Breakdown of Fermi-degeneracy in the simplest liquid metal*

The phase diagram of Fermi-matter can be demarcated by regions where quantum or classical effects are uniquely dominant, as determined by the temperature of the system relative to its Fermi temperature, T_F. Studies of this cross-over have been limited to dilute systems such as liquid ³He or ultracold alkali gases. Here, we report the observation of the breakdown of Fermi degeneracy in the simplest element: deuterium shock compressed to the metallic state. Above the insulator–metal transition, the optical reflectance shows the distinctive temperature-independent saturation, which is prescribed by the Mott minimum scattering limit, in agreement with previous experiments. However, at T > 0.4 T_F, the reflectance of metallic deuterium rises with a temperature-dependent slope that is characteristic of a classical Landau–Spitzer plasma. The onset of the crossover coincides with the fluid transitioning from a strongly coupled metal to a moderately coupled plasma. Our results provide an invaluable benchmark for quantum statistical models of coulomb systems, over a wide range of temperature relevant to dense astrophysical objects and ignition physics.