Electronic collective excitations in compressed lithium from ab initio calculations: Importance and anisotropy of local-field effects at large momenta

Abstract

According to our ab initio calculations, the recently reported undamped plasmon in compressed fcc lithium emerges not only due to the fact that the dielectric matrix determinant vanishes at long wavelengths at the plasmon energy, i.e., satisfying the ideal condition for an undamped plasmon in a crystal but also extends at large momenta specially along the ΓL direction. Different from the case of simple metals, the local-field effects dominate, leading to the striking periodicity exhibited by the low-energy plasmon for momentum transfers beyond the first Brillouin zone. Remarkably, pressure-induced electronic anisotropy and localization increase the impact of local-field effects on the electronic response even in the compact fcc structure. Interestingly, the importance of local-field effects is similar in lithium under pressure and in materials with confined geometries such as MgB2.