Enhanced excitonic effects in the energy loss spectra of LiF and Ar at large momentum transfer

TL;DR

This study demonstrates that excitonic effects in LiF and Ar are significantly enhanced at large momentum transfer, influenced by exciton interactions with high-energy electron-hole transitions, using the bootstrap kernel approach.

Contribution

The paper introduces the application of the bootstrap kernel to analyze excitonic effects at finite momentum transfer, highlighting the matrix character's importance and the enhancement of excitonic effects in LiF and Ar.

Findings

Excitonic effects are enhanced in LiF and Ar at large ${f q}$.

The bootstrap kernel effectively captures the ${f q}$ dependence of spectra.

Interactions with high-energy transitions cause the excitonic enhancement.

Abstract

It is demonstrated that the bootstrap kernel [\onlinecite{sharma11}] for finite values of ${\bf q}$ crucially depends upon the matrix character of the kernel and gives results of the same good quality as in the ${\bf q} \rightarrow 0$ limit. The bootstrap kernel is further used to study the electron loss as well as absorption spectra for Si, LiF and Ar for various values of ${\bf q}$. The results show that the excitonic effects in LiF and Ar are enhanced for values of ${\bf q}$ away from the $\Gamma$-point. The reason for this enhancement is the interaction between the exciton and high energy inter-band electron-hole transitions. This fact is validated by calculating the absorption spectra under the influence of an external electric field. The electron energy loss spectra is shown to change dramatically as a function of ${\bf q}$.