Magnetic properties of strongly frustrated and correlated systems

While consequences of frustration of magnetic interactions are much studied in localized spin systems, much less studies have been performed on frustrated metallic systems. However, several effects of strong geometrical frustration in metallic correlated system have also been experimentally observed in rare-earth or transition metal compounds: coexistence of magnetic and non-magnetic sites in ordered magnetic structure, heavy fermion behaviour and anomalous Hall effect due to spin chirality are consequences of frustration. An overview of the experimental observations and of the proposed models is given. Other interesting effects due to magnetic frustration in metallic systems, which have been predicted theoretically, are also reviewed.     

Competing spin waves and superconducting fluctuations in strongly correlated electron systems

A special diagram technique recently proposed for strongly correlated electron systems is used to study the peculiarities of a spin-density wave (SDW) in competition with superconductivity. This method allows formulation of the Dyson equations for the renormalized electron propagators of the co-existing phases of SDW antiferromagnetism and superconductivity. We find the surprising result that triplet superconductivity appears provided that we have the co-existence of singlet superconductivity and SDW antiferromagnetism. A special ansatz, which takes into account the full Green's functions as well as the dynamical structure of the correlations, is used to establish the equations determining the gap functions and order parameters.     

de Haas–van Alphen effect in strongly correlated electron systems

We present the Fermi surface properties in strongly correlated electron systems of rare earth and uranium compounds via de Haas–van Alphen experiments. The conduction electrons with large cyclotron effective masses over 100m₀ (m₀: rest mass of an electron) are detected in CeRu₂Si₂, CeCoIn₅ and UPt₃. These electrons move slowly in the crystal. The topology of the Fermi surface and the cyclotron mass are compared to those of energy band calculations.     

Magnetic and resonant X-ray scattering investigations of strongly correlated electron systems

Resonant X-ray scattering is a method which combines high- resolution X-ray elastic diffraction and atomic core-hole spectroscopy for investigating electronic and magnetic long-range ordered structures in condensed matter. During recent years the development of theoretical models to describe resonant X-ray scattering amplitudes and the evolution of experimental techniques, which include the control and analysis of linear photon polarization and the introduction of extreme environment conditions such as low temperatures, high magnetic field and high pressures, have opened a new field of investigation in the domain of strongly correlated electron systems. To cite this article: L. Paolasini, F. de Bergevin, C. R. Physique 9 (2008).     

Spin, charge, orbital and lattice degrees of freedom in quasi-cubic manganites: A brief review

The rich phase diagram of La_1-xCa_xMnO₃ is the consequence of the interplay of spin, charge, orbital and lattice degrees of freedom, giving rise to several magnetic phases, Jahn–Teller distortions, spin-canting and phase separation, and orbital and charge order. Each Mn ion is in a mixed valent state of two magnetic configurations, Mn⁴⁺ and Mn³⁺. We study the phase diagram using mean-field slave bosons for the itinerant e_g-electrons in two orbitals with excluded multiple occupancy of each site, Hund's rule interaction between the e_g and t_2g states and Jahn–Teller coupling of the e_g orbitals to the lattice.     

Microscopic interactions in CuGeO 3 and organic Spin-Peierls systems deduced from their pretransitional lattice fluctuations

CuGeO₃ exhibits a Spin-Peierls (SP) transition, at T _SP = 14.3 K, which is announced above 19 K by an important regime of one-dimensional (1D) pretransitional lattice fluctuations which can be detected until about 40 K using X-ray diffuse scattering investigations. A quantitative analysis of this scattering shows that in this 1D direction the correlation length follows the “universal” behaviour expected for the thermal fluctuations of a real order parameter which characterizes the lattice dimerization. This allows to define a 1D mean-field temperature, T _SP ^MF , of about 60 K and invalidates any mean field scenario for the SP transition of CuGeO₃. As T _SP ^MF is as high as 4 T _SP we propose that the 3D-SP order is achieved by the interchain coupling between 1D solitons which form below about 16-20 K. CuGeO₃ being in the non-adiabatic regime, it is also suggested that the observed pretransitional fluctuations of CuGeO₃ originate from the X-ray scattering on a very broad damped critical response of lower frequency than the “critical” phonon modes. From the quantitative analysis of the 1D fluctuations we also estimate the microscopic parameters of the SP chain. These parameters allow to locate CuGeO₃ close to the quantum critical boundary separating the gapped SP ground state to the ungapped anti-ferromagnetic ground state. The vicinity of a quantum critical point emphasizes the role of the quantum and non-adiabatic fluctuations and the importance of the interchain coupling in the physics of CuGeO₃. Finally we compare these findings with those obtained for the organic SP systems (BCPTTF)₂PF₆, (TMTTF)₂PF₆ and MEM(TCNQ)₂. From a similar analysis of the pretransitional lattice fluctuations it is found that (BCPTTF)₂PF₆ and (TMTTF)₂PF₆ are located on the SP gapped classical-quantum boundary and are in the adiabatic regime where the fluctuations lead to the formation of a pseudo-gap in the spin degrees of freedom. Differently, we place MEM(TCNQ)₂ inside the SP quantum phase around the crossover line between the adiabatic and non-adiabatic regimes. Received 13 September 2000 and Received in final form 6 February 2001     

Role of the d-f Coulomb interaction in intermediate valence and Kondo systems: a numerical renormalization group study

Using the numerical renormalization group method, the dependences on temperature of the magnetic susceptibility χ(T) and specific heat C(T) are obtained for the single-impurity Anderson model with inclusion of d-f the Coulomb interaction. It is shown that the exciton effects caused by this effect (charge fluctuations) can significantly change the behaviour of C(T) in comparison with the standard Anderson model at moderately low temperatures, whereas the behaviour of χ(T) remains nearly universal. The ground-state and temperature-dependent renormalizations of the effective hybridization parameter and f-level position caused by the d-f interaction are calculated, and satisfactory agreement with the Hartree-Fock approximation is derived.