769 research outputs found
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 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 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
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