Full Temperature-Dependent Potential and Anharmonicity in Metallic Hydrogen: Colossal NQE and the Consequences

TL;DR

This paper extends the temperature-dependent effective potential method to full potential cases using path integrals, revealing colossal nuclear quantum effects in metallic hydrogen that challenge traditional phonon models and suggest new lattice dynamics phenomena.

Contribution

It introduces a generalized TDEP method combined with path integrals for anharmonic phonon dispersion, addressing low-temperature limitations and uncovering significant NQE in metallic hydrogen.

Findings

Discovery of colossal nuclear quantum effects in metallic hydrogen.

Large anharmonicity causes significant proton drift and slow phonon convergence.

Potential breakdown of the phonon picture due to NQE in metallic hydrogen.

Abstract

The temperature-dependent effective potential (TDEP) method for anharmonic phonon dispersion is generalized to the full potential case by combining with path integral formalism. This extension naturally resolves the intrinsic difficulty in the original TDEP at low temperature. The new method is applied to solid metallic hydrogen at high pressure. A colossal nuclear quantum effect (NQE) and subsequent anharmonicity are discovered, which not only leads to unexpectedly large drift of protons, but also slows down the convergence rate substantially when computing the phonon dispersions. By employing direct ab initio path integral molecular dynamics simulations as the benchmark, a possible breakdown of phonon picture in metallic hydrogen due to colossal NQE is indicated, implying novel lattice dynamical phenomena might exist. Inspired by this observation, a general theoretical formalism for quantum lattice dynamics beyond phonon is sketched, with the main features being discussed.