Current issues in fulleride superconductivity

Alkali fullerides exhibit superconductivity with T-c's as high as 33 K at ambient pressure. The transition temperatures scale monotonically with unit cell size, an observation rationalized as arising from the modulation of the electronic density-of-states at the Fermi level, N(epsilon(F)) by the interfullerene spacing, a. Deviations from this relationship leading to suppressed T-c's are apparent at small interfullerene spacings for sodium fullerides, Na-2(A,A')C-60 and may be linked to the formation of quasi-one-dimensional bridged fulleride structures. Superconductivity is also suppressed at large interfullerene spacings for (NH3)K3C60 whence magnetic origin effects play an important role and provide a connection with the established phenomenology in high-T-c and organic superconductors.