Excitons on a microscopic level: The mixed dynamic structure factor

Abstract

The dynamic structure factor of materials is proportional to their linear electronic response and it displays their excitation spectra. Usually the response is measured on the same length scale as the perturbation. Here, we illustrate that much can be gained by studying also the mixed dynamic structure factor, which connects different spatial components of perturbation and response. We extend state-of-the-art ab initio calculations to access the mixed dynamic structure factor, including excitonic effects. Using bulk silicon and lithium fluoride as prototype examples, we show that these effects play a crucial role above and below the quasiparticle gap, and are needed in order to explain coherent inelastic x-ray scattering experiments. Our approach also yields important information concerning the microscopic structure of time-dependent density functional theory.

Design of auxiliary systems for spectroscopy

“…excitonic) effects, f abs (ω) also has to account for the band-gap opening from the LDA to the GWA. As also previously found 70,71 , both kernels are strongly frequency dependent. In the low-energy region (before strong oscillations start to develop in correspondence to poles of the response functions), the signs of the two kernels f abs (ω) and f abs,mb (ω) are opposite: they effectively shift the weight of the absorption spectra to larger or smaller energies, respectively.…”

Low-cost alternatives to the Bethe-Salpeter equation: Towards simple hybrid functionals for excitonic effects in solids

“…For most practical BSE calculations, a widely used treatment is to ignore the frequency dependence of the dielectric function and use ε −1 G,G (q, ω = 0) [4,5,61]. However, the ωdependent dynamic effects of the dielectric screening may have a non-negligible influence on the electron dynamics [62,63], which can be estimated by methods such as generalized plasmon-pole models [64][65][66]. In the present work we use the static approximation throughout and refer to this as the standard BSE approach or simply as the BSE.…”

Dynamic Structure Factor and Dielectric Function of Valence Electrons in Lithium Hydride: An Inelastic X‐Ray Scattering Study at Finite Momentum Transfer

“…Approximations based on the BSE of MBPT have become the state‐of‐the‐art method to calculate the dielectric function of semiconductors and insulators . The BSE is an in‐principle exact Dyson equation for the two‐particle correlation function, from which can be inferred.…”

“…[3,[32][33][34] Approximations based on the BSE of MBPT have become the state-of-the-art method to calculate the dielectric function of semiconductors and insulators. [1][2][3][18][19][20]22,26,35] The BSE is an in-principle exact Dyson equation for the two-particle correlation function, from which ϵ M ðq, ωÞ can be inferred. But in calculations of real materials, approximations have to be invoked.…”