Emergence of flat bands in the quasicrystal limit of boron nitride twisted bilayers

TL;DR

This study reveals that near 30-degree twisted boron nitride bilayers develop universal flat bands and strong excitonic effects, independent of stacking, due to moiré-induced K-valley scattering, with implications for other 2D materials.

Contribution

The paper introduces a simple triangular model to explain the emergence of flat bands in 30-degree twisted bilayers, validated against DFT, highlighting stacking-independent electronic features.

Findings

Flat bands appear above the conduction minimum near 30° twist.

These flat bands cause intense optical absorption peaks.

The phenomenon is due to moiré-induced K-valley scattering.

Abstract

We investigate the electronic structure and the optical absorption onset of close-to-30\degree twisted hexagonal boron nitride bilayers. Our study is carried out with a purposely developed tight-binding model validated against DFT simulations. We demonstrate that approaching 30\degree (quasicrystal limit), all bilayers sharing the same moir\'e supercell develop identical band structures, irrespective of their stacking sequence. This band structure features a bundle of flat bands laying slightly above the bottom conduction state which is responsible for an intense peak at the onset of the absorption spectrum. These results suggest the presence of strong, stable and stacking-independent excitons in boron nitride 30\degree-twisted bilayers. By carefully analyzing the electronic structure and its spatial distribution, we elucidate the origin of these states as moir\'e-induced K-valley scattering due to interlayer B$-$B coupling. We take advantage of the the physical transparency of the tight-binding parameters to derive a simple triangular model based on the B sublattice that accurately describes the emergence of the bundle. Being our conclusions very general, we predict that a similar bundle should emerge in other close-to-30{\degree} bilayers, like transition metal dichalcogenides, shedding new light on the unique potential of 2D materials.