Magnetoplasmons in quasi-neutral epitaxial graphene nanoribbons

We report on infrared transmission spectroscopy study of magnetoplasmons in quasi-neutral epitaxial graphene nanoribbon arrays. The energy of the $ L_{0(-1)}\to L_{1(0)}$ inter-Landau level transitions deviates from the characteristic $\sqrt{B}$ dependence observed in two-dimensional graphene. This behavior is explained as a signature of the upper hybrid mode formed between the Landau level transition and the plasmon resonance. Studying the hybrid mode allows us to probe the zero magnetic field plasmon resonance in the interacting regime, when coupling to electron-holes excitations results in strong decay of plasmons. We observe a deviation of the plasmon frequency from the standard $\omega_{pl}\propto q^{1/2}$ dispersion relation, and attribute it to the finite length of the graphene ribbons.