paper

Quantum Hydrogen-Bond Symmetrization and High-Temperature Superconductivity in Hydrogen Sulfide

arXiv:1512.02933 · doi:10.1038/nature17175

Abstract

Hydrogen compounds are peculiar as the quantum nature of the proton can crucially affect their structural and physical properties. A remarkable example are the high-pressure phases of HO, where quantum proton fluctuations favor the symmetrization of the H bond and lower by 30 GPa the boundary between the asymmetric structure and the symmetric one. Here we show that an analogous quantum symmetrization occurs in the recently discovered sulfur hydride superconductor with the record superconducting critical temperature K at 155 GPa. In this system, according to classical theory, superconductivity occurs via formation of a structure of stoichiometry HS with S atoms arranged on a body-centered-cubic (bcc) lattice. For GPa, the H atoms are predicted to sit midway between two S atoms, in a structure with symmetry. At lower pressures the H atoms move to an off-center position forming a short HS covalent bond and a longer HS hydrogen bond, in a structure with symmetry. X-ray diffraction experiments confirmed the HS stoichiometry and the S lattice sites, but were unable to discriminate between the two phases. Our present ab initio density-functional theory (DFT) calculations show that the quantum nuclear motion lowers the symmetrization pressure by 72 GPa. Consequently, we predict that the phase is stable over the whole pressure range within which a high was measured. The observed pressure-dependence of is closely reproduced in our calculations for the phase, but not for the phase. Thus, the quantum nature of the proton completely rules the superconducting phase diagram of HS.

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