Evolution of ac Conductivity in Nonequilibrium Warm Dense Gold

Using a chirped pulse probe technique, we have obtained single-shot measurements of temporal evolution of ac conductivity at 1.55 eV (800 nm) during electron energy relaxation in nonequilibrium warm dense gold with energy densities up to $4.1\mathrm{MJ}/\mathrm{kg}$ ($8\times {10}^{10}\mathrm{J}/{\mathrm{m}}^3$). The results uncover important changes that have been masked in an earlier experiment. Equally significant, they provide valuable tests of an ab initio model for the calculation of electron heat capacity, electron-ion coupling, and ac conductivity in a single, first principles framework. While measurements of the real part of ac conductivity corroborate our theoretical temperature-dependent electron heat capacity, they point to an electron-ion coupling factor of $\sim 2.2\times {10}^{16}\mathrm{W}/{\mathrm{m}}^3\mathrm{K}$, significantly below that predicted by theory. In addition, measurements of the imaginary part of ac conductivity reveal the need to improve theoretical treatment of intraband contributions at very low photon energy.

Figure 1

Temporal changes in probe reflectivity and transmissivity for excitation energy densities of (a),(b) $4.0–4.1\mathrm{MJ}/\mathrm{kg}$, and (c), (d) $0.53–0.58\mathrm{MJ}/\mathrm{kg}$. Dotted lines are lineouts taken from measured spectra, and solid lines are the corresponding underlying intensity signals.

Figure 2

(a) Real and (b) imaginary parts of ac conductivity as a function of pump-probe delays.

Figure 3

Comparison of our calculated $C_e(T_e)$ and $g_{ei}(T_e)$ to those of Lin et al. [35].

Figure 4

(a) Real and (b) imaginary parts of initial ac conductivity as a function of excitation energy density.

Figure 5

Comparison of measured ${\sigma }_r(t)$ and ${\sigma }_i(t)$ to theoretical calculations for excitation energy densities of (a),(e) $4.0–4.1\mathrm{MJ}/\mathrm{kg}$, (b), (f) $2.4–2.7\mathrm{MJ}/\mathrm{kg}$, (c), (g) $1.4\mathrm{MJ}/\mathrm{kg}$, and (d), (h) $0.53–0.58\mathrm{MJ}/\mathrm{kg}$.