Theory of complex fluids in the warm-dense-matter regime, and   application to an unusual phase-transitions in liquid carbon
  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. 
M.W.C. Dharma-wardana (NRC-Canada)
cond-mat.stat-mech astro-ph.EP physics.plasm-ph