3,298 research outputs found
Stoner-Wohlfart model applied to bicrystal magnetoresistance hysteresis
We calculate numerically the magnetization direction as function of magnetic
field in the Stoner-Wohlfart theory and are able to reproduce the shape of the
low-field magnetoresistance hysteresis observed in manganite grain boundary
junctions. Moreover, we show that it is necessary to include biaxial
magnetocrystalline anisotropy to fully describe the grain boundary
magnetoresistance in LaSrMnO/SrTiO bicrystal tunnel
junctions.Comment: 5 pages, 4 figure
Dielectric screening in doped Fullerides
For conventional superconductors the electron-electron interaction is
strongly reduced by retardation effects, making the formation of Cooper pairs
possible. In the alkali-doped Fullerides, however, there are no strong
retardation effects. But dielectric screening can reduce the electron-electron
interaction sufficiently, if we assume that the random-phase approximation
(RPA) is valid. It is not clear, however, if this assumption holds, since the
alkali-doped Fullerides are strongly correlated systems close to a Mott
transition. To test the validity of the RPA for these systems we have
calculated the screening of a test charge using quantum Monte Carlo.Comment: 4 pages, 1 eps figure included; to be published in the proceedings of
the International Winterschool on Electronic Properties of Novel Materials,
Kirchberg/Tirol, 1998; additional information is available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Metal-Insulator transitions in generalized Hubbard models
We study the Mott transition in Hubbard models with a degenerate band on
different 3-dimensional lattices. While for a non-degenerate band only the
half-filled system may exhibit a Mott transition, with degeneracy there can be
a transition for any integer filling. We analyze the filling dependence of the
Mott transition and find that (the Hubbard interaction at which the
transition takes place) decreases away from half-filling. In addition we can
change the lattice structure of the model. This allows us to study the
influence of frustration on the Mott transition. We find that frustration
increases , compared to bipartite systems. The results were obtained from
fixed-node diffusion Monte Carlo calculations using trial functions which allow
us to systematically vary the magnetic character of the system. To gain a
qualitative understanding of the results, we have developed simple hopping
arguments that help to rationalize the doping dependence and the influence of
frustration on the Mott transition. Choosing the model parameters to describe
the doped Fullerides, we can make contact with experiment and understand why
some of the Fullerides are metals, while others, which according to density
functional theory should also be metallic, actually are insulators.Comment: 4 pages LaTeX with 4 eps figures; submitted to Computer Physics
Communications, Proceedings of the CPP'99/Centennial Meeting, Atlanta, GA;
additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Phonon spectral function for an interacting electron-phonon system
Using exact diagonalzation techniques, we study a model of interacting
electrons and phonons. The spectral width of the phonons is found to be reduced
as the Coulomb interaction U is increased. For a system with two modes per
site, we find a transfer of coupling strength from the upper to the lower mode.
This transfer is reduced as U is increased. These results give a qualitative
explanation of differences between Raman and photoemission estimates of the
electron-phonon coupling constants for A3C60 (A= K, Rb).Comment: 4 pages, RevTeX, 2 eps figur
Filling dependence of the Mott transition in the degenerate Hubbard model
Describing the doped Fullerenes using a generalized Hubbard model, we study
the Mott transition for different integer fillings of the t_1u band. We use the
opening of the energy-gap E_g as a criterion for the transition. E_g is
calculated as a function of the on-site Coulomb interaction U using fixed-node
diffusion Monte Carlo. We find that for systems with doping away from
half-filling the Mott transitions occurs at smaller U than for the half-filled
system. We give a simple model for the doping dependence of the Mott
transition.Comment: 7 pages RevTeX with 10 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Metal-insulator transitions: Influence of lattice structure, Jahn-Teller effect, and Hund's rule coupling
We study the influence of the lattice structure, the Jahn-Teller effect and
the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb).
The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60
(bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors
A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases)
the value Uc of the Coulomb integral at which the metal-insulator transition
occurs. There is an important partial cancellation between the Jahn-Teller
effect and the Hund's rule coupling.Comment: 4 pages, RevTeX, 3 eps figure, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Screening, Coulomb pseudopotential, and superconductivity in alkali-doped Fullerenes
We study the static screening in a Hubbard-like model using quantum Monte
Carlo. We find that the random phase approximation is surprisingly accurate
almost up to the Mott transition. We argue that in alkali-doped Fullerenes the
Coulomb pseudopotential is not very much reduced by retardation
effects. Therefore efficient screening is important in reducing
sufficiently to allow for an electron-phonon driven superconductivity. In this
way the Fullerides differ from the conventional picture, where retardation
effects play a major role in reducing the electron-electron repulsion.Comment: 4 pages RevTeX with 2 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
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