**New experiments elucidate the crossover between a degenerate quantum system and classical one in the simplest metal.**

Liquids and plasmas constitute two of the primary states of matter. We have successfully demonstrated the first experimental observation of the crossover between a dense liquid metal and a dense plasma. A key distinguishing feature between these two states at high density, is that metals are quantum, i.e their properties are described by Fermi-Dirac statistics, while plasmas assume classical Maxwellian properties. This Fermi-Driac statistics was introduced independently by E. Fermi and P. Dirac in 1926 to describe indistinguishable systems with antisymmetric eigenfunctions such as electrons. The distinction between the two forms the foundation of our description of Fermi matter and gives rises to electron conduction in metals, the structure of the periodic table and the degeneracy pressures in compact stars.

Although being a canonical textbook example, the crossover between the Fermi and Maxwellian statistics wasn’t observed in a dense system before.

Although being a canonical textbook example, the crossover between these two statistics wasn’t observed in a dense system before. By employing the powerful 40 kJ lasers of the OMEGA system, we have studied the optical properties of the simplest metal, metallic deuterium as a function of temperature. The experiments elucidate this crossover between the quantum and the classical statistics around 0.4 of the Fermi temperature, TF. Besides their fundamental significance, the data would help benchmark transport models of dense plasmas used in describing stellar objects, and could aid in the design of inertial fusion capsules.

**Implications **

Our result should allow a predictive criterion for the degeneracy condition occurring in compact astrophysical bodies and provide an invaluable benchmark for theoretical models.

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