Unified ab initio description of the ultrafast reversible and irreversible laser-induced structural changes occurring in antimony: from coherent phonons up to nonthermal melting

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Femtosecond-laser pulses are known to create extreme nonequilibrium conditions in solids: Whereas the electrons are heated to several 10000 K, the ions remain close to room temperature. In this proposal we plan to perform, for the first time, a detailed study of the atomic pathways triggered by the changes in the bonding properties of solid antimony due to the laser excitation of hot electrons over a broad range of laser fluences up to the (nonthermal) melting threshold. Antimony is a particularly interesting material, since it is expected to exhibit all so far known laser-induced ultrafast structural phenomena, namely, coherent phonons, thermal phonon squeezing, solid-solid phase transitions, and nonthermal melting. In order to resolve the microscopic pathways followed by the atoms we intend to employ a molecular dynamics code based on Mermin's electronic-temperature-dependent density functional theory that has been recently written in our group. This so-called Code for Highly-excited Valence Electron Systems (CHIVES) uses local basis sets and order(N) methods for the construction of the Hamiltonian and overlap matrices and has been shown to be about two hundred times faster than state-of-the-art plane-wave codes. At present we can perform simulations of supercells with up to 1200 atoms. Ultrafast structural phenomena can be experimentally detected by all-optical pump-probe measurements of the reflectivity and more recently also directly using time-resolved electron- or x-ray-diffraction. Our analysis will yield time-dependent structure factors, which will, among other applications, be important for the interpretation of experiments planned at new x-ray free-electron-laser facilities, such as, FLASH in Hamburg and LCLS in Stanford.