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

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          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.

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          Generalized Gradient Approximation Made Simple

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            Soft self-consistent pseudopotentials in a generalized eigenvalue formalism

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              Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials

              Quantum ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn Source Package for Research in Electronic Structure, Simulation, and Optimization". It is freely available to researchers around the world under the terms of the GNU General Public License. Quantum ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively-parallel architectures, and a great effort being devoted to user friendliness. Quantum ESPRESSO is evolving towards a distribution of independent and inter-operable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
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                Author and article information

                Journal
                2015-12-09
                Article
                10.1038/nature17175
                1512.02933
                dc442008-015c-4ba2-abb6-c176efbbbe91

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

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                Custom metadata
                Nature 532, 81-84 (2016)
                cond-mat.supr-con

                Condensed matter
                Condensed matter

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