SUPERCONDUCTING AND NORMAL STATE PROPERTIES OF THE A3C60 COMPOUNDS

The A3C60 compounds, where A = alkali atom, form fcc -lattices (cryolite structure) and are highly ionic [A+]3.[C60]3-. The C60 icosahedra are located in sites of local cubic symmetry thereby preserving the degeneracy of the t1u orbitals, allowing for the formation of a narrow half filled band of a width comparable to or smaller than the various molecular excitation energies. The T(c)-s of the dozen or so compounds already synthesized span the range from 2 - 33 K; the variation of T(c) with pressure and from material to material suggests that the attraction responsible for the Cooper pair formation is a local property of the C60 molecules, so that variations of the density-of-state rho(epsilonF) at the Fermi level (i.e. bandwidth) determine T(c). The values of the superconducting parameters, lambda(L) and xi(o), determined from critical field and muSR measurements, favor a local pairing picture, but are at best only marginally supportive for the expected density-of-state variations. The so far available C-13 nuclear relaxation, susceptibility and ESR measurements in the normal state manifest several features that are more related to the complex correlated nature of the C60 molecules than to the free-electron band effects of the simple lattice they are arranged in.