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The Kondo lattice model including nearest-neighbor magnetic exchange interactions is studied here in a mean-field approximation describing both the Kondo state and the intersite magnetic correlations. In the case of antiferromagnetic correlations, our model yields a decrease of the Kondo temperature compared to the one-impurity value, giving, therefore, a ''revisited'' version of the Doniach diagram with a rather flat Kondo temperature in the nonmagnetic case, as observed in several cerium compounds. Our model shows also that short-range magnetic correlations can appear at a temperature clearly larger than the Kondo temperature, as observed in compounds such as CeCu 6 or CeRu 2 Si 2 . ͓S0163-1829͑97͒04642-0͔

A theory is presented that describes a spin-glass phase at finite temperatures in Kondo-lattice systems with an additional Ruderman-Kittel-Kasuya-Yosida interaction represented by long range, random couplings among localized spins as in the Sherrington-Kirkpatrick ͑SK͒ spin-glass model. The problem is studied within the functional integral formalism where the spin operators are represented by bilinear combinations of fermionic ͑anticommuting͒ Grassmann variables. The Kondo and spin-glass transitions are both described with the mean-field-like static ansatz that reproduces good results in the two well-known limits. At high temperatures and low values of the Kondo coupling there is a paramagnetic ͑disordered͒ phase with vanishing Kondo and spin-glass order parameters. By lowering the temperature, a second order transition line is found at T SG to a spin-glass phase. For larger values of the Kondo coupling there is a second order transition line at roughly T k to a Kondo ordered state. For TϽT SG the transition between the Kondo and spin-glass phases becomes first order.

We present theoretical results for a Kondo-lattice model with spin-1/2 localized moments, including both the intrasite Kondo coupling and an intersite antiferromagnetic exchange interaction, treated within an extended mean-field approximation. We describe here the case of a non-integer conduction-band filling for which an "exhaustion" problem arises when the number of conduction electrons is not large enough to screen all the lattice spins. This is best seen in the computed magnetic susceptibility. The Kondo temperature so obtained is different from the single-impurity one, and increases for small values of the intersite interaction, but the Kondo-effect disappears abruptly for low band filling and/or strong intersite coupling; a phase diagram is presented as a function of both parameters. A discussion of experimental results on cerium Kondo compounds is also given.

The temperature dependence of the thermoelectric power ͑TEP͒ of metallic systems with cerium and ytterbium ions exhibits characteristic features which we explain by the Coqblin-Schrieffer model ͑CSM͒. We specify a given system by the degeneracy and splitting of the crystal-field ͑CF͒ levels, the strength of the exchange and potential scattering, and the number of f electrons or f holes; for cerium and ytterbium ions we assume n f р1 and n f hole р1, respectively. The Kondo temperature T K is then generated by the ''poor man's PHYSICAL REVIEW B 68, 104432 ͑2003͒

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We present theoretical results for the underscreened Kondo lattice model with localized S = 1 spins coupled to a conduction band through a Kondo coupling, J K , and interacting among them ferromagnetically. We use a fermionic representation for the spin operators and expand the Hamiltonian in terms of bosonic fields. For large values of J K , we obtain a ferromagnetically ordered solution and a Kondo regime with a Kondo temperature, T K , larger than the Curie temperature, T C . This finding suggests a scenario for a coexistence of Kondo effect and ferromagnetic order. In some uranium compounds, such as UTe or UCu 0.9 Sb 2 , this kind of coexistence has been experimentally observed: they order ferromagnetically with a Curie temperature of order T C ϳ 100 K and exhibit a Kondo behavior for T Ͼ T C . The proposed underscreened Kondo lattice model accounts well for the coexistence between magnetic order and Kondo behavior and yields to a new "ferromagnetic Doniach diagram."