Comment on “Isochoric, isobaric, and ultrafast conductivities of aluminum, lithium, and carbon in the warm dense matter regime”

Dharma-wardana et al. [M. W. C. Dharma-wardana et al., Phys. Rev. E 96, 053206 (2017)] recently calculated dynamic electrical conductivities for warm dense matter as well as for nonequilibrium two-temperature states termed “ultrafast matter” (UFM) [M. W. C. Dharma-wardana, Phys. Rev. E 93, 063205 (2016)]. In this Comment we present two evident reasons why these UFM calculations are neither suited to calculate dynamic conductivities nor x-ray Thomson scattering spectra in isochorically heated warm dense aluminum. First, the ion-ion structure factor, a major input into the conductivity and scattering spectra calculations, deviates strongly from that of isochorically heated aluminum. Second, the dynamic conductivity does not show a non-Drude behavior which is an essential prerequisite for a correct description of the absorption behavior in aluminum. Additionally, we clarify misinterpretations by Dharma-wardana et al. concerning the conductivity measurements of Gathers [G. R. Gathers, Int. J. Thermophys. 4, 209 (1983)].

Figure 1

$S_{ii}\left(k\right)$ of aluminum measured at LCLS by Sperling et al. [3] (red squares) and Neumayer et al. [8] (blue circles) within ultrashort 25–50 fs pulses in comparison with predictions of DWD [1] and DFT-MD.

Figure 2

(a) Real part of $\sigma$ in warm dense aluminum at $\rho =2.7\mathrm{g}/{\mathrm{cm}}^3$. The UFM-NPA model [1] does not capture non-Drude behavior in contrast to DFT-MD [4] or AA [20, 21]. (b) Absorption coefficient shows non-Drude behavior in the dielectric function (compare Birken et al. [22], Gullikson et al. [17], and Scott et al. [23]) appearing slightly above the plasma frequency, ${\omega }_{pl}=15.8$ eV (figure taken from [7]).

Figure 3

(a) dc conductivity from DFT-MD simulations [7], the AA model [24, 25], and a mean field calculation [27] compared to the NPA of DWD [2] and experimental results of Milchberg et al. [26]. (b) Comparison of dc conductivities in isobaric aluminum from the NPA model [2] to experimental data. DWD claim good agreement with the Gathers points (red), although NPA predicts more than 20% higher conductivities compared to the isobaric measurements of Gathers [5] (green), Desai et al. [28] (brown), or Grigoriev et al. [29] (purple); cf. text.

Figure 4

Convergence test for DFT-MD simulations of liquid aluminum at solid density. (a) Dynamic conductivity for different $E_{\text{c}}$ and (b) relative deviation to the calculation with $E_{\text{c}}=1000$ eV. (c) dc conductivity convergence test for particle number and k-point sampling. (d) Convergence test of particle numbers for $S_{ii}\left(k\right)$.