Compressed H₃S: inter-sublattice Coulomb coupling in a high-T_C superconductor

Upon thermal annealing at or above room temperature (RT) and at high hydrostatic pressure P ∼ 155 GPa, sulfur trihydride H₃S exhibits a measured maximum superconducting transition temperature T _C ∼ 200 K. Various theoretical frameworks incorporating strong electron–phonon coupling and Coulomb repulsion have reproduced this record-level T _C. Of particular relevance is that experimentally observed H–D isotopic correlations among T _C, P, and annealed order indicate an H–D isotope effect exponent α limited to values  ⩽  0.183, leaving open for consideration unconventional high-T _C superconductivity with electronic-based enhancements. The work presented herein examines Coulombic pairing arising from interactions between neighboring S and H species on separate interlaced sublattices constituting H₃S in the Im$\overline{3}$m structure. The optimal value of the transition temperature is calculated from T _C0  =  $k_B^{-1}$Λe ²/$\ell$ ζ, with Λ  =  0.007465 Å, inter-sublattice S–H separation spacing ζ  =  a ₀/$\sqrt{2}$, interaction charge linear spacing $\ell$ =  a ₀ (3/σ)^1/2, average participating charge fraction σ  =  3.43  ±  0.10 estimated from calculated H-projected 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 mid-infrared 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 scattering-induced pair breaking. Correspondences between H₃S and layered high-T _C superconductor structures are also discussed, and a model considering Compton scattering of virtual photons of energies  ⩽  e ²/ζ by inter-sublattice electrons is introduced, illustrating that Λ  ∝  ƛ _C, where ƛ _C is the reduced electron Compton wavelength. (paper)