Three-dimensional Superconductivity in Fullerene Compounds

In spite of many experiments, with other methods as well [3], a clear answer to the question of the coupling mechanism and thus of the mechanism of the superconductivity itself has not been found, although there are many indications that a phonon-induced coupling exists and and therefore conventional BCS behaviour. 

Especially for the strongly anisotropic materials, there are however experiments which seem to indicate unconventional superconductivity. As coupling mechanisms for the Cooper pairs, non-phononic interactions, e.g. magnetic correlations, have been discussed. It is also not completely certain whether the pair wavefunc-tions have s-state symmetry or instead have p- or d-w2cve symmetry. There are many parallels to the inorganic high-temperature superconductors. 

The current state of knowledge can be summarised as follows there is as yet no complete consensus as to the nature of the superconducting state in organic superconductors. More on this subject can be foimd in [3] and [Ml]. 

Pure Fullerene (Cgo, Fig. 10.11) crystallises in a face-centered-cubic structure with a lattice parameter of 14.15 A. Crystallised Cgo is an insulator. It can however be 

The alkali metals act here as donors, which make a metal of the semiconductor Ceo as acceptor, with an energy band gap of ca. 2.3 eV via half-filling of the conduction band. The system behaves similarly to the radical-anion salts which we have already treated (Sects. 9.2 and 9.3). Ceo is a good electron acceptor. It consists entirely of carbon atoms and is thus not actually an organic molecule. Superconducting Fullerene salts however have properties which are like those of the organic molecular salts. These are above all the important role played by the Jt electrons in charge transport, and the existence of relatively narrow bands with a low electron density. 

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