Valence electron excitations in LiH investigated by IXS spectroscopy

Full text: The simplicity of LiH regarding crystal structure and electronic configuration has established this compound as an ideal system for studying electronic structure and electron excitation in condensed matter while probing different theoretical approaches. The dielectric response of electrons, due to an external perturbation, can be probed by means of inelastic x-ray scattering (IXS) spectroscopy. The information about the excited electron system is obtained by measuring the energy-loss spectrum of scattered hard x-rays, while transferring energy ω and momentum q to the sample. In the present work the effects of electron-hole interaction in the dynamic structure factor S(q,ω) and in the complex dielectric function ε(q,ω) of valence electrons in lithium hydride at finite momentum transfer were investigated by means of IXS spectroscopy and ab initio theoretical methods. Experiments were carried out at the XDS beamline of the Brazilian Synchrotron Light Laboratory (LNLS). Calculations of S(q,ω) and ε(q,ω) were performed within time-dependent density-functional theory (TDDFT) in the the adiabatic local density approximation (ALDA) and many-body perturbation theory (MBPT) based on the Bethe-Salpeter equation (BSE). Our findings reveal that for low-q an explicit treatment of electron-hole interactions by using the BSE formalism slightly modify low-energy structures in S(q,ω) in comparison to ALDA results, but affects strongly the macroscopic dielectric functions. A very good agreement between experimental and theoretical S(q ,ω) and ε(q,ω) in all the range of investigated q values was achieved for calculations based on BSE after taking into account the full excitonic Hamiltonian. Present results demonstrate the potential of approximations based on the Bethe-Salpeter equation to accurately describe the valence excitation spectra, including the near-onset region, and the dielectric response in insulating systems where excitonic effects are relevant. (author)