B. Coqblin

Spin fluctuations in metals: Application to the actinides

Abstract We present here a review of the spin fluctuation theory and of its applications to transition and actinide systems, with a particular emphasis to the latter where some very anomalous properties find an explanation in terms of spin fluctuation effects. Firstly, we summarize the development of the spin fluctuation model which had been initially applied to transition metals and alloys such as palladium or Pd–Ni alloys. Then, we present the extension of the paramagnon model to nearly magnetic actinide systems by taking into account explicitly the temperature dependence of the Stoner susceptibility, because the 5f-band of actinides is much narrower than the d-band of transition metals. As a result the paramagnon contribution to the resistivity departs from the usual T 2 and T power laws at temperatures higher than the spin fluctuation one and saturates at high temperatures, with eventually the presence of a maximum at intermediate temperatures. We present also the calculation of the other properties o...

Spin-glass freezing in Kondo-lattice compounds

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 ~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.

Theory of Kondo lattice

In order to understand the low temperature anomalous properties of some rare‐earth compounds (such as CeAl2, CeAl3, TmSe), we introduce a Kondo lattice hamiltonian in which a localized 1/2 spin per site interacts antiferromagnetically with the conduction electron spins. We investigate the ground state properties of this hamiltonian by using a real space renormalization group method. After studying a simpler one dimensional analog we study the original hamiltonian in one dimension when there is one conduction electron per site.

Transport properties of SmO

Abstract The electrical resistivity and thermoelectric power of SmO are measured between 4.2 and 300 K. The existence of a low temperature T2 law in the resistivity and the large and roughly constant magnetic susceptibility suggest that SmO presents large spin fluctuations.

Theory of the Kondo lattice : competition between Kondo effect and magnetic order

We present here a review of some theoretical features of the Kondo lattice problem, which can describe anomalous experimental properties of strongly correlated electron systems with mainly cerium, ytterbium or uranium. In such Kondo systems, there exist both a Kondo effect on each site and strong intersite magnetic interactions. We present firstly the general case of the Kondo lattice and the mean-field approximation. We then discuss the case of the “underscreened” Kondo lattice with a strong coexistence between the Kondo effect and a ferromagnetic order, as for example observed in uranium compounds such as UTe. We also discuss the Kondo–spin glass competition with eventually an additional magnetically ordered phase (ferromagnetic or antiferromagnetic one) and we can account for phase diagrams of some cerium and uranium disordered alloys, such as CeNi1 − x Cu x . Finally, the introduction of a transverse magnetic field in the Kondo–spin glass problem yields a quantum critical point and allows a better description of the phase diagrams of disordered alloys.

Virtual bound state model for the exchange interaction in semimagnetic semiconductors such as Cd1−xMnxTe

Abstract The difference in both sign (i. e. ferromagnetic interaction in the conduction band and antiferromagnetic one in the valence band) and magnitude of the exchange interaction constant in semiconductors such as CdMnTe is explained by a virtual bound state model involving the resonance of the Mn d-level in the valence band.

Spin glass and ferromagnetism in disordered cerium compounds

The competition between spin glass, ferromagnetism and Kondo effect is analysed here in a Kondo lattice model with an inter-site random coupling

Magnetic and transport properties of cerium Kondo compounds

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Crystal field and valence change effects in anomalous rare-earth compounds

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

Thermal conductivity of YbCu4Ag and YbCu4In compounds

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