2,253 research outputs found

    Spin glass freezing in Kondo lattice compounds

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    It is presented a theory that describes a spin glass phase at finite temperatures in Kondo lattice systems with an additional RKKY interaction represented by long range, random couplings among localized spins like in the Sherrington- Kirkpatrick (SK) spin glass model. The problem is studied within the functional integral formalism where the spin operators are represented by bilinear combinations of fermionic (anticommuting) 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 (disordered) phase with vanishing Kondo and spin glass order parameters. By lowering the temperature a second order transition line is found at Tsg to a spin glass phase. For larger values of the Kondo coupling there is a second order transition line at roughly Tk to a Kondo ordered state. For T<Tsg the transition between the Kondo and spin glass phases becomes first order.Comment: 21 pages, 1 figure, to appear on Phys. Rev.

    A van Hemmen-Kondo model for disordered strongly correlated electron systems

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    We present here a theoretical model in order to describe the competition between the Kondo effect and the spin glass behavior. The spin glass part of the starting Hamiltonian contains Ising spins with an intersite exchange interaction given by the local van Hemmen model, while the Kondo effect is described as usual by the intrasite exchange JKJ_K. We obtain, for large JKJ_K values, a Kondo phase and, for smaller JKJ_K values, a succession, with decreasingComment: 14 pages, 4 figures, accepted for publication in Phys. Rev.

    Spin Glass and antiferromagnetism in Kondo lattice disordered systems

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    The competition between spin glass (SG), antiferromagnetism (AF) and Kondo effect is studied here in a model which consists of two Kondo sublattices with a gaussian random interaction between spins in differents sublattices with an antiferromagnetic mean Jo and standard deviation J. In the present approach there is no hopping of the conduction electrons between the sublattices and only spins in different sublattices can interact. The problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields which can be solved at mean field level within the static approximation and the replica symmetry ansatz. The obtained phase diagram shows the sequence of phases SG, AF and Kondo state for increasing Kondo coupling. This sequence agrees qualitatively with experimental data of the Ce_{2} Au_{1-x} Co_{x} Si_{3} compound.Comment: 7 pages, 1 figure, submitted to EPJ

    Spin Glass and ferromagnetism in disordered Cerium compounds

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    The competition between spin glass, ferromagnetism and Kondo effect is analysed here in a Kondo lattice model with an inter-site random coupling JijJ_{ij} between the localized magnetic moments given by a generalization of the Mattis model which represents an interpolation between ferromagnetism and a highly disordered spin glass. Functional integral techniques with Grassmann fields have been used to obtain the partition function. The static approximation and the replica symmetric ansatz have also been used. The solution of the problem is presented as a phase diagram giving T/JT/{J} {\it versus} JK/JJ_K/J where TT is the temperature, JKJ_{K} and J{J} are the strengths of the intrasite Kondo and the intersite random couplings, respectively. If JK/JJ_K/{J} is small, when temperature is decreased, there is a second order transition from a paramagnetic to a spin glass phase. For lower T/JT/{J}, a first order transition appears between the spin glass phase and a region where there are Mattis states which are thermodynamically equivalent to the ferromagnetism. For very low T/J{T/{J}}, the Mattis states become stable. On the other hand, it is found as solution a Kondo state for large JK/JJ_{K}/{J} values. These results can improve the theoretical description of the well known experimental phase diagram of CeNi1xCuxCeNi_{1-x}Cu_{x}.Comment: 17 pages, 5 figures, accepted Phys. Rev.

    Spin Glass and Ferromagnetism in Kondo lattices compounds

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    The Kondo lattice model has been analyzed in the presence of a random inter-site interaction among localized spins with non zero mean Jo and standard deviation J. Following the same framework previously introduced by us, the problem is formulated in the path integral formalism where the spin operators are expressed as bilinear combinations of Grassmann fields. The static approximation and the replica symmetry ansatz have allowed us to solve the problem at a mean field level. The resulting phase diagram displays several phase transitions among a ferromagnetically ordered region,a spin glass one, a mixed phase and a Kondo state depending on Jo, J and its relation with the Kondo interaction coupling Jk. These results could be used to address part of the experimental data for the CeNi_{1-x}Cu_x compound, when x =< 0.8.Comment: 7 pages, 2 figures. accepted for publication in The European Physical Journal

    Fitotoxicidade causada por herbicidas na fase inicial de desenvolvimento da cultura do sorgo.

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    A utilização de herbicidas em pós-emergência inicial ou tardia em jato dirigido à cultura de sorgo cresceu muito com o aumento da área plantada da cultura. No entanto. estes herbicidas podem causar injúrias às plantas de sorgo, quando não são totalmente seletivos. Como esses herbicidas são basicamente bloqueadores de processos metabólicos, surge a dúvida sobre quais serão os efeitos dessas injúrias na produção final de grãos de sorgo. Assim, o objetivo deste trabalho foi estudar o efeito da toxicidade à cultura causada pela aplicação de herbicidas na fase inicial e em pós-emergência tardia em jato dirigido e o seu efeito na produção de grãos. Este estudo foi conduzido durante os anos agrícolas 1994/95 e 1995/96, utilizando-se o híbrido BR 700, no delineamento experimental de blocos ao acaso. com 12 tratamentos e quatro repetições. Os tratamentos consistiram da aplicação da combinação dos seguintes herbicidas em diferentes doses: cyanazine + simazine com e sem óleo mineral, aplicados nos estádios de crescimentos de 4 e 6 folhas, e paraquat + espalhante adesivo e ametryn + óleo mineral, aplicados em jato dirigido no estádio de 12 folhas. Foram incluídas também testemunhas com e sem capina, além da retirada mecânica das folhas do Iº, 2º e 3º pares de folhas. Foram avaliados área foliar, peso da matéria seca da parte aérea. teor de clorofila nas folhas, altura da planta, diâmetro do colmo, estande, peso de 1.000 grãos, peso de panículas e produção de grãos. A aplicação de cyanazine + simazine + óleo mineral, no estádio de 4 folhas, reduziu drasticamente o estande das plantas de sorgo, nos dois períodos avaliados. O desenvolvimento das plantas, medido pela área foliar e pelo diâmetro do caule, foi pouco afetado, sendo os piores desempenhos verificados na testemunha sem capina e com a aplicação da mistura cyanazine + simazine. A redução no estande afetou diretamente a produção de panículas e de grãos. Os tratamentos que proporcionaram os mais altos rendimentos foram: paraquat + espalhante adesivo, ametryn + óleo mineral, retirada mecânica de folhas e cyanazine + simazine aplicados no estádio de 6 folhas. À exceção do tratamento cyanazine + simazine aplicado no estádio de 4 folhas, o uso de herbicidas em pós-emergência inicial e área total na cultura do sorgo mostrou-se seletiva. Mesmo quando houve injúrias, decorrentes da ação dos herbicidas, elas não foram suficientes para prejudicar a produção. O bom controle das plantas daninhas proveniente do uso dos herbicidas possibilitou ao sorgo expressar melhor seu potencial produtivo
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