Interplay between many-body effects and charge transfers in Cr2AlC bulk plasmon excitation

The bulk plasmon excitation anisotropy of Cr2AlC, a nanolaminated ternary carbide, is investigated by electron energy-loss spectroscopy (EELS) and ab initio calculations. Depending on the crystallographic orientation, the valence EELS signal is dominated either by a single plasmon or split into a superposition of two independent plasmons with mutually orthogonal momentum transfers. This splitting arises from the electron-hole interaction anisotropy, important along the hexagonal structure c axis and negligible within the basal plane. It is shown that within the basal plane, the plasmon behavior can be reproduced from an effective medium theory considering Cr2AlC as an atomic scale superlattice built from pure aluminum and CrC planes.