Flat-band plasmons in twisted bilayer transition metal dichalcogenides: twist angle, relaxation and band cutoff effects

Abstract

Twisted bilayer transition metal dichalcogenides are ideal platforms to study flat-band phenomena. In this paper, we investigate flat-band plasmons in doped twisted bilayer MoS2 by employing a full tight-binding model and the random phase approximation. Twist-angle effects on flat-band plasmons in twisted bilayer MoS2 show that plasmons at a smaller angle exhibit lower energy due to a flatter band, which is similar to the twisted bilayer graphene case. Besides, effects of band cutoff and doping on plasmons are studied. We find that, in relaxed samples, low-damped and quasi-flat plasmons emerge only in one-band calculation where merely intraband transitions in the doped flat band are considered, whereas classical plasmons of two dimensional electron gas emerge in multi-band calculations. In rigid samples that possess isolated flat bands, classical and quasi-flat plasmons appear in both slightly-doped and highly-doped cases. So one-band calculation is only accurate in system with isolated flat bands. Especially, isolation of the flat band from its adjacent bands is essential to observe a low-damped and quasi-flat plasmon mode. On the contrary, for relaxed samples, contribution of interband transitions between flat band and other bands to plasmons makes a significant influence in transforming plasmons dispersion and energy. We hope that our study on flat-band plasmons can be instructive for studying other twisted systems with flat bands.