Structural transition dynamics of the formation of warm dense gold: From an atomic scale view

With the highly optimized embedded-atom-method (EAM) potential and electron-phonon coupling factor obtained from experimental data, the dynamics of the formation of warm dense gold and the nuclear response of gold foils upon intense laser excitation were investigated using two-temperature molecular dynamics simulations. Considering laser energy densities ranging from 0.18 to 1.17 MJ/kg, we provide a microscopic picture of the formation of warm dense gold. A threshold (0.19 MJ/kg) for the laser energy density was determined, identifying two different melting mechanisms. For an energy density below 0.19 MJ/kg, the melting of the foil is controlled by the propagation of melt fronts from external surfaces, which results in heterogeneous melting on the time scale of hundreds of picoseconds. For an energy density above 0.19 MJ/kg, homogeneous nucleation and growth of liquid regions inside the foil play the leading role, and homogeneous melting occurs with several picoseconds. Compared with previous simulations and experimental measurements, the evaluated different threshold value indicates that the improvement in the electron heat capacity for the two-temperature model by including the kinetic information of electrons may predict better laser-matter interactions under such extreme non-equilibrium conditions.