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We investigate the influence of electron-phonon coupling on the superexchange interaction of magnetic insulators. Both the Holstein-Hubbard model where the phonons couple to the electron density, as well as an extended Su, Schrieffer,…
This article gives a comprehensive review on the recent studies of the double exchange systems using non-perturbative approaches; the dynamical mean-field theory and the Monte Carlo method. Investigations beyond mean-field type treatments…
A system defined by two coupled Ising models, with a bimodal random field acting in one of them, is investigated. The interactions among variables of each Ising system are infinite-ranged, a limit where mean field becomes exact. This model…
We consider a model of Raman scattering for a two--dimensional $S=1/2$ Heisenberg Anti-Ferromagnet which includes a {\it dynamical} spin--phonon interaction. We observe a broadening of the line shape due to increased coupling with excited…
Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic tendencies. However, experimentally the opposite effect has been recently observed: ferromagnetism appears in some nano-sized…
The competition of charge, orbital, and ferromagnetic interactions in layered manganites is investigated by magneto-Raman scattering spectroscopy. We find that the colossal magnetoresistance effect in the layered compounds results from the…
The metallic or insulating nature of the paramagnetic phase of the colossal-magnetoresistance manganites is investigated via a double exchange Hamiltonian with diagonal disorder. Mobility edge trajectory is determined with the transfer…
A double-exchange mechanism for the emergence of ferromagnetism in cubic uranium compounds is proposed on the basis of a $j$-$j$ coupling scheme. The idea is {\it orbital-dependent duality} of $5f$ electrons concerning itinerant…
The Monte Carlo Ferromagnetic Ising model was used to study the electrical properties of manganese oxides due to the charge ordering phase occurring at doping, x = 0.5. The half-doped manganites have an insulator antiferromagnetic ground…
The effects of bond randomness on the phase diagram and critical behavior of the square lattice ferromagnetic Blume-Capel model are discussed. The system is studied in both the pure and disordered versions by the same efficient two-stage…
Giant oxygen isotope effects observed in colossal magnetoresistance manganites are investigated by employing the combined model of the double exchange and interacting lattice polaron mechanism. We have shown that the isotope effects on $…
We predict a double-resonant feature in the magnetic field dependence of the phonon-mediated mobility of a two-subband quasi-two-dimensional electron system. These resonances take place when two Landau levels corresponding to different…
Ferromagnetic transition in three-dimensional double-exchange models is studied by the Monte Carlo method. Critical temperature $T_{\rm c}$ is precisely determined by finite-size scaling analysis. Strong spin fluctuations in this itinerant…
In the manganites $RE_{1-x}AE_{x}$MnO$_{3}$ ($RE$ and $AE$ being rare-earth and alkaline-earth elements, respectively) the random distribution of $RE^{3+}$ and $AE^{2+}$ induces random, but correlated, shifts of site energies of charge…
We calculate the value of the Fr\"ohlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction is much stronger than any relevant magnetic interaction. A polaron shift…
A substantial energy gap of charge excitations induced by strong correlations is the characteristic feature of Mott insulators. We study how the Mott gap is affected by long-range antiferromagnetic order. Our key finding is that the Mott…
In response to the experimental discovery of double perovskite Ru- base material, The magnetic and electronic properties of the double exchange mechanism are studied theoretically. The electron band structure and the spin wave spectrum…
Monte-Carlo simulations and ground-state calculations have been used to map out the phase diagram of a system of classical spins, on a simple cubic lattice, where nearest-neighbor pairs of spins are coupled via competing antiferromagnetic…
Dielectric and magnetic phenomena in spin and charge frustrated system RFe2O4 (R is a rare-earth metal ion) are studied. An electronic model for charge, spin and orbital degrees in a pair of triangular-lattice planes is derived. We analyze…
Experimental results on the metal-insulator transitions and the anomalous properties of strongly interacting two-dimensional electron systems are reviewed and critically analyzed. Special attention is given to recent results for the…