Evidence for out-of-equilibrium states in warm dense matter probed by X-ray Thomson scattering

TL;DR

This paper investigates unexpected discrepancies in X-ray Thomson scattering data for aluminum, suggesting the presence of out-of-equilibrium states with decoupled ion and electron temperatures, using orbital free molecular dynamics simulations.

Contribution

It introduces a novel interpretation of XRTS data indicating out-of-equilibrium states in warm dense matter, supported by new simulation approaches.

Findings

Ion-ion structure factor indicates strongly coupled out-of-equilibrium state.

Orbital free molecular dynamics simulations with colder ions match experimental data.

Decoupled ion and electron temperatures may mimic transient or metastable states.

Abstract

A recent and unexpected discrepancy between \textit{ab initio} simulations and the interpretation of a laser shock experiment on aluminum, probed by X-ray Thomson scattering (XRTS), is addressed. The ion-ion structure factor deduced from the XRTS elastic peak (ion feature) is only compatible with a strongly coupled out-of-equilibrium state. Orbital free molecular dynamics simulations with ions colder than the electrons are employed to interpret the experiment. The relevance of decoupled temperatures for ions and electrons is discussed. The possibility that it mimics a transient, or metastable, out-of-equilibrium state after melting is also suggested.