Electron-phonon coupling and exchange-correlation effects in superconducting H₃S under high pressure

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We investigate the H₃S phase of sulphur hydride under high pressure ≃ 200 GPa by means of ab-initio calculations within the framework of the density-functional theory (DFT) with the PBE0 hybrid exchange-correlation (E xc) approximation. The choice of E xc has the largest effect on the calculated electron-phonon coupling (EPC) matrix elements; the high pressure equation of state and phonon frequencies are only slightly modified. Mode-dependent EPC correction factors are determined from PBE0 using a frozen-phonon supercell approach, while standard density-functional perturbation theory is used to determine the EPC with PBE generalized-gradient approximation E xc. Our principle finding is that the calculated PBE0 Tc is enhanced by 25% compared to PBE. This is similar in magnitude, but in opposite direction, to the proposed suppression of Tc by anharmonic effects [Errea et al., Phys. Rev. Lett. 114, 157004 (2015)]. Our calculations demonstrate the importance of considering exchange-correlation approximations for calculations of superconducting properties for this class of materials.