Inducing topological flat bands in bilayer graphene with electric and magnetic superlattices

TL;DR

This paper investigates how applying magnetic and electric superlattices to bilayer graphene can induce topological flat bands, revealing new ways to engineer quantum states with high Chern numbers.

Contribution

It introduces the concept of magnetic superlattices acting on bilayer graphene and explores their combined effects with electric superlattices to create flat Chern bands.

Findings

Magnetic superlattices alone can induce topological flat bands.

Combined magnetic and electric superlattices produce bands with high Chern numbers.

Proposed experimental setup using vortex lattices and magnetoelectric materials to realize these effects.

Abstract

It was recently argued that Bernal stacked bilayer graphene (BLG) exposed to a 2D superlattice (SL) potential exhibits a variety of intriguing behaviors [Ghorashi et al., Phys. Rev. Lett. 130, 196201 (2023)]. Chief among them is the appearance of flat Chern bands that are favorable to the appearance of fractional Chern insulator states. Here, we explore the application of spatially periodic out-of-plane orbital magnetic fields to the model of Ghorashi et al. to find additional means of inducing flat Chern bands. We focus on fields that vary on length scales much larger than the atomic spacing in BLG, generating what we refer to as magnetic SLs. The magnetic SLs we investigate either introduce no net magnetic flux to the SL unit cell, or a single quantum of flux. We find that magnetic SLs acting on their own can induce topological flat bands, but richer behavior, such as the appearance of flat and generic bands with high Chern numbers, can be observed when the magnetic SLs act in conjunction with commensurate electric SLs. Finally, we propose a method of generating unit-flux-quantum magnetic SLs along with concomitant electric SLs. The magnetic SL is generated by periodic arrays of flux vortices originating from type II superconductors, while the electric SL arises due to a magnetic SL-induced charge density on the surface of a magnetoelectric material. Tuning the vortex lattice and the magnetoelectric coupling permits control of both SLs, and we study their effects on the band structure of BLG.