Observation of new plasmons in the fractional quantum Hall effect: interplay of topological and nematic orders(*)*

Collective modes of exotic quantum fluids reveal underlying physical mechanisms responsible for emerging ground states. We observe unexpected collective modes in the fractional quantum Hall (FQH) regime of the second Landau level (LL): intra-Landau-level plasmons measured by resonant inelastic light scattering. The plasmons herald nematic phases in the second LL and uncover the nature of long-range translational invariance in these phases. The fascinating dependence of plasmon features on filling factor provides new insights on interplays between topological quantum Hall order and nematic electronic liquid crystal phases. At LL filling factor v = 7/3, a sharp and strong plasmon peak that links to emerging macroscopic coherence supports the proposed model of a FQH nematic state at this filling factor. A marked intensity minimum in the plasmon spectrum at v = 5/2 strongly suggests that the paired state overwhelms competing nematic phases, unveiling the robustness of the superfluid state.