Optical properties of calcium under pressure from first-principles calculations

Abstract

Here we present theoretical ab initio calculations based on time-dependent density-functional theory of the macroscopic dielectric function of calcium from 0 to 110 GPa in the fcc, bcc, and sc phases. All the dielectric functions are calculated using very fine reciprocal space meshes since both eigenvalues and matrix elements entering in the density-response functions are interpolated using Wannier functions. Our calculations correctly predict the energies of the low-and high-energy plasmons observed in the fcc phase at room pressure. Moreover, we predict that in the sc phase a very low-energy interband plasmon emerges at around 50 GPa that dramatically modifies the behavior of the reflectivity making it almost vanish at the plasmon energy. As a consequence, calcium becomes an example of how optical properties can become complex under pressure.

Ab initioanalysis of plasmon dispersion in sodium under pressure

“…This figure reveals that the anomalous behavior of R(ω) in the optical range originates from a low-energy plasmon emerging at around 0.25 eV. This plasmon shares common features with the one theoretically predicted in calcium under pressure 12 , which also induces a dip in the calculated reflectivity. It is essentially undamped since both Im ˆ q (ω) 00 and Re ˆ q (ω) 00 become almost zero at ω 0.25 eV, making the plasmon linewidth vanishingly small at this energy.…”

“…In particular, the high and uniform reflectivity (characteristic of good metals) of the bcc, fcc, and cI16 phases has been measured to drop drastically in the high pressure phases oP8 and tI19, accompanied by a decrease of the metallic character [4]. The origin of such behavior may lie on the emergence of low-energy interband plasmons arising from the increasing localization of the valence electrons [10], as it is the case of Li and Ca [11,12].…”

“…3 The origin of such behavior may lie on the emergence of low-energy interband plasmons arising from the increasing localization of the valence electrons, 10 as it is the case of Li and Ca. 11,12 In this paper, we perform a detailed ab-initio analysis of the electronic properties of Na in a pressure domain ranging from 0 to 180 GPa, covering all the metallic phases of sodium at room temperature. In the spirit of Ref.…”

Transmittance and optical constants of Ca films in the 4–1000 eV spectral range

“…Regarding this k minimum over the literature, the energy for the mentioned absorption minimum below the M 2;3 edge is only reached by Potter and Green, but their minimum is shifted toward smaller energies and is much wider. Langkowski [33] calculated Ca optical constants from EELS data (not plotted in Figs. 2 and 3), but the referred minimum is also shifted to smaller energies and to much larger k values.…”

“…Lopez-Rios and Sommers [32] calculated ε 2 of Ca in the 2-9 eV range. More recently, Errea et al [33] performed theoretical ab initio calculations of the dielectric function of fcc Ca (along with Ca under pressure) in the 0-15 eV spectral range, although the plotted data is difficult to read above ∼9 eV.…”