Ultrafast Thermal Melting in Nonequilibrium Warm Dense Copper.

The ultrafast dynamics of the loss of crystalline periodicity is investigated in femtosecond laser heated warm dense copper, by the original use of x-ray absorption near-edge specific structures just above the $L3$ edge. The characteristic time is observed near 1 ps, for specific energy density ranging from 1 to $5\text{ }\text{ }\mathrm{MJ}/\mathrm{kg}$, using ps-resolution x-ray absorption spectroscopy. The overall experimental data are well reproduced with two-temperature hydrodynamic simulations, supporting a thermal phase transition.