The superconducting fullerenes
The discovery of superconductivity in alkali-metal doped fullerenes (A3C60) compounds at relatively high temperatures has created considerable interest. Despite a variety of experiments, several issues were controversial either for the normal or for the superconducting state properties of A3C60. The central question, of course, was about the nature of the pairing mechanism. There has been debate about whether electron-electron interactions on the C60 ball or electron-phonon (e-ph) coupling mediate the pairing. Concerning the latter mechanism, there was quite a bit of controversy with respect to arguments, favouring electron-phonon interactions with low frequency intermolecular vibrations or with high frequency intramolecular modes. The energy scale of the various modes which mediate the e-ph coupling are different, and therefore, which of these are important, could in principle be decided by examining whether the weak or strong coupling limit applies. In this context, optical investigations are a powerful experimental tool in order to single out whether the weak or strong coupling limit of the BCS theory is more appropriate to describe the superconducting state. Our work (Fig.2.1) was mainly focused on the attempts to determine optically the relevant energy scale (i.e. the superconducting gap) of the alkali doped superconducting fullerenes. The analysis of our optical results in the superconducting state within the standard Eliashberg electron phonon theory of superconductivity strongly supports a pairing mechanism mediated by high-frequency intramolecular phonon modes, in accord with the weak-coupling limit.