Superconducting and normal state properties of the A 3C 60 compounds

Abstract

This report is a critical review of the measurements and their interpretations of the normal and superconducting state of the A 3C 60 compounds, where A = alkali atom. These compounds are highly ionic [A +] 3 · [C 60] 3− and form fcc lattices (cryolite structure) which locate the C 60 icosahedra in sites of local cubic symmetry, thereby preserving the degeneracy of the t lu orbitals, allowing for the formation of a narrow half-filled band of a width comparable to or smaller than the different molecular excitation energies. The T c -s of the more than a dozen compounds synthesized so far span the range 2–33 K; the variation of T c with pressure and from material to material is assessed as an empirical T c lattice parameter relation, suggestive that the attraction responsible for the Cooper pair formation is a local property of the C 60 molecules and variations of the density of state ρ( ϵ f ) at the Fermi level (i.e., band width) determine T c . The superconducting parameters, λ L and ξ 0 determined from critical field and μSR measurements, favoring a local pairing image, are marginally supportive of the expected density of state variations. The so far available 13C nuclear relaxation, susceptibility and ESR measurements in the normal state manifest several features more related to the complex correlated nature of the C 60 molecules than free electron band effects of the simple lattice they are arranged in. The paper emphasizes these unusual characteristics.