Insulator-to-Conducting Transition in Dense Fluid Helium

By combining diamond-anvil-cell and laser-driven shock wave techniques, we produced dense He samples up to 1.5  g/cm3 at temperatures reaching 60 kK. Optical measurements of reflectivity and temperature show that electronic conduction in He at these conditions is temperature-activated (semiconducting). A fit to the data suggests that the mobility gap closes with increasing density, and that hot dense He becomes metallic above ∼1.9  g/cm3. These data provide a benchmark to test models that describe He ionization at conditions found in astrophysical objects, such as cold white dwarf atmospheres.