HighT_{C} Superconductivity in Cs_{3}C_{60} Compounds Governed by Local CsC_{60} Coulomb Interactions, D. R. Harshman and A. T. Fiory [arXiv]
Unique among alkalidoped A_{3}C_{60} fullerene compounds, the A15 and fcc forms of Cs_{3}C_{60} exhibit superconducting states varying under hydrostatic pressure with highest transition temperatures at T_{C}^{meas} = 38.3 and 35.2 K, respectively. Herein it is argued that these two compounds under pressure represent the optimal materials of the A_{3}C_{60} family, and that the C_{60}associated superconductivity is mediated through Coulombic interactions with charges on the alkalis. A derivation of the interlayer Coulombic pairing model of highT_{C} superconductivity employing nonplanar geometry is introduced, generalizing the picture of two interacting layers to an interaction between charge reservoirs located on the C_{60} and alkali ions. The optimal transition temperature follows the algebraic expression, T_{C0} = (12.474 nm^{2} K)/ℓζ, where ℓ relates to the mean spacing between interacting surface charges on the C_{60} and ζ is the average radial distance between the C_{60} surface and the neighboring Cs ions. Values of T_{C0} for the measured cation stoichiometries of Cs_{3–x}C_{60} 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^{2}/ζ are shown to agree quantitatively with findings from nuclearspin relaxation and midinfrared optical conductivity. In the fcc form, suppression of T_{C}^{meas} below T_{C0} is ascribed to native structural disorder. Phononic effects in conjunction with Coulombic pairing are discussed.
Measured optimal transition temperature T_{C}^{meas} versus (ℓζ)^{–1} for A15 Cs_{2.85(1)}C_{60} and fcc Cs_{2.901(6)}C_{60} (legend upper left), compared to other highT_{C} superconductors: cuprates; iron pnictides and chalcogenides; intercalated group4metal nitridechlorides; and RuO and ETbased compounds (legend lower right). The line represents the theoretical T_{C0}. 

Maximum measured transition temperature T_{C}^{max} plotted against Rb content x in Rb_{x}Cs_{3–x}C_{60} for x = 0 and x = 0.35, 0.5, 0.75 and 1), denoted by circle symbols. Triangles denote corresponding measured V_{per C60}. Dashed curves are guides to the eye. After Refs. [8] and [13] of paper. 
Dale R. Harshman and Anthony T. Fiory, J. Phys.: Condens. Matter 29, 145602 (2017).