PM-FM transition in a DE model

We study paramagnetic - ferromagnetic transition due to exchange interaction between classical localized magnetic moments and conduction electrons. By solving the Dynamical Mean Field Approximation equations we find explicit formula for the transition temperature $T_c$ for arbitrary electron dispersion law, concentration and re lation between exchange coupling and the electron band width. We present the results of calculations of the $T_c$ for the semi-circular electron density of states.Comment: 2 pages, latex, submitted for the SCES'0

Electron self-trapping at quantum and classical critical points

Using Feynman path integral technique estimations of the ground state energy have been found for a conduction electron interacting with order parameter fluctuations near quantum critical points. In some cases only \textit{singular} perturbation theory in the coupling constant emerges for the electron ground state energy. It is shown that an autolocalized state (quantum fluctuon) can be formed and its characteristics have been calculated depending on critical exponents for both weak and strong coupling regimes. The concept of fluctuon is considered also for the classical critical point (at finite temperatures) and the difference between quantum and classical cases has been investigated. It is shown that, whereas the quantum fluctuon energy is connected with a true boundary of the energy spectrum, for classical fluctuon it is just a saddle-point solution for the chemical potential in the exponential density of states fluctuation tail.Comment: 45 pages, 1 eps figure, elsart, submitted to Annals of Physic

CPA density of states and conductivity in a double-exchange system containing impurities

We study density of states and conductivity of the doped double-exchange system, treating interaction of charge carriers both with the localized spins and with the impurities in the coherent potential approximation. It is shown that under appropriate conditions there is a gap between the conduction band and the impurity band in paramagnetic phase, while the density of states is gapless in ferromagnetic phase. This can explain metal-insulator transition frequently observed in manganites and magnetic semiconductors. Activated conductivity in the insulator phase is numerically calculated.Comment: 5 pages, 2 eps figures, LaTex2e, EPJ macro package (style files included). The calculations done for realistic model predict that a gap appears between the conduction band and the impurity band in paramagnetic phase, while the density of states is gapless in ferromagnetic phas

Localization and Dephasing Driven by Magnetic Fluctuations in Low Carrier Density Colossal Magnetoresistance Materials

Localization and dephasing of conduction electrons in a low carrier density ferromagnet due to scattering on magnetic fluctuations is considered. We claim the existence of the "mobility edge", which separates the states with fast diffusion and the states with slow diffusion; the latter is determined by the dephasing time. When the "mobility edge" crosses the Fermi energy a large and sharp change of conductivity is observed. The theory provides an explanation for the observed temperature dependence of conductivity in ferromagnetic semiconductors and manganite pyrochlores.Comment: 4 pages, 1 eps figure, LaTex2e, EPJ macro package (style files included); final version, submitted to EPJ