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1
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Kondo Lattice and Magnetic Properties in Strongly Correlated Electron Systems

100%
Coqblin B.
Acta Physica Polonica A
|
2009
|
vol. 115
|
issue 1
13-17
EN
We review here the magnetic properties of some strongly correlated electron systems and especially cerium and uranium compounds. The normal Kondo lattice model with a localized S=1/2 spin can account for the Kondo-magnetism competition observed in cerium or ytterbium systems, while the underscreened Kondo lattice model with a localized S=1 spin can well account for the ferromagnetism - Kondo coexistence observed in some uranium compounds such as UTe. Then, we discuss the spin glass-Kondo competition and present the resulting phase diagrams showing spin glass, Kondo and magnetically ordered phases observed in disordered heavy fermion cerium alloys such as CeNi_xCu_{1-x} alloys.
2
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Kondo Lattice Models for Rare-Earth and Actinide Systems

100%
Coqblin B.
Acta Physica Polonica A
|
2012
|
vol. 121
|
issue 5-6
1005-1013
EN
We present here some works on the strong competition between the Kondo effect, magnetic order and eventually spin glass or frustration effect in anomalous rare-earth and actinide systems. First, we develop an underscreened Kondo lattice model with S_{f} = 1 spins for the 5f-electrons and we have recently improved it by deriving, by the Schrieffer-Wolff transformation, a 5f-band with a finite bandwidth. The underscreened Kondo lattice model can account for properties of some uranium and neptunium compounds, like UTe, Np_2PdGa_3 or UCu_2Si_2 which have a large Curie temperature T_{c} of order 100 K and present also a Kondo behavior. In particular, we can account for the observed maximum of T_{c} under pressure in UTe and the magnetization curves of NpNiSi_2 showing the occurrence of the Kondo effect at low temperatures below T_{c}. Second, we have studied the properties of disordered cerium alloys like CeCu_xNi_{1-x} or CeRh_xPd_{1-x} by considering the Kondo effect, a ferromagnetic order and a spin glass behavior described by several approaches. The van Hemmen approach gives a good explanation of the properties of cerium alloys and we are describing the magnetic glass clusters which occur in both spin glass and ferromagnetic phases. Third, we present a new description of a frustrated Kondo lattice model, which can account for the behavior under pressure or doping of some ytterbium compounds like Yb_2Pd_2Sn and YbAgGe.
3
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Magnetism and Kondo Effect in Cerium and Uranium Compounds

100%
Coqblin B.
Acta Physica Polonica A
|
2008
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vol. 113
|
issue 1
391-394
EN
A brief review of the Kondo lattice problem is presented, with a special emphasis on the underscreened Kondo effect in ferromagnetic-Kondo uranium compounds and on the spin glass-Kondo competition in disordered heavy fermion cerium alloys.
4
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The Kondo Lattice Models and Magnetism for Cerium and Uranium Systems

100%
Coqblin B.
|
|
issue 5
913-918
EN
The interplay between the Kondo effect and magnetism is important in anomalous rare-earth and actinide systems and a brief review of the different Kondo lattice models is presented here. The first studied case is the classical Kondo lattice model with S = 1/2 spins for the 4f localized electrons, which describes the strong competition between the Kondo effect and magnetic ordering leading to small ordering temperatures less than roughly 10 K. The second reviewed case describes the underscreened Kondo lattice model with S = 1 spins for the 5f localized electrons, which can account for the coexistence of both the Kondo effect and the ferromagnetic order and, therefore, the large Curie temperatures observed in actinide compounds like UTe and NpNiSi_2; the question of the delocalization of the 5f electrons is also discussed here. The last case describes the competition between the "spin glass" behaviour, the Kondo effect and the magnetic ordering in disordered cerium alloys like CeNi_{x}Cu_{1 - x} or CeRh_{x}Pd_{1 - x} and several models used for the spin glass state are reviewed, as well as the occurrence of magnetic glass clusters.
5
Content available remote

Indirect RKKY-Type Interaction by Direct Oxygen Hopping

51%
Rodriguez-Nuńez J., Coqblin B., Beck H., Konior J.
Acta Physica Polonica A
|
1994
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vol. 85
|
issue 2
317-322
EN
The three-band Emery model, describing the holes in the CuO_{2} planes of the high-temperature superconducting oxides, is considered. The model includes the direct oxygen-oxygen hopping integral t_{pp}. The exact Bogolyubov transformation is used to exclude one oxygen band and obtain a two-dimensional Anderson model. Afterward, the effective Hamiltonian is obtained by eliminating the second oxygen band with the use of two approximate canonical transformations. The effective Hamiltonian describes the spins residing on the copper sites and interacting through an indirect interaction J_{SX}(R), where R is the distance between two copper ions. J_{SX}(R) depends on the doping rate δ and is a decaying function of R. Numerical results for J_{SX}(R) are given for different doping rates δ for the case of parabolic bands. The obtained interaction J_{SX}(R), when added to the original antiferromagnetic interaction (present in oxides at δ = 0), might lead to a frustration of the long-range antiferromagnetic ordering upon doping.
6
Content available remote

Kondo Screening Effect and Ferromagnetic Order in UCu_2Si_2

51%
Troć R., Samsel-Czekała M., Coqblin B.
Acta Physica Polonica A
|
2012
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vol. 121
|
issue 5-6
1023-1025
EN
Electrical resistivity and low temperature magnetoresistivity measurements made on a single crystal of UCu_2Si_2 are reported. By using as a phonon reference the temperature dependence of the electrical resistivity of ThCu_2Si_2 we could establish that UCu_2Si_2 has both a ferromagnetic and a Kondo behaviour. Such a phenomenon can be described by the underscreened Kondo lattice model. The magnetoresistivity revealed the presence of magnetic fluctuations within the ferromagnetic order as it was reported previously for UGe_2. Also one of the calculated Fermi surface sheets exhibits nesting properties, being in perfect agreement with the previous neutron diffraction data, supporting the possibility of a presence of the spin density wave phase. In this ternary silicide, where the strong ferromagnetic behaviour exists, this phase is signalised by magnetic fluctuations.
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