Collective Modes in Multilayer Graphene/α-RuCl3 Heterostructures

Collective modes in multilayer graphene, such as plasmons and phonons, exhibit sensitivity to displacement fields and interlayer coupling, distinguishing them from their counterparts in single-layer graphene. Here, we engineer collective modes in charge-Transfer heterostructures composed of multilayer graphene and α-RuCl3. In heterostructures with a single α-RuCl3 interface, the charge transfer generates displacement fields up to 7 V/nm at the interface between α-RuCl3 and the adjacent graphene layer-the highest value achieved through charge-Transfer methods. As a result of the broken inversion symmetry, we discover enhanced nonlinear optical response and modified phonon selection rules. Conversely, we find that multilayer graphene sandwiched between two α-RuCl3 flakes causes displacement fields to cancel. There, we achieve carrier densities as high as 8×1013 cm-2 in multilayer graphene and restore the phonon selection rules to their unperturbed state. Meanwhile, we demonstrate that plasmonic properties derive from the depletion of multiple valence bands. As a result of the quasilinear band dispersion, these "Dirac multiband plasmons"are relatively unaffected by displacement fields. On the other hand, the inverted heterostructure sequence-two multilayer graphene sheets encapsulating α-RuCl3-activates significant alteration of the plasmons via interlayer plasmon-plasmon coupling. Hence, multilayer graphene and α-RuCl3 heterostructures offer a gate-free platform for engineering collective modes derived from inversion symmetry and interlayer coupling.