Hydrogen sulphide at high pressure: a strongly-anharmonic phonon-mediated superconductor

TL;DR

This study uses first principles calculations to explore the structural, vibrational, and superconducting properties of high-pressure hydrogen sulfide, revealing significant anharmonic effects that influence electron-phonon interactions and superconducting transition temperatures.

Contribution

It provides the first detailed analysis of anharmonic effects in high-pressure H$_2$S, showing their impact on superconductivity and electron-phonon coupling.

Findings

Anharmonicity reduces electron-phonon coupling by 30%.

Superconducting transition temperature T$_c$ becomes pressure-independent due to anharmonic effects.

Harmonic approximation overestimates electron-phonon interaction and T$_c$.

Abstract

We use first principles calculations to study structural, vibrational and superconducting properties of H$_2$S at pressures $P\ge 200$ GPa. The inclusion of zero point energy leads to two different possible dissociations of H$_2$S, namely 3H$_2$S $\to$ 2H$_3$S + S and 5H$_2$S $\to$ 3H$_3$S + HS$_2$, where both H$_3$S and HS$_2$ are metallic. For H$_3$S, we perform non-perturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction ($\lambda\approx 2.64$ at 200 GPa) and T$_c$. Anharmonicity hardens HS bond-stretching modes and softens H--S bond-bending modes. As a result, the electron-phonon coupling is suppressed by $30\%$ ($\lambda\approx 1.84$ at 200 GPa). Moreover, while at the harmonic level T$_c$ decreases with increasing pressure, the inclusion of anharmonicity leads to a T$_c$ that is almost independent of pressure. High pressure hydrogen sulfide is a strongly anharmonic superconductor.