# Intrinsically Undamped Plasmon Modes in Narrow Electron Bands

## TL;DR

This paper predicts that in narrow-band 2D electron systems like moiré graphene, plasmon modes can be intrinsically undamped due to strong coupling effects, leading to enhanced optical coherence and observable interference patterns.

## Contribution

It introduces the concept of intrinsically undamped plasmon modes in narrow electron bands, highlighting their emergence when plasmon dispersion exceeds the particle-hole continuum.

## Key findings

- Landau damping is quenched in narrow-band systems with large coupling constants.
- Undamped plasmon modes extend into the energy gap, avoiding electron-hole pair excitation.
- Enhanced optical coherence and interference signatures are predicted in moiré graphene at magic angles.

## Abstract

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 $\alpha=e^2/\hbar \kappa v_{\rm F}$. 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 moir\'e graphene, where at magic twist-angle values, flat bands feature $\alpha\gg1$. 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.