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 $U_c$ (the Hubbard interaction $U$ 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 $U_c$, 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