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

TL;DR

This paper demonstrates that quantum proton fluctuations in hydrogen sulfide under high pressure induce symmetrization of hydrogen bonds, which crucially influences its high-temperature superconductivity, and predicts the stable phase responsible for observed superconducting properties.

Contribution

The study reveals that quantum nuclear motion lowers the symmetrization pressure in H3S, confirming the stable phase responsible for high-temperature superconductivity through ab initio calculations.

Findings

Quantum proton fluctuations lower symmetrization pressure by 72 GPa.

The stable phase is the symmetric Im-3m structure across the superconducting pressure range.

Quantum effects fully determine the superconducting phase diagram of H3S.

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 H$_2$O, 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 $T_c=203$ K at 155 GPa. In this system, according to classical theory, superconductivity occurs via formation of a structure of stoichiometry H$_3$S with S atoms arranged on a body-centered-cubic (bcc) lattice. For $P \gtrsim 175$ GPa, the H atoms are predicted to sit midway between two S atoms, in a structure with $Im\bar3m$ symmetry. At lower pressures the H atoms move to an off-center position forming a short H$-$S covalent bond and a longer H$\cdots$S hydrogen bond, in a structure with $R3m$ symmetry. X-ray diffraction experiments confirmed the H$_3$S 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 $Im\bar3m$ phase is stable over the whole pressure range within which a high $T_c$ was measured. The observed pressure-dependence of $T_c$ is closely reproduced in our calculations for the $Im\bar3m$ phase, but not for the $R3m$ phase. Thus, the quantum nature of the proton completely rules the superconducting phase diagram of H$_3$S.