  We are reporting the observation of the breakdown of electrons degeneracy and
emergence of classical statistics in the simplest element: metallic deuterium.
We have studied the optical reflectance, shock velocity and temperature of
dynamically compressed liquid deuterium up to its Fermi temperature, TF. Above
the insulator-metal transition, the optical reflectance shows the distinctive
temperature-independent resistivity saturation, which is prescribed by Mott
minimum metallic limit, in agreement with previous experiments. At T > 0.4 TF,
however, the reflectance of metallic deuterium starts to rise with a
temperature-dependent slope, consistent with the breakdown of the Fermi
surface. The experimentally inferred collisional time in this region exhibits
the characteristic temperature dependence expected for classical Landau-Spitzer
plasma. Our observation of electron degeneracy lifting extends studies of
degeneracy to new fermionic species, electron Fermi systems and offers an
invaluable benchmark for quantum statistical models of Coulomb systems over a
wide range of temperatures relevant to dense astrophysical objects and ignition
physics.
