Proton crystallization in a dense hydrogen plasma

Complete text of publication follows. In a recent letter we have predicted that in a dense hydrogen plasma at sufficiently low temperatures and high densities protons would spontaneously order into a lattice which is embedded into a highly degenerate electron gas. There the conditions for the stability of such an ion crystal in a neutral two-component plasma were derived. In particular, we found that there exists a minimal mass ratio of the heavy and light charges of about 80. Thus the effect should also be observable in various semiconductor materials. Here we concentrate on the phase diagram of the proton crystal for which only a rough estimate of the phase diagram is known. We present extensive new simulation results which allow to predict the temperature and density range for proton crystallization in dense laboratory experiments. We simulate a macroscopic spatially homogeneous fully ionized two-component electron-proton plasma in thermodynamic equilibrium from first principles using direct fermionic PIMC simulations, for an overview, see [3]. Our results for the phase diagram differ substantially from the previous predictions based on the one-component plasma (OCP) model: In the classical part of the phase diagram the crystal appears to be stabilized compared to the OCP prediction. In contrast, in the quantum part of the phase diagram the crystal appears to be de-stabilized and vanishes at lower densities compared to the OCP prediction. Finally, the maximum temperature for the proton crystal is found to be around 40 000K, slightly below the previous prediction. Our results indicate that the OCP treatment of the liquid-solid transition in a two-component plasma has to be questioned. The OCP-assumption of a homogeneous rigid neutralizing background gives rise to substantial deviations of the critical parameters. More simulations are underway to verify and generalize these predictions. The results are of relevance for a large variety of plasmas, including dwarf stars, laser-cooled expanding plasmas, warm dense matter, dusty plasmas, and semiconductors. We expect that the necessary parameters will be achievable in laser or ion beam compression experiments of hydrogen.