Collective modes in narrow electron bands: over-the-band plasmon excitations and unconventional superconductivity in moire graphene

Plasmon modes in moire graphene, as a result of strong interactions, can pierce through the particle-hole continuum of the flat band, reimerge above it and acquire strong over-the-band character [1]. We will discuss various implications of this interesting behavior. One is the opportunity to realize plasmons that are not subject to Landau damping. Eliminating damping is central for the ongoing quest for low-loss plasmons and dissipationless light–matter coupling. These modes feature enhanced optical coherence and spatial interference, directly testable by state-of-the-art near-field techniques. Another implication is the appearance of novel optical plasmon excitations [2]. These modes are uniquely sensitive to the correlated insulating order in a flat band, becoming strongly dipole-active and developing a gap within the correlated insulator gap. Strong dipole moments and sensitivity to charge order make these modes readily accessible by optical and microwave measurements, offering a convenient diagnostic of the correlated states. Lastly, we will consider superconductivity enabled by these excitations [3]. The new pairing mechanism relies on the high-energy degrees of freedom, the far-out pairs in the empty band positioned above the conduction band, and is fully repulsion-dominated.

1. Cyprian K Lewandowski and LL, PNAS 116, 20869 (2019)
2. Ali Fahimniya, Cyprian K Lewandowski and LL, arXiv:2011.02982
3. Zhiyu Dong and LL, to be published