Superconducting and normal state properties of the A₃C₆₀ compounds

This report is a critical review of the measurements and their interpretations of the normal and superconducting state of the A₃C₆₀ compounds, where A = alkali atom. These compounds are highly ionic [A⁺]₃ · [C₆₀]^3- and form fcc lattices (cryolite structure) which locate the C₆₀ 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 C₆₀ molecules and variations of the density of state ρ(ε{lunate}_f) at the Fermi level (i.e., band width) determine T_c. The superconducting parameters, λ_L and ξ₀ 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 C₆₀ molecules than free electron band effects of the simple lattice they are arranged in. The paper emphasizes these unusual characteristics.