Karasiev Valentin V, Hu S X
Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623 USA.
Phys Rev E. 2021 Mar;103(3-1):033202. doi: 10.1103/PhysRevE.103.033202.
We present a free-energy density functional theory (DFT)-based methodology for optical property calculations of warm dense matter to cover a wide range of thermodynamic conditions and photon energies including the entire x-ray range. It uses Mermin-Kohn-Sham density functional theory with exchange-correlation (XC) thermal effects taken into account via a fully temperature dependent generalized gradient approximation XC functional. The methodology incorporates a combination of the ab initio molecular dynamics (AIMD) snapshotted Kubo-Greenwood optic data with a single atom in simulation cell calculations to close the photon energy gap between the L and K edges and extend the K-edge tail toward many-keV photon energies. This gap arises in the standard scheme due to a prohibitively large number of bands required for the Kubo-Greenwood calculations with AIMD snapshots. Kubo-Greenwood data on snapshots provide an accurate description of optic properties at low photon frequencies slightly beyond the L edge and x-ray absorption near edges structure (XANES) spectra, while data from periodic calculations with single atoms cover the tail regions beyond the edges. To demonstrate its applicability to mid-Z materials where the standard DFT-based approach is not computationally feasible, we have applied it to opacity calculations of warm dense silicon plasmas. These first-principles calculations revealed a very interesting phenomenon of redshift-to-blueshift in K-L (1s→2p) and K-edge absorptions along both isotherm and isochore, which are absent in most continuum-lowering models of traditional plasma physics. This new physics phenomenon can be attributed to the underlying competition between the screening of deeply bound core electrons and the screening of outer-shell electrons caused by warm-dense-plasma conditions. We further demonstrate that the ratio of 1s→2p to the K-edge x-ray absorptions can be used to characterize warm-dense-plasma conditions. Eventually, based on our absorption calculations, we have established a first-principles opacity table (FPOT) for silicon in a wide range of material densities and temperatures.
我们提出了一种基于自由能密度泛函理论（DFT）的方法，用于计算热密物质的光学性质，以涵盖广泛的热力学条件和光子能量，包括整个X射线范围。该方法采用Mermin-Kohn-Sham密度泛函理论，并通过完全依赖温度的广义梯度近似交换关联（XC）泛函来考虑交换关联（XC）热效应。该方法将从头算分子动力学（AIMD）快照的久保-格林伍德光学数据与模拟单元计算中的单个原子相结合，以弥合L边和K边之间的光子能隙，并将K边尾部扩展到多keV光子能量。由于使用AIMD快照进行久保-格林伍德计算需要大量的能带，在标准方案中会出现这种能隙。快照上的久保-格林伍德数据提供了低光子频率下略高于L边的光学性质以及近边X射线吸收结构（XANES）光谱的准确描述，而单个原子的周期性计算数据则覆盖了边以外的尾部区域。为了证明其对中等原子序数材料的适用性，在基于标准DFT的方法计算上不可行，我们将其应用于热密硅等离子体的不透明度计算。这些第一性原理计算揭示了一个非常有趣的现象，即在等温线和等容线上，K-L（1s→2p）和K边吸收存在红移到蓝移的现象，这在传统等离子体物理学的大多数连续体降低模型中是不存在的。这种新的物理现象可归因于热密等离子体条件下深层束缚的核心电子屏蔽与外层电子屏蔽之间的潜在竞争。我们进一步证明，1s→2p与K边X射线吸收的比值可用于表征热密等离子体条件。最终，基于我们的吸收计算，我们建立了硅在广泛的材料密度和温度范围内的第一性原理不透明度表（FPOT）。
