Quantum Hydrogen-Bond Symmetrization and High-Temperature Superconductivity in Hydrogen Sulfide (1512.02933v1)

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.

• The paper demonstrates that quantum proton fluctuations lower the symmetrization pressure by 72 GPa, stabilizing the high-symmetry Im3m phase in H3S.
• It employs ab initio DFT and the stochastic self-consistent harmonic approximation to accurately model anharmonic phonon spectra and quantum effects.
• The study reveals a pressure-dependent Tc aligning with experimental data, suggesting that quantum effects are crucial for high-temperature superconductivity.

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

The paper "Quantum Hydrogen-Bond Symmetrization and High-Temperature Superconductivity in Hydrogen Sulfide" presents a detailed investigation into the remarkable properties of the superconducting hydrogen sulfide compound, H$_3$S, under high-pressure conditions. This research is grounded in a robust theoretical framework supported by ab initio density-functional theory (DFT) calculations, with a specific focus on understanding the effects of quantum nuclear motion on the phase stability and superconducting behavior of H$_3$S.

A key aspect of this paper is the examination of quantum hydrogen-bond symmetrization within H$_3$S. It is demonstrated that quantum fluctuations of the proton can significantly lower the symmetrization pressure by 72 GPa, thereby enhancing the stability of the high-symmetry $Im\bar{3}m$ phase across a broader pressure range where superconductivity with a critical temperature $T_c$ of 203 K at 155 GPa has been reported. The paper's findings suggest that proton quantum dynamics are integral to regulating the superconducting phase diagram of H$_3$S, which challenges the conventional understanding based purely on structural predictions without considering quantum effects.

The authors utilized the stochastic self-consistent harmonic approximation (SSCHA) to accurately capture the anharmonic phonon spectra and vibrational contributions critical to this analysis. This methodology enables the consideration of quantum and anharmonic effects at finite pressures, overcoming limitations associated with imaginary phonon modes in harmonic approximations.

One of the notable outcomes is the calculated variation of $T_c$ in relation to pressure for the $Im\bar{3}m$ phase, which aligns closely with experimental data. The paper reveals that the $Im\bar{3}m$ phase persists across the pressure range significant for high-$T_c$ superconductivity, as evidenced by the matched pressure dependence of $T_c$ from both theoretical predictions and experimental observations. This success is not observed for the $R3m$ phase, implying that only the symmetrized $Im\bar{3}m$ phase can explain the high-$T_c$ superconducting phase observed experimentally.

The implications of this work suggest potential adjustments in the search for other high-$T_c$ materials, emphasizing the need to consider quantum effects in hydrogen-rich compounds. In a broader sense, this research contributes to our understanding of electron-phonon coupling in metallic hydrogen systems, which could redefine strategies for discovering new superconducting materials. Future developments could explore alternative settings and pressure conditions, pushing the boundaries of known superconducting states and investigating their practical applications under varying environmental parameters.

Overall, this paper underscores the paramount influence of quantum mechanics in defining material properties at extreme conditions, providing a refined perspective that may guide subsequent experimental and theoretical advancements in high-pressure superconductivity and materials science.