Resolving the Mechanism of Acoustic Plasmon Instability in Graphene Doped by Alkali Metals

Graphene doped by alkali atoms (AC [Formula: see text]) supports two heavily populated bands ([Formula: see text] and [Formula: see text]) crossing the Fermi level, which enables the formation of two intense two-dimensional plasmons: the Dirac plasmon (DP) and the acoustic plasmon (AP). Although the mechanism of the formation of these plasmons in electrostatically biased graphene or at noble metal surfaces is well known, the mechanism of their formation in alkali-doped graphenes is still not completely understood. We shall demonstrate that two isoelectronic systems, KC [Formula: see text] and CsC [Formula: see text] , support substantially different plasmonic spectra: the KC [Formula: see text] supports a sharp DP and a well-defined AP, while the CsC [Formula: see text] supports a broad DP and does not support an AP at all. We shall demonstrate that the AP in an AC [Formula: see text] is not, as previously believed, just a consequence of the interplay of the [Formula: see text] and [Formula: see text] intraband transitions, but a very subtle interplay between these transitions and the background screening, caused by the out-of-plane interband [Formula: see text] transitions.