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 (ACx) supports strong Dirac and weak acoustic plasmons. Here we show that when ACx 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 ACx layer from the metal surface recovers the ACx 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.