavy electron materials by which at some onset temperature the itinerant coherent heavy electron state emerges out of the local moments that make up the Kondo lattice. It has recently proved possible to develop a two-fluid description that describes this emergent... 

In the second part of the lecture we want to demonstrate in the simplest possible way the problem of strongly correlated f-electrons. It is intimately connected with the Kondo lattice problem, or alternatively with the formation of a singlet state (5,6). 

An alternative approach to accounting for the maxima in the temperature dependence of p is based on the Kondo-lattice model (Lavagna et al. 1982). The periodic array of independent Kondo impurities, described by the single-ion Kondo temperature TK, provides a proper description at elevated temperatures, while a coherent state yielding a drop of the resistivity is attained when the system is cooled to below another characteristic temperature coh- Although this approach is suitable particularly for Ce compounds where the Kondo regime was identified inequiv-ocally, the coherence effects are probably significant also in narrow-band actinide materials, as indicated by an extreme sensitivity of the lower-temperature decrease of the resistivity to the presence of impurities. 

The exact diagonalization method has been widely exploited in the study of polyenes as well as small conjugated molecules. It has also been employed in studying spin systems and systems with interacting fermions and spins such as Kondo lattices. These studies have been mainly confined to low- dimensions. The exact diagonalization techniques also allow bench-marking various approximate many-body techniques for model quantum cell Hamiltonians. 

Some of the Kondo systems investigated in this section are not in fact dilute magnetic alloys. Instead they are Kondo lattices in which the magnetic moments lie on a sublattice of the RI compound (e.g. CeAy. The R-atom in all these compounds is Ce for which the d-f admixture interaction mentioned in section 3.1.2 is dominant. Such RI compounds are therefore Kondo systems in which eq. (30) for the resistivity holds. However, for temperatures T < Tr the magnetic moment is usually not totally compensated in a Kondo lattice since now there are too few conduction electrons to achieve full compensation on every magnetic lattice site. 

Kondo s treatnient omitted several important interaction terms but the latter authors were able to show that the thermopower depends in an interconnected manner on exchange coupling, potential scattering, and the range of the interaction (see Blatt et al., 1976). No theoretical work has as yet been done for Kondo lattices. However, the thermopower of CeAl2 which is a Kondo lattice does in fact show a giant thermopower anomaly similar to that found in dilute Kondo alloys. 

The CeAlj compound shows a VltW value of 1.2JK /Ce atom, but the C /T ratio increases up to 2 JK"VCe atom at 0.5 K, which signals the transition to the coherent coupling of the Kondo resonances in a Kondo lattice (Bredl et al. 1984). Under an applied field the C nlT maximum value is reduced to 1.7 J K /Ce atom for H = 4 T and lightly shifted to lower temperatures (see also Bredl et al. 1984). The effect of pressure on /ltCO) is similar to that in CeCug, reducing its value to 0.55 JK VCe atom under 8.2 kbar, but it is significant that the C /T maximum disappears under only 0.4 kbar of pressure. 

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. 

PrInAgz. Specific heat, magnetic susceptibility and neutron scattering (Galera et al. 1984) carried out on this Kondo-lattice material indicate a non-Kramers doublet (Fs,... 

YbPdSb. Transport, bulk magnetic and caloric measurements (Le Bras et al. 1995, Suzuki et al. 1995, Bonville et al. 1997) clearly show that cubic YbPdSb is a Kondo-lattice compound. CEF interaction puts a Fg quartet lowest on which the Kondo coupling acts with a strength of T 7 K. 

The first 5 materials are pseudo-binary or ternary compounds based on established HF or Kondo-lattice materials. The last 3 are independent intermetallics on their own for which NFL behavior has been claimed. 

The groimd-state properties of Kondo-lattice systems, for example, are expected to be very sensitive to external pressure. The latter may modify the strength of the exchange interaction J between the localized 4f and the conduction electrons and thereby the competition between the intersite (RKKY) and intrasite (Kondo) interactions. The balance between these effects has been theoretically described by Doniach (1977) for a magnetic phase diagram depending on the exchange parameter J. 

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