Two-Temperature Pair Potentials and Phonon Spectra for Simple Metals in the Warm Dense Matter Regime

L Harbour and MWC Dharma-wardana and DD Klug and LJ Lewis, CONTRIBUTIONS TO PLASMA PHYSICS, 55, 144-151 (2015).

DOI: 10.1002/ctpp.201400092

We develop ion-ion pair potentials for Al, Na and K for densities and temperatures relevant to the warm-dense-matter (WDM) regime. Furthermore, we emphasize non-equilibrium states where the ion temperature T-i differs from the electron temperature T-e. This work focuses mainly on ultra-fast laser-metal interactions where the energy of the laser is almost exclusively transferred to the electron sub- system over femtosecond time scales. This results in a two-temperature system with T-e > T-i and with the ions still at the initial room temperature T-i = T-r. First-principles calculations, such as density functional theory (DFT) or quantum Monte Carlo, are as yet not fully feasible for WDM conditions due to lack of finite-T features, e.g. pseudopotentials, and extensive CPU time requirements. Simpler methods are needed to study these highly complex systems. We propose to use two- temperature pair potentials U-ii(r, T-i, T-e) constructed from linear- response theory using the non-linear electron density n(r) obtained from finite-T DFT with a single ion immersed in the appropriate electron fluid. We compute equilibrium phonon spectra at T-r which are found to be in very good agreement with experiments. This gives credibility to our non-equilibrium phonon dispersion relations which are important in determining thermophysical properties, stability, energy-relaxation mechanisms and transport coefficients. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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