High-Tc Superconductivity in Cs3C60 Compounds Governed by Local Cs-C60 Coulomb Interactions

High-T_C Superconductivity in Cs₃C₆₀ Compounds Governed by Local Cs-C₆₀ Coulomb Interactions, D. R. Harshman and A. T. Fiory [arXiv]

Unique among alkali-doped A₃C₆₀ fullerene compounds, the A15 and fcc forms of Cs₃C₆₀ 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 A₃C₆₀ family, and that the C₆₀-associated superconductivity is mediated through Coulombic interactions with charges on the alkalis. A derivation of the interlayer Coulombic pairing model of high-T_C superconductivity employing non-planar geometry is introduced, generalizing the picture of two interacting layers to an interaction between charge reservoirs located on the C₆₀ and alkali ions. The optimal transition temperature follows the algebraic expression, T_C0 = (12.474 nm² K)/ℓζ, where ℓ relates to the mean spacing between interacting surface charges on the C₆₀ and ζ is the average radial distance between the C₆₀ surface and the neighboring Cs ions. Values of T_C0 for the measured cation stoichiometries of Cs_3–xC₆₀ 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 e²/ζ are shown to agree quantitatively with findings from nuclear-spin relaxation and mid-infrared 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)C₆₀ and fcc Cs_2.901(6)C₆₀ (legend upper left), compared to other high-T_C superconductors: cuprates; iron pnictides and chalcogenides; intercalated group-4-metal nitride-chlorides; and RuO- and ET-based 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_xCs_3–xC₆₀ 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.