Floquet engineering of twisted double bilayer graphene

TL;DR

This paper investigates how circularly polarized light, both in free space and confined in a waveguide, can be used to tune the electronic band structure and topological properties of twisted double bilayer graphene, revealing new ways to control its electronic phases.

Contribution

It introduces two Floquet protocols for tuning TDBG properties, providing analytical and numerical insights into their effects on band gaps and topology, including flat band generation.

Findings

Selective gap closing and opening in TDBG with different stacking configurations.

Generation of flat bands through strong Floquet drives.

Bandwidth engineering of quasienergy bands without symmetry breaking.

Abstract

Motivated by the recent experimental realization of twisted double bilayer graphene (TDBG) samples we study, both analytically and numerically, the effects of circularly polarized light propagating in free space and confined into a waveguide on the band structure and topological properties of these systems. These two complementary Floquet protocols allow us to selectively tune different parameters of the system by varying the intensity and the frequency of the light. For the drive protocol in free space, in the high-frequency regime, we find that in TDBG with AB/BA stacking, we can selectively close the zone-center quasienergy gaps around one valley while increasing the gaps near the opposite valley by tuning the parameters of the drive. In TDBG with AB/AB stacking, a similar effect can be obtained upon the application of a perpendicular static electric field. Furthermore, we study the topological properties of the driven system in different settings, provide accurate effective Floquet Hamiltonians, and show that relatively strong drives can generate flat bands. On the other hand, longitudinal light confined into a waveguide couples to the components of the interlayer hopping that are perpendicular to the TDBG sheet, allowing for selective engineering of the bandwidth of Floquet zone center quasienergy bands without breaking the symmetries of the static system.