Superconducting and Normal State Properties of the A3C60 Compounds

The A 3 C 60 compounds, where A=alkali atom, form fcc -lattices (cryolite structure) and are highly ionic [ A +]3·[ C 60]3−. The C 60 icosahedra are located in sites of local cubic symmetry thereby preserving the degeneracy of the t 1u 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 C 60 molecules, so that variations of the density-of-state ρ(ε F ) at the Fermi level (i.e. bandwidth) determine T c . The values of the superconducting parameters, λ L and ξ o , determined from critical field and µSR measurements, favor a local pairing picture, but are at best only marginally supportive for the expected density-ofstate variations. The so far available 13 C nuclear relaxation, susceptibility and ESR measurements in the normal state manifest several features that are more related to the complex correlated nature of the C 60 molecules than to the freeelectron band effects of the simple lattice they are arranged in.