Dipole-active collective excitations in moir\'e flat bands

TL;DR

This paper investigates unique collective plasma excitations in moiré flat bands, revealing their strong dipole activity and potential for optical detection of correlated electronic states.

Contribution

It introduces the concept of over-the-band plasmons in moiré systems and predicts their dipole activity upon charge ordering, a novel insight into moiré plasmon behavior.

Findings

Moiré plasmons pierce through the flat-band continuum.

Charge ordering induces strong dipole activity in high-frequency modes.

Low-frequency modes are gapped within the correlated insulator gap.

Abstract

Collective plasma excitations in moir\'e flat bands display unique properties reflecting strong electron-electron interactions and unusual carrier dynamics in these systems. Unlike the conventional two-dimensional plasmon modes, dispersing as $\sqrt{k}$ at low frequencies and plunging into particle-hole continuum at higher frequencies, the moir\'e plasmons pierce through the flat-band continuum and acquire a strong over-the-band character. Due to the complex structure of the moir\'e superlattice unit cell, the over-the-band plasmons feature several distinct branches connected through zone folding in the superlattice Brillouin zone. Using a toy Hubbard model for the correlated insulating order in a flat band, we predict that these high-frequency modes become strongly dipole-active upon the system undergoing charge ordering, with the low-frequency modes gapped out within the correlated insulator gap. Strong dipole moments and sensitivity to charge order make these modes readily accessible by optical measurements, providing a convenient diagnostic of the correlated states.