Helium gap in the warm dense matter regime and experimental reflectivity measurements

Abstract

We revisit the reflectivity measurements of Celliers et al. [P. M. Celliers, P. Loubeyre, J. H. Eggert, S. Brygoo, R. S. McWilliams, D. G. Hicks, T. R. Boehly, R. Jeanloz, and G. W. Collins, Phys. Rev. Lett. 104, 184503 (2010)] for He at high density by taking into account a temperature dependence, based on existing ab initio calculations, in the functional form of the gap energy inferred from these data. We show that, when accounting for such a temperature dependence, the predicted behavior of the gap energy at high density differs significantly from the one suggested by Celliers et al. and that He is predicted to become metallic (gap closure) at much higher densities, around 10 g/cm${}^{3}$. High-pressure experiments over a larger temperature and density range are mandatory to correctly determine this gap behavior, a problem of crucial importance for astrophysical applications.