Theory of complex fluids in the warm-dense-matter regime, and application to an unusual phase-transitions in liquid carbon

TL;DR

This paper compares advanced simulation methods for complex liquid carbon in warm-dense matter, demonstrating the effectiveness of the NPA+HNC approach in capturing covalent bonding and phase transitions not seen in simpler models.

Contribution

It introduces the NPA+HNC method as a reliable tool for modeling complex bonding and phase transitions in warm-dense carbon, outperforming traditional average-atom models.

Findings

NPA+HNC accurately reproduces pair-distribution functions from DFT+MD.

Evidence of simultaneous liquid-vapor and metal-semi-metal transitions in carbon.

Discontinuous changes in ionization Z linked to phase transitions.

Abstract

Data from recent laser-shock experiments, density-functional theory (DFT) with molecular-dynamics (MD), and path-integral Monte Carlo (PIMC) simulations on carbon are compared with predictions from the neutral-pseudo-atom (NPA)+ hyper-netted-chain (HNC) approach for carbon, a complex liquid in the warm-dense matter regime. The NPA results are in good agreement, not only with high-density regimes that have been studies via PIMC, but even at low densities and low temperatures where transient covalent bonding dominates ionic correlations. Thus the `pre-peak' due to the C-C bond at $\sim$1.4-1.6 \AA$\,$ and other features found in the pair-distribution function from DFT+MD simulations at 0.86 eV and 3.7 g/cm$^3$ etc., are recovered accurately in the NPA+HNC calculations. Such C-C bonding peaks have not been captured via average-atom ion-sphere (IS) models. Evidence for an unusual liquid $\to$ vapor and metal$\to$ semi-metal transition occurring simultaneously is presented. Here a strongly correlated metallic-liquid with transient C-C bonds, i.e., carbon at density $\sim$ 1.0 g/cm$^3$ and mean ionization $Z=4$ transits abruptly to a disordered mono-atomic vapour at 7 eV, with $Z\simeq$ 3. Other cases where $Z$ drops abruptly are also noted. The nature of $Z$, its discontinuities, and the role of exchange correlation, are reviewed. The limitations of IS models in capturing the physics of transient covalent bonding in warm dense matter are discussed.