High-T_Csuperconductivity in Cs₃C₆₀compounds governed by local Cs–C₆₀Coulomb interactions

Abstract

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.

Compressed H3S: inter-sublattice Coulomb coupling in a high-TCsuperconductor

“…In Coulomb-mediated (high-T C ) superconductors, phononic interactions have been generally observed to be associated with pair breaking [19]. While the model presented herein fully explains the superconductivity in H 3 S and accurately determines T C0 , experimental uncertainty in α leaves open the possibility of a hybrid mechanism incorporating Coulombic enhancements to a BCS-based model.…”

“…The maximum in the pressure dependence of T C for H 3 S resembles that for compressed Cs 3 C 60 , where the pressure for maximum T C has been identified as being optimal for high-T C Coulombic coupling [19]. Proposing such a comparison recognizes that the R3m-Im m transition in H 3 S at 140 GPa [7] occurs in a flat, pressureindependent region of T C (P) (see figure 1).…”

“…In A15 Cs 3 C 60 , a given C 60 also has 12 Cs nearest neighbors located at the 12 vertices of a regular convex icosahedron. In the case of A15 Cs 3 C 60 near optimal compression, a maximum near ~2500 cm 1 (0.31 eV) in the mid-infrared optical conductivity at ambient temperature [32] (figure 1I therein) was previously identified with an excitation scaling with the energy e 2 /ζ [19]. For H 3 S at P = 150 GPa, the normal-state reflectance shows a depressed band with minimum at ~0.46 eV [6].…”

“…Section 2 presents a brief synopsis of existing T C data and relevant phononic theoretical predictions. Section 3 presents the Coulombbased pairing model and its application to H 3 S. Consideration of the pairing mechanism from the perspective of the Compton scattering of virtual photons, optimal superconductivity of Im m H 3 S in relation to compressed A15 Cs 3 C 60 [19], and pair-breaking induced suppression of T C below T C0 for annealing temperatures < RT are discussed in section 4, and the work is summarized in section 5. Figure 1 shows experimental results for T C of H 3 S over a range of applied pressure, determined from onset points in resistance measurements.…”

High-TC Superconductivity in Hydrogen Clathrates Mediated by Coulomb Interactions Between Hydrogen and Central-Atom Electrons

“…Mass isotope data for DH 10 reported in [12], deemed inconclusive in [4], are shown to more likely reflect the inverse correlation between T C and the fractional transition width T C /T C , as is found for D 3 S. Consequently, and notwithstanding the predictions of Éliashberg theory, the precise role played by phonons in the superconductivity of MH n awaits further investigation. Since quantitative scaling with the theoretical e 2 / is noted from optical evidence for compressed A15 Cs 3 C 60 [49] and H 3 S [41], and plausibly extending to LaH 10 , reflectivity measurements of MH n may prove invaluable in determining the nature of the mediating boson.…”