Modeling thermoreflectance in Au and Ni from molecular dynamics

Experimental thermoreflectance measurements using femto-second laser irradiation (Hopkins et al 2011 J. Heat Transfer 133 044505) can be used to shed light on the electron-phonon coupling in metals through a selective excitation of electrons. In these experiments the energy transfer occurs at a time scale of pico-seconds which corresponds to the typical time scale of molecular dynamics (MD) simulations. However since the electron-phonon coupling is, generally, not taken into account in MD simulations, it is in principle not possible to model thermoreflectance as well as other properties related to electron-phonon coupling such as electric conductivity and thermal transport. Here we show that it is however possible to extend MD using a method proposed by Finnis, Agnew and Foreman (FAF) (Finnis et al 1991 Phys. Rev. B 44 567-74), originally implemented in order to account for electronic stopping power in particle irradiation. Although the FAF method was devoted to model high energy atomic displacements yielding local melt of the crystal, we have been able to reproduce pulsed-laser irradiation experiments at room temperature. Our computations were realized in both Au and Ni to exemplify the transferability of our results. The agreement between the calculations and the experimental results allowed us to discuss different theories for computing the amplitude of electron-phonon coupling and to select the more appropriate according to FAF. Our work paves the way to re-introduce the phenomenology of electric conductivity in MD simulations for metals.