The discovery in 1991 of high temperature superconductivity (SC) in A3C60
compounds, where A is an alkali ion, has been rapidly ascribed to a BCS
mechanism, in which the pairing is mediated by on ball optical phonon modes.
While this has lead to consider that electronic correlations were not important
in these compounds, further studies of various AnC60 with n=1, 2, 4 allowed to
evidence that their electronic properties cannot be explained by a simple
progressive band filling of the C60 six-fold degenerate t1u molecular level.
This could only be ascribed to the simultaneous influence of electron
correlations and Jahn-Teller Distortions (JTD) of the C60 ball, which
energetically favour evenly charged C60 molecules. This is underlined by the
recent discovery of two expanded fulleride Cs3C60 isomeric phases which are
Mott insulators at ambient pressure. Both phases undergo a pressure induced
first order Mott transition to SC with a (p, T) phase diagram displaying a dome
shaped SC, a common situation encountered nowadays in correlated electron
systems. NMR experiments allowed us to study the magnetic properties of the
Mott phases and to evidence clear deviations from BCS expectations near the
Mott transition. So, although SC involves an electron-phonon mechanism, the
incidence of electron correlations has an importance on the electronic
properties, as had been anticipated from DMFT calculations.Comment: Small review article 10 pages, 12 figures. Talk given at the 2011
Eurasia-Pacific Summer School and Conference on Correlated Electrons, in
Tutunc (Turkey
