Abstract
We propose a tunable optical setup to engineer topologically nontrivial flat bands in twisted bilayer graphene under circularly polarized light. Using both analytical and numerical calculations, we demonstrate that nearly flat bands can be engineered at small twist angles near the magic angles of the static system. The flatness and the gaps between these bands can be tuned optically by varying laser frequency and amplitude. We study the effects of interlayer hopping variations on Floquet flat bands and find that changes associated with lattice relaxation favor their formation. Furthermore, we find that, once formed, the flat bands carry nonzero Chern numbers. We show that at currently known values of parameters, such topological flat bands can be realized using circularly polarized UV laser light. Thus, our work opens the way to creating optically tunable, strongly correlated topological phases of electrons in moiré superlattices.Received 21 October 2019Revised 5 May 2020Accepted 2 November 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.043275Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasChern insulatorsFractional quantum Hall effectPhysical SystemsFloquet systemsGrapheneTopological materialsCondensed Matter, Materials & Applied Physics
Highlights
Despite the simplicity of its structure, graphene and its multilayers have been proven to support a remarkable diversity of electronic behaviors [1,2,3]
While there has been significant progress, a complete understanding of the physical origin of the magic angle flatness remains elusive [44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59] In particular, magic angles are obtained at mechanically fine-tuned values, which cannot be changed once a sample is prepared [60]
We demonstrate that twisted bilayer graphene (TBG) as a platform for correlated electronic states is enriched by circularly polarized light impinging on the system
Summary
Despite the simplicity of its structure, graphene and its multilayers have been proven to support a remarkable diversity of electronic behaviors [1,2,3]. The potential for these systems to support extraordinarily flat bands at “magic” twist angles [9,10,22] has been verified experimentally, and the system demonstrated to display interaction physics in the forms of Mott insulating behavior and superconductivity [23,24] The implications of this single-particle structure for collective electron states is an area of intense investigation [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]. This combination of properties—extreme flatness, gaps, and nonvanishing Chern numbers—makes these bands near-perfect analogs of Landau levels, the energy spectrum of two-dimensional electrons in a static magnetic field This offers the potential that electrons in this setting can support states akin to those found in the fractional quantized Hall effect [61,62], stabilized by repulsive interactions among the electrons. TBG is special in that the large size of the moiré unit cell allows the flat Chern band to emerge at relatively low excitation energies relative to that needed to induce topological behavior in more microscopic honeycomb lattices such as single layer graphene [64,65]. (This possibility was recently explored at larger twist angles [66], where no flat bands were observed.) our work shows that irradiated TBG represents an extremely attractive setting to search for fractional Chern insulators and other correlated electron states
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