Abstract
The knowledge of the metallization of warm dense helium has important implications for understanding the thermal histories, stellar structure and magnetic field environment of giant planets. However, it is also a pendent scientific topic. For a revisiting into the properties of warm dense helium, we performed extensive quantum Langevin molecular dynamic simulations and electronic structure calculations to study helium over a very wide range of density (ρ = 1~24 g/cm3) and temperature (T = 10~160 kK). The dependencies of helium band gap on ρ and T were presented and a metallization boundary of helium was thus determined by gap closure. Such a boundary is further identified by the calculated electrical conductivity and optical reflectivity based on Kubo-Greenwood formula: along the boundary, the electrical conductivities are found to be 7.0 × 105~1.3 × 106 Ω−1 m−1 and the optical reflectivity value at 532 nm is about 0.55, which are typical values for true metal.
Highlights
As the second most-abundant chemical element in the universe, helium makes up a large fraction of giant gaseous planets[1] and most extrasolar planets[2] which have been discovered so far
In shock wave experiments, some typical low Z molecular fluids achieved their metallic states under pressures only about 100 GPa6,7 indicated that metalized pressure could be significantly reduced by the high temperature produced in shock wave compression[6]
The measured electrical conductivity of dense fluid He under multiple shock compression could be achieved to typical liquid alkali metals and the metalized density was estimated to be around 1 g/cm[3 10,11] which was identified by quantum molecular dynamics simulation of the day[12]
Summary
As the second most-abundant chemical element in the universe, helium makes up a large fraction of giant gaseous planets[1] and most extrasolar planets[2] which have been discovered so far. By considering the electron-phonon coupling effects, a recent theoretical results by Monserrat et al gave a much higher metalized pressure of 32.9 TPa9 Such a high pressure is far from experimental reach within current static pressure technique. The measured electrical conductivity of dense fluid He under multiple shock compression could be achieved to typical liquid alkali metals and the metalized density was estimated to be around 1 g/cm[3 10,11] which was identified by quantum molecular dynamics simulation of the day[12]. Combining diamond-anvil-cell and laser-driven shock wave techniques, a recent experimental work by Celliers et al reported that the hot dense He could become metallic above about 1.9 g/cm[3], this conclusion was obtained by fitting their optical measurements of reflectivity with a simple semiconducting Drude model without considering the thermal effects[13]. Wide range Ddirect-Current (DC) conductivity and optical reflectivity data were obtained which are essential input for modeling He-rich astrophysical objects
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