Equation of state, phonons, and lattice stability of ultra-fast warm dense matter

Abstract

Using the two-temperature model for ultrafast matter (UFM), we compare the equation of state, pair-distribution functions $g(r)$, and phonons using the neutral pseudoatom (NPA) model with results from density-functional theory (DFT) codes and molecular-dynamics (MD) simulations for Al, Li and Na. The NPA approach uses state-dependent first-principles pseudopotentials from an `all-electron' DFT calculation with finite-$T$ XCF. It provides pair potentials, structure factors, the `bound' and `free' states, as well as a mean ionization $\bar{Z}$ unambiguously. These are not easily accessible {\it via} DFT+MD calculations which become prohibitive for $T/T_F$ exceeding $\sim 0.6$, where $T_F$ is the Fermi temperature. Hence, both DFT+MD and NPA methods can be compared up to $\sim 8$ eV, while higher $T$ can be addressed $via$ the NPA. The high-$T_e$ phonon calculations raise the question of UFM lattice stability and surface ablation in thin UFM samples. The ablation forces in a UFM slab are used to define an "ablation time" competing with phonon formation times in thin UFM samples. Excellent agreement for all properties is found between NPA and standard DFT codes, even for Li where a strongly non-local pseudopotential is used in DFT codes. The need to use pseudopotentials appropriate to the ionization state $\bar{Z}$ is emphasized. The effect of finite-$T$ exchange-correlation functional is illustrated via its effect on the pressure and the electron-density distribution at a nucleus.

arXiv comment13 apges, 9 figures

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• Ion-ion dynamic structure factor, acoustic modes and equation of state of two-temperature warm dense aluminum
• Lattice stability of ordered Au-Cu alloys in the warm dense matter regime
• Lattice dynamics for isochorically heated metals: A model study
• Yukawa Friedel-Tail pair potentials for warm dense matter applications
• Simple pair-potentials and pseudo-potentials for warm-dense matter and general applications
• Pseudo-Fermi surface and phonon softening in sodium with a stepwise electron distribution