Flat-band plasmons in twisted bilayer transition metal dichalcogenides

TL;DR

This study investigates flat-band plasmons in twisted bilayer MoS2, revealing how lattice relaxations and doping influence plasmon dispersion, with implications for plasmon-mediated superconductivity.

Contribution

It provides a comprehensive analysis of flat-band plasmons considering lattice relaxation and doping effects in twisted bilayer MoS2, highlighting conditions for low-damped plasmon modes.

Findings

Flat-band plasmons are tunable by twist angle and doping.

Lattice relaxations prevent the flat band from isolating, affecting plasmon behavior.

Interband transitions significantly modify plasmon dispersion.

Abstract

Twisted bilayer transition metal dichalcogenides are ideal platforms to study flat-band phenomena. In this paper, we investigate flat-band plasmons in the hole-doped twisted bilayer MoS$_2$ (tb-MoS$_2$) by employing a full tight-binding model and the random phase approximation. When considering lattice relaxations in tb-MoS$_2$, the flat band is not separated from remote valence bands, which makes the contribution of interband transitions in transforming the plasmon dispersion and energy significantly different. In particular, low-damped and quasi-flat plasmons emerge if we only consider intraband transitions in the doped flat band, whereas a $\sqrt q$ plasmon dispersion emerges if we also take into account interband transitions between the flat band and remote bands. Furthermore, the plasmon energies are tunable with twist angles and doping levels. However, in a rigid sample that suffers no lattice relaxations, lower-energy quasi-flat plasmons and higher-energy interband plasmons can coexist. For rigid tb-MoS$_2$ with a high doping level, strongly enhanced interband transitions quench the quasi-flat plasmons. Based on the lattice relaxation and doping effects, we conclude that two conditions, the isolated flat band and a properly hole-doping level, are essential for observing the low-damped and quasi-flat plasmon mode in twisted bilayer transition metal dichalcogenides. We hope that our study on flat-band plasmons can be instructive for studying the possibility of plasmon-mediated superconductivity in twisted bilayer transition metal dichalcogenides in the future.