The discovery in 1991 of high temperature superconductivity in A3C60 compounds, where A is an alkali ion, has been rapidly ascribed to a BCS mechanism, in which the SC 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 detailed studies of various AnC60 compounds with n =1, 2, 4 [1-3] allowed to evidence that the electronic properties of these compounds 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 influence of electron correlations and Jahn-Teller Distortions (JTD) of the C60 ball, which energetically favour evenly charged C60 molecules [1].
We shall report the recent discovery of two expanded fulleride Cs3C60 isomeric phases which are Mott insulators at ambient pressure [4-5]. Both phases undergo a pressure induced first order Mott transition to superconductivity with a (p, T) phase diagram displaying a dome shaped superconductivity, a common situation encountered nowadays in correlated electron systems [5].
NMR experiments allowed us to study the magnetic properties of the Mott phases, to evidence the onset of large magnetic fluctuations with decreasing pressure towards the Mott transition, and to follow the phase diagram up to a critical point near room T, analogous to that observed at the liquid-gas transition. So, although superconductivity admittedly results from an electron-phonon mechanism, the incidence of electron correlations has an importance on the electronic properties, as had been anticipated from DMFT calculations [6].
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Phys. Rev. Lett. 104, 256402 (2010) and http://arxiv.org/abs/1102.5693, submitted to EPL.
[6] M. Cappone, M. Fabrizio, C. Castellani and E. Tosatti, Rev. Modern Physics, 81, 943 (2009).
Laboratoire de Physique du Solide, Université Paris-Sud/CNRS