Intrinsically undamped plasmon modes in narrow electron bands

Surface plasmons in 2-dimensional electron systems with narrow Bloch bands feature an interesting regime in which Landau damping (dissipation via electron–hole pair excitation) is completely quenched. This surprising behavior is made possible by strong coupling in narrow-band systems characterized by large values of the “fine structure” constant α=e2/ℏκvF. Dissipation quenching occurs when dispersing plasmon modes rise above the particle–hole continuum, extending into the forbidden energy gap that is free from particle–hole excitations. The effect is predicted to be prominent in moiré graphene, where at magic twist-angle values, flat bands feature α≫1. The extinction of Landau damping enhances spatial optical coherence. Speckle-like interference, arising in the presence of disorder scattering, can serve as a telltale signature of undamped plasmons directly accessible in near-field imaging experiments.