92 research outputs found
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
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
Optical conductivity in A3C60 (A=K, Rb)
We study the optical conductivity in A3C60 (A =K, Rb). The effects of the
electron-phonon interaction is included to lowest order in the coupling
strength lambda. It is shown that this leads to a narrowing of the Drude peak
by a factor 1+lambda and a transfer of weight to a mid-infrared peak at
somewhat larger energies than the phonon energy. Although this goes in the
right direction, it is not sufficient to describe experiment.Comment: 5 pages, revtex, 2 figures more information at
http://librix.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
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