Beyond the random phase approximation (RPA): First principles calculation of the valence EELS spectrum for KBr including local field, quasiparticle, excitonic and spin orbit coupling effects

• Published in: Ultramicroscopy 2025-01, Vol.268, p.114070, Article 114070
• Main Author: Keast, V.J.
• Format: Article
• Language: English
• Subjects: Dielectric function; Electron-energy loss spectroscopy; Excitons; Local field effects; Quasiparticles
• ISSN: 0304-3991; 1879-2723; 1879-2723
• DOI: 10.1016/j.ultramic.2024.114070

•Calculation of valence EELS for KBr including quasiparticle and excitonic effects.•Spin orbit coupling explains splitting of the excitonic peak.•Review of the complex theory behind computational methods.•Practical guidance on performing calculations. The low energy region (< 50 eV) of the electron energy loss spectrum (EELS) can contain a great deal of spectral detail associated with excitations of the valence electrons. Calculation of the spectra from first principles can assist with interpretation and the most widely used method is the random phase approximation (RPA), usually neglecting local field effects (LFE). For KBr this approach is insufficient due to the importance of quasiparticle and excitonic effects. Calculations including these multi-electron effects are shown to give much improved agreement with the experimental spectra, and the inclusion of spin-orbit coupling (SOC) reproduces the excitonic doublet just above band-edge onset. A review of the complex theory behind these methods is given along with practical guidance on performing these calculations.