Lattice-induced double peak in the dielectric response of Be metal

Abstract
Measuring the inelastic scattering of synchrotron x rays by Be metal we found a pronounced double peak in the electronic dynamic structure factor S(q,ω) for q parallel [110]. We attribute the double peak to the existence of an excitation gap for final states on the (110) Bragg plane. For q=0.77 a.u. a zone-boundary collective state could be identified by means of a Kramers-Kronigdeduced zero of Reε(q,ω). Applications to band analysis are discussed.