Quantum symmetrization transition in superconducting sulfur hydride from quantum Monte Carlo and path integral molecular dynamics

TL;DR

This study combines advanced computational methods to analyze the pressure-induced structural transition in sulfur hydride, linking local dipole formation and quantum effects to the maximum superconducting temperature.

Contribution

It provides a quantitative pressure estimate for hydrogen bond symmetrization using a novel combination of density functional theory, quantum Monte Carlo, and path integral molecular dynamics.

Findings

Maximum T_c correlates with local dipole moments forming on hydrogen sites.

Quantum nuclear effects significantly lower the ferroelectric transition pressure.

Isotope substitution shifts the transition pressure, affecting T_c maximum.

Abstract

We study the structural phase transition, originally associated with the highest superconducting critical temperature $T_c$ measured in high-pressure sulfur hydride. A quantitative description of its pressure dependence has been elusive for any \emph{ab initio} theory attempted so far, raising questions on the actual mechanism leading to the maximum of $T_c$. Here, we estimate the critical pressure of the hydrogen bond symmetrization in the Im$\bar{3}$m structure, by combining density functional theory and quantum Monte Carlo simulations for electrons with path integral molecular dynamics for quantum nuclei. We find that the $T_c$ maximum corresponds to pressures where local dipole moments dynamically form on the hydrogen sites, as precursors of the ferroelectric Im$\bar{3}$m-R3m transition, happening at lower pressures. For comparison, we also apply the self-consistent harmonic approximation, whose ferroelectric critical pressure lies in between the ferroelectric transition estimated by path integral molecular dynamics and the local dipole formation. Nuclear quantum effects play a major role in a significant reduction ($\approx$ 50 GPa) of the classical ferroelectric transition pressure at 200K and in a large isotope shift ($\approx$ 25 GPa) upon hydrogen-to-deuterium substitution of the local dipole formation pressure, in agreement with the corresponding change in the $T_c$ maximum location.