Strong acoustic plasmons in chemically doped graphene induced by a nearby metal surface

Recent theoretical considerations have demonstrated that free-standing graphene doped with alkali metals ($A{\mathrm{C}}_x$) supports strong Dirac and weak acoustic plasmons. Here we show that when $A{\mathrm{C}}_x$ is deposited on a metal surface the effective Coulomb screening produced by the substrate can completely wash out these collective modes. However, even tiny detachment of the $A{\mathrm{C}}_x$ layer from the metal surface recovers the $A{\mathrm{C}}_x$ plasmonic properties. Especially the acoustic plasmon intensity is strongly enhanced in comparison to a free-standing case. We further provide the physical background of these intriguing phenomena. The studied systems consist of lithium- and cesium-doped graphene layers deposited on the Ir(111) surface.