title: Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system.
abstract: The quantum nature of the proton can crucially affect the structural and physical properties of hydrogen compounds. For example, in the high-pressure phases of H2O, quantum proton fluctuations lead to symmetrization of the hydrogen bond and reduce the boundary between asymmetric and symmetric structures in the phase diagram by 30 gigapascals (ref. 3). Here we show that an analogous quantum symmetrization occurs in the recently discovered sulfur hydride superconductor with a superconducting transition temperature Tc of 203 kelvin at 155 gigapascals--the highest Tc reported for any superconductor so far. Superconductivity occurs via the formation of a compound with chemical formula H3S (sulfur trihydride) with sulfur atoms arranged on a body-centred cubic lattice. If the hydrogen atoms are treated as classical particles, then for pressures greater than about 175 gigapascals they are predicted to sit exactly halfway between two sulfur atoms in a structure with Im3m symmetry. At lower pressures, the hydrogen atoms move to an off-centre position, forming a short H-S covalent bond and a longer H···S hydrogen bond in a structure with R3m symmetry. X-ray diffraction experiments confirm the H3S stoichiometry and the sulfur lattice sites, but were unable to discriminate between the two phases. Ab initio density-functional-theory calculations show that quantum nuclear motion lowers the symmetrization pressure by 72 gigapascals for H3S and by 60 gigapascals for D3S. Consequently, we predict that the Im3m phase dominates the pressure range within which the high Tc was measured. The observed pressure dependence of Tc is accurately reproduced in our calculations for the phase, but not for the R3m phase. Therefore, the quantum nature of the proton fundamentally changes the superconducting phase diagram of H3S.
authors.affiliation: Fisika Aplikatua 1 Saila, EUITI Bilbao, University of the Basque Country (UPV/EHU), Rafael Moreno "Pitxitxi" Pasealekua 3, 48013 Bilbao, Spain.|Donostia International Physics Center (DIPC), Manuel Lardizabal Pasealekua 4, 20018 Donostia/San Sebastián, Spain.
authors.forename: Ion
authors.initials: I
authors.lastname: Errea
authors.affiliation: IMPMC, UMR CNRS 7590, Sorbonne Universités - UPMC Université Paris 06, MNHN, IRD, 4 Place Jussieu, F-75005 Paris, France.
authors.forename: Matteo
authors.initials: M
authors.lastname: Calandra
authors.affiliation: Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK.
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Pickard
authors.affiliation: Theory of Condensed Matter Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Nelson
authors.affiliation: Theory of Condensed Matter Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Needs
authors.affiliation: School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China.
authors.forename: Yinwei
authors.initials: Y
authors.lastname: Li
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA.
authors.forename: Hanyu
authors.initials: H
authors.lastname: Liu
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China.
authors.forename: Yunwei
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China.
authors.forename: Yanming
authors.initials: Y
authors.lastname: Ma
authors.affiliation: IMPMC, UMR CNRS 7590, Sorbonne Universités - UPMC Université Paris 06, MNHN, IRD, 4 Place Jussieu, F-75005 Paris, France.|Dipartimento di Fisica, Università di Roma "La Sapienza", Piazzale Aldo Moro 5, I-00185 Roma, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Mauri