Metallization in hydrogen-helium mixtures

Hydrogen-helium mixtures have long been predicted to undergo demixing at high pressures of several megabars which occur in the interiors of giant planets such as Jupiter and Saturn. This effect is most important to understand their evolution and current interior structure. Ab initio calculations have already proven their potential to give precise predictions for the demixing line for pressures above 4 Mbar, well above a first-order phase transition from molecular to metallic hydrogen. Here we calculate the miscibility gap for lower pressures between 1 and 2 Mbar using ab initio molecular dynamics simulations. By using the Kubo-Greenwood formula we obtain the electrical conductivity and reveal the close connection between metallization in the hydrogen subsystem and the location of the miscibility gap. Especially, we find direct evidence for H-He demixing by performing simulations for large particle numbers.