Floquet-engineering topological transitions in a twisted transition metal dichalcogenide homobilayer

TL;DR

This paper explores how different monochromatic light sources can induce topological phase transitions in twisted transition metal dichalcogenide bilayers, revealing unique effects of circularly polarized and longitudinal light on the system's electronic properties.

Contribution

It demonstrates that longitudinal light can tune the twist angle and induce topological transitions in twisted TMD bilayers, unlike circularly polarized light which only shifts quasi-energy.

Findings

Circularly polarized light shifts quasi-energy without creating new terms.

Longitudinal light renormalizes tunneling, enabling in-situ twist angle tuning.

Strong drives cause band gap closings unrelated to topological transitions.

Abstract

Motivated by the recent experimental realization of twisted transition metal dichalcogenide bilayers, we study a simplified model driven by different forms of monochromatic light. As a concrete and representative example we use parameters that correspond to a twisted MoTe$_2$ homobilayer. First, we consider irradiation with circularly polarized light in free space and demonstrate that the corresponding Floquet Hamiltonian takes the same form as the static Hamiltonian, only with a constant overall shift in quasi-energy. This is in stark contrast to twisted bilayer graphene, where new terms are typically generated under an analagous drive. Longitudinal light, on the other hand, which can be generated from the transverse magnetic mode in a waveguide, has a much more dramatic effect--it renormalizes the tunneling strength between the layers, which effectively permits the tuning of the twist angle {\em in-situ}. We find that, by varying the frequency and amplitude of the drive, one can induce a topological transition that cannot be obtained with the traditional form of the Floquet drive in free space. Furthermore, we find that strong drives can have a profound effect on the layer pseudospin texture of the twisted system, which coincides with multiple simultaneous band gap closings in the infinite-frequency limit. Surprisingly, these bandgap closings are not associated with topological transitions. For high but finite drive frequencies near $0.7$eV, the infinite-frequency band crossings become band gap minima of the order of $10^{-6}$ eV or smaller.