Dipole-active collective excitations in moiré flat bands as a probe of the correlated insulating and superconducting states*

Collective plasma excitations in moiré flat bands display unique properties reflecting strong electron-electron interactions and unusual carrier dynamics in these systems. Unlike the conventional 2D plasmons, dispersing as k^1/2 at low frequencies and plunging into particle-hole continuum at higher frequencies, the moiré plasmons pierce through the flat-band continuum and acquire a strong over-the-band character [1,2,3]. Due to the complex structure of the moiré superlattice (SL) unit cell, the over-the-band plasmons feature several distinct branches connected through zone folding in the SL Brillouin zone. Using a toy Hubbard model for the correlated insulating (CI) 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 CI gap. We also predict a similar behavior for the superconducting state. Strong dipole moments and sensitivity to charge order make these modes readily accessible by optical and microwave measurements, offering a convenient diagnostic of these states.
1. Lewandowski, Levitov, PNAS 116, 20869 (2019)
2. Fahimniya, Lewandowski, Levitov, arXiv:2011.02982
3. to be published