Ab-Initio Simulations of the Optical Properties of Warm Dense Gold

Abstract

Using a combination of classical and ab-initio molecular dynamics simulations, we calculate the structure and the electrical conductivity of warm dense gold during the first picoseconds after a short-pulse laser illumination. We find that the ions remain in their initial fcc structure for several picoseconds, despite electron temperatures ranging from a few to several eV after the laser illumination. The electrical conductivities calculated under these nonequilibrium conditions and using the latter assumption are in remarkable agreement with recent measurements using a short-pulse laser interacting with gold thin films.

Nonthermal solid-to-solid phase transitions in tungsten

“…4). We have performed similar calculations for tantalum, another approximately half-filled d-band transition bcc metal, and found that Ta exhibits the same kind of behavior as a function of T e as W. On the other hand, previous calculations for Au have shown that Au undergoes phonon hardening as the electronic temperature increases [50], if the unit cell volume is kept at V eq (300 K), FIG. 4.…”

“…We note that different structural transformations have been proposed caused by a soft mode on the -N line of the bcc Brillouin zone [54,56,57], particularly bcc-to-fcc and bcc-to-hcp transformations caused by the ,0]. We would like to emphasize that these transformations are totally different from the nonthermal melting of silicon, where phonon softening is observed in the whole Brillouin zone [50].…”

“…For instance, a hardening of all phonon branches at elevated electronic temperatures has been reported for gold, when calculated at the room-temperature atomic volume [50] V eq (300 K), and a strong softening of all branches, when calculated [36] at V eq (T e ). We performed calculations for both cases for W without noticing significantly different behavior.…”

“…A dynamical instability, also called mechanical instability in the literature [49], will manifest itself by imaginary phonon frequencies in some points of the Brillouin zone (BZ). For instance, in silicon a softening of the whole transverse acoustic branch at an elevated electronic temperature is considered as the origin of a nonthermal melting transition [50]. Similarly, a softening of the longitudinal optical branch at the point in bismuth is considered as the origin of coherent optical phonon generation [51,52].…”

Transport and optical properties of warm dense aluminum in the two-temperature regime: Ab initio calculation and semiempirical approximation

“…The study of the optical properties is possible both using modern picosecond diagnostics 1,2 and computational methods 1,7 .…”