Free electron properties of metals under ultrafast laser-induced electron-phonon nonequilibrium: a first-principles study

The dynamics of laser-excited materials is an area of intense research as diagnostics of laser-matter experiments can be discussed by back-tracking the transient properties of the irradiated material. Particularly, the primary phenomena of transient electronic excitation and energy transport are of utmost importance. Irradiating a metal by a short laser pulse (� 100 fs) can lead to a significant rise of the electronic temperature with respect to the ionic lattice as the energy of the laser pulse can be deposited before the material system starts dissipating energy by thermal or mechanical ways. The electronic excitation can affect both electronic and structural properties of the solid, impacting optical coupling, transport and phase transitions. The confinement of the absorbed energy at solid density pushes the matter into an extreme nonequilibrium state and new thermodynamic regimes are triggered. The interplay between the ultrafast excitation and the material response still requires a comprehensive theoretical description for highly excited solid materials including in particular the excitation-dependent band structure evolution as this influence the response to laser action. 1 Recent advances in studying processes induced by short laser pulses have revealed the determinant role of primary excitation events. Their accurate comprehension is necessary to correctly describe ultrafast structural dynamics, 2,3 phase transitions, 4,5 nanostructure formation, 6 ablation dynamics, 7,8 or strong shock propagation. 9 In such nonequilibrium conditions, conduction electrons participating to energy exchange are expected to evolve in time, depending on the excitation degree. 10 They largely determine the material transient properties and transformation paths. In this context, they are a crucial parameter required to describe complex ultrafast phenomena involving relaxation of excited states. Particularly, pure electronic effects (population and band distribution) determining transient coefficients before structural transitions set in are of interest and we will follow excitation influence in the form of nonequilib