VALENCE ELECTRON EXCITATIONS IN LiH INVESTIGATED BY IXS SPECTROSCOPY

S.A. CEPPI; KOSKELO, JAAKKO; O.A. PAREDES MELLONE; G.E. STUTZ

Congreso; 30th Annual Users Meeting LNLS/CNPEM; 2020

The simplicity of LiH regarding crystal structure and electronic configuration hasestablished this compound as an ideal system for studying electronic structure andelectron excitation in condensed matter while probing different theoretical approaches.The dielectric response of electrons, due to an external perturbation, can be probed bymeans of inelastic x-ray scattering (IXS) spectroscopy. The information about the excitedelectron system is obtained by measuring the energy-loss spectrum of scattered hard xrays,while transferring energy ω and momentum q to the sample. In the present workthe effects of electron-hole interaction in the dynamic structure factor S(q,ω) and in thecomplex dielectric function ε(q,ω) of valence electrons in lithium hydride at finitemomentum transfer were investigated by means of IXS spectroscopy and ab initiotheoretical methods. Experiments were carried out at the XDS beamline of the BrazilianSynchrotron Light Laboratory (LNLS). Calculations of S(q,ω) and ε(q,ω) were performedwithin time-dependent density-functional theory (TDDFT) in the the adiabatic localdensity approximation (ALDA) and many-body perturbation theory (MBPT) based on theBethe-Salpeter equation (BSE). Our findings reveal that for low-q an explicit treatmentof electron-hole interactions by using the BSE formalism slightly modify low-energystructures in S(q,ω) in comparison to ALDA results, but affects strongly the macroscopicdielectric functions. A very good agreement between experimental and theoreticalS(q ,ω) and ε(q,ω) in all the range of investigated q values was achieved for calculationsbased on BSE after taking into account the full excitonic Hamiltonian. Present results [1]demonstrate the potential of approximations based on the Bethe-Salpeter equation toaccurately describe the valence excitation spectra, including the near-onset region, andthe dielectric response in insulating systems where excitonic effects are relevant.