Coupled pair of one- and two-dimensional magnetoplasmons on electrons on helium

Electrons on the liquid helium surface form an extremely clean two-dimensional system where different plasmon excitations can coexist. Under a magnetic field, time-reversal symmetry is broken, and all the bulk magnetoplasmons become gapped at frequencies below cyclotron resonance while chiral one-dimensional edge magnetoplasmons appear at the system perimeter. We theoretically show that the presence of a homogeneous density gradient in the electron gas leads to the formation of a delocalized magnetoplasmon mode in the same frequency range as the lowest-frequency edge-magnetoplasmon mode. We experimentally confirm its existence by measuring the corresponding resonance peak in frequency dependence of the admittance of the electron gas. This allows us to realize a prototype system to investigate the coupling between a chiral one-dimensional mode and a single delocalized bulk mode. Such a model system can be important for the understanding of transport properties of topological materials where states of different dimensionality can coexist.