Isotope quantum effects in the metallization transition in liquid hydrogen*

Quantum effects in condensed matter are often associated with low temperatures. In our work we show a significant isotope effect in the liquid-liquid transition in high-pressure hydrogen. This transition between an insulating molecular liquid and an atomic metallic liquid occurs at temperatures on the order of thousands of Kelvin, beyond where quantum effects are often assumed to be relevant. We show that this transition nevertheless exhibits a significant isotope effect on the order of hundreds of Kelvin when compared to deuterium, arising from the large zero-point energy difference between hydrogen and deuterium.

Using ab-initio density functional theory combined with path integral molecular dynamics to accurately include nuclear quantum effects, we identify significant isotopic shifts in the dissociation, metallization and thermodynamic boundaries. This highlights shortcomings of the widely used Born-Oppenheimer molecular dynamics method, which implicitly treats the nuclei classically. Our results show that nuclear quantum effects remain significant in hydrogen even at these high temperatures, meaning that experiments performed on deuterated samples are likely to significantly overestimate transition pressures and temperatures.