High-T superconductivity in CsC compounds governed by local Cs-C Coulomb interactions.

Unique among alkali-doped A C fullerene compounds, the A15 and fcc forms of CsC exhibit superconducting states varying under hydrostatic pressure with highest transition temperatures at [Formula: see text]  =  38.3 and 35.2 K, respectively. Herein it is argued that these two compounds under pressure represent the optimal materials of the A C family, and that the C-associated superconductivity is mediated through Coulombic interactions with charges on the alkalis. A derivation of the interlayer Coulombic pairing model of high-T superconductivity employing non-planar geometry is introduced, generalizing the picture of two interacting layers to an interaction between charge reservoirs located on the C and alkali ions. The optimal transition temperature follows the algebraic expression, T   =  (12.474 nm K)/ℓζ, where ℓ relates to the mean spacing between interacting surface charges on the C and ζ is the average radial distance between the C surface and the neighboring Cs ions. Values of T for the measured cation stoichiometries of Cs C with x  ≈  0 are found to be 38.19 and 36.88 K for the A15 and fcc forms, respectively, with the dichotomy in transition temperature reflecting the larger ζ and structural disorder in the fcc form. In the A15 form, modeled interacting charges and Coulomb potential e/ζ are shown to agree quantitatively with findings from nuclear-spin relaxation and mid-infrared optical conductivity. In the fcc form, suppression of [Formula: see text] below T is ascribed to native structural disorder. Phononic effects in conjunction with Coulombic pairing are discussed.