Flat and Correlated Plasmon Bands in Graphene/${\alpha}$-RuCl${_3}$ Heterostructures.

We develop a microscopic theory for plasmon excitations of graphene/${\alpha}$-RuCl${_3}$ heterostructures. Within a Kondo-Kitaev model with various interactions, a heavy Fermi liquid hosting flat bands emerges in which the itinerant electrons of graphene effectively hybridize with the fractionalized fermions of the Kitaev quantum spin liquid. We find novel correlated plasmon bands induced by the interplay of flat bands and interactions and argue that our theory is consistent with the available experimental data on graphene/${\alpha}$-RuCl${_3}$ heterostructures. We predict novel plasmon branches beyond the long-wavelength limit and discuss the implications for probing correlation phenomena in other flat band systems.