High-$T_{\text{C}}$ superconductivity in Cs${}_3$C${}_{60}$ compounds governed by local Cs-C${}_{60}$ Coulomb interactions

Abstract

Unique among alkali-doped $\text{A}$${}_3$C${}_{60}$ fullerene compounds, the A15 and fcc forms of Cs${}_3$C${}_{60}$ exhibit superconducting states varying under hydrostatic pressure with highest transition temperatures at $T_{\text{C}}$${}^{\text{meas}}$ = 38.3 and 35.2 K, respectively. Herein it is argued that these two compounds under pressure represent the optimal materials of the $\text{A}$${}_3$C${}_{60}$ family, and that the C${}_{60}$-associated superconductivity is mediated through Coulombic interactions with charges on the alkalis. A derivation of the interlayer Coulombic pairing model of high-$T_{\text{C}}$ superconductivity employing non-planar geometry is introduced, generalizing the picture of two interacting layers to an interaction between charge reservoirs located on the C${}_{60}$ and alkali ions. The optimal transition temperature follows the algebraic expression, $T_{\text{C0}}$ = (12.474 nm${}^2$ K)/$\ell$$\zeta$, where $\ell$ relates to the mean spacing between interacting surface charges on the C${}_{60}$ and $\zeta$ is the average radial distance between the C${}_{60}$ surface and the neighboring Cs ions. Values of $T_{\text{C0}}$ for the measured cation stoichiometries of Cs${}_{3-\text{x}}$C${}_{60}$ with x $\approx$ 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 $\zeta$ and structural disorder in the fcc form. In the A15 form, modeled interacting charges and Coulomb potential e${}^2$/$\zeta$ are shown to agree quantitatively with findings from nuclear-spin relaxation and mid-infrared optical conductivity. In the fcc form, suppression of $T_{\text{C}}$${}^{\text{meas}}$ below $T_{\text{C0}}$ is ascribed to native structural disorder. Phononic effects in conjunction with Coulombic pairing are discussed.

14 pages, 3 figures (Updated Metadata to correct typographical errors, 12 March 2017)

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Cited by in corpus (2)

• High-$T$${}_{\text{C}}$ Superconductivity in Hydrogen Clathrates Mediated by Coulomb Interactions between Hydrogen and Central-Atom Electrons
• Superconducting $I$$\overline{4}$3$m$ CSH${}_7$ model applied to resistive transition temperature data for compressed C-S-H at high pressure