Compressed H_{3}S: intersublattice Coulomb coupling in a highT_{C} superconductor, D. R. Harshman and A. T. Fiory [arXiv]
Upon thermal annealing at or above room temperature (RT) and at high hydrostatic pressure P ~ 155 GPa, sulfur trihydride H_{3}S exhibits a measured maximum superconducting transition temperature T_{C} ~ 200 K. Various theoretical frameworks incorporating strong electronphonon coupling and Coulomb repulsion have reproduced this recordlevel T_{C}. Of particular relevance is that experimentally observed HD isotopic correlations among T_{C}, P, and annealed order indicate an HD isotope effect exponent α limited to values ≤ 0.183, leaving open for consideration unconventional highT_{C} superconductivity with electronicbased enhancements. The work presented herein examines Coulombic pairing arising from interactions between neighboring S and H species on separate interlaced sublattices constituting H_{3}S in the Im3m structure. The optimal value of the transition temperature is calculated from T_{C0} = k_{B}^{–1}Λe^{2}/ℓζ, with Λ= 0.007465 Å, intersublattice SH separation spacing ζ = a_{0}/√2, interaction charge linear spacing ℓ = a_{0} (3/σ)^{1/2}, average participating charge fraction σ = 3.43 ± 0.10 estimated from calculated Hprojected electron states, and lattice parameter a_{0} = 3.0823 Å at P = 155 GPa. The resulting value of T_{C0} = 198.5 ± 3.0 K is in excellent agreement with transition temperatures determined from resistivity (196 – 200 K onsets, 190 – 197 K midpoints), susceptibility (200 K onset), and critical magnetic fields (203.5 K by extrapolation). Analysis of midinfrared reflectivity data confirms the expected correlation between boson energy and ζ^{–1}. Suppression of T_{C} below T_{C0}, correlating with increasing residual resistance for < RT annealing, is treated in terms of scatteringinduced pair breaking. Correspondences between H_{3}S and layered highT_{C} superconductor structures are also discussed, and a model considering Compton scattering of virtual photons of energies ≤ e^{2}/ζ by intersublattice electrons is introduced, illustrating that Λ ∝ ƛ_{C}, where ƛ_{C} is the reduced electron Compton wavelength.
Illustration of Im3m unit cell of compressed H_{3}S with color contrast distinguishing the two simple cubic sublattices; basis H_{3}S is shown with S larger than H. Lattice parameter a_{0} is cube edge; intersublattice SH distance ζ is onehalf cube face diagonal. 

Variation of measured T_{C} of H_{3}S with applied pressure P for ≥ RT (room temperature) anneal, green circles with center dot, and < RT anneal, blue circles (from [1, 5, 6]). Gray symbols are various theoretical calculations denoted as (D) [2], (E) [4], (P) [8], (A) [10], (K) [11], (G) [13], (F) [14], (S) [16], (К) [17], and (J) [18]; structure indicated where available. Red square symbol (H) corresponds to T_{C0} from this work. 
Dale R. Harshman and Anthony T. Fiory, Journal of Physics: Condensed Matter 29, 445702 (2017).