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

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.

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."

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Different models of capital exchange among economic agents have been proposed recently trying to explain the emergence of Pareto's wealth power law distribution. One important factor to be considered is the existence of risk aversion. In this paper we study a model where agents posses different levels of risk aversion, going from uniform to a random distribution. In all cases the risk aversion level for a given agent is constant during the simulation. While for a uniform and constant risk aversion the system self-organizes in a distribution that goes from an unfair "one takes all" distribution to a Gaussian one, a random risk aversion can produce distributions going from exponential to log-normal and power-law. Besides, interesting correlations between wealth and risk aversion are found. (J. R. Iglesias). 1 J.R.I. acknowledges support from CNPq (Brazil) and the hospitality and support and G.A. thanks the hospitality of the Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil. We acknowledge partial support from SETCYP (Argentina) and CAPES (Brazil) through the Argentine-Brazilian Cooperation Agreement BR 18/00.