From Weakly to Strongly Correlated Electrons in the Alkali Fulleride Family

Abstract

We discuss the effect of strong electron-vibron interaction and static or dynamic Jahn-Teller distortion on the electromc properties of the AnC60 family, where A is alkali metal and 0 < n < 6. Repulsive electron-electron correlations are expected for even n, but attractive ones for odd n. Such a model seems able to make compatible weak electron correlations for n = 3, fairly strong correlations for n = 1 and an insulating behavior for n = 2 or 4. An important due for understanding the electronic properties of the Alkali Fulleride fam- ily is the answer to the following question how large are the electron-electron correlations compared to the one-electron energy bandwidth II~? The C60 molecule forms, with the alkali metal series, salts with chemical formula AnC60 where A is an alkali metal and n an integer (0 < n < 6). In icosahedral synimetry, the LUMO of the C60 molecule is a three-fold degen- erate tiu orbital, giving rise, in the crystal, to a narrow baud of about o-a-o-a eV. If there is a general agreement about this order of magnitude for W, the value of the intraniolecular electron-electron repulsion (U in the Hubbard model) is rather controversial. The experimen- tal estimations are indirect and range from more than eV (from photoemission) iii to values much smaller than the bandwidth (from analysis of Fermi hquid properties). This question is conceptually important since the relevant theoretical description of the Alkali fullerides could be either a weakly correlated Fermi hquid or a strongly correlated one, or even a (possibly doped) Mott insulator, depending on the strength of the ratio Uliv. As far as we know, there is no reliable ab mitio calculation of U, taking into accourt many- electron eflects. The effective value of U is certainly smaller than the bare carbon atom on-site electron-electron Coulomb repulsion, typically a few eV's. It is certainly strongly reduced by electronic screening and by the delocalization of the electron wave function over the 60 atoms. However, given the order of magnitude of W, it seems diflicult to beheve, at first sight, that U cari be much smaller than the bandwith iv. It is, therefore, important to discuss the possible eflects of electron correlations on the electronic properties. (*) Author for correspondence je-mail: victorof%Ips.u-psud.fr)