3/2 magic-angle quantization rule of flat bands in twisted bilayer graphene and relationship with the Quantum Hall effect

TL;DR

This paper reveals a universal quantization rule for high-order magic angles in twisted bilayer graphene, linking flat band modes to Landau states and the quantum Hall effect through a detailed theoretical analysis.

Contribution

It introduces a new quantization rule for high-order magic angles and establishes a connection between flat band modes and quantum Hall states in twisted bilayer graphene.

Findings

Zero flat band modes form coherent Landau states at high-order magic angles.

The Hamiltonian squared is equivalent to a 2D quantum harmonic oscillator.

A quantization rule for magic angles: 5_{m+1}-5_{m}=3/2.

Abstract

Flat band electronic modes in twisted graphene bilayers are responsible for superconducting and other highly correlated electron-electron phases. Although some hints were known of a possible connection between the quantum Hall effect and zero flat band modes, it was not clear how such connection appears. Here the electronic behavior in twisted bilayer graphene is studied using the chiral model Hamiltonian. As a result, it is proved that for high-order magic angles, the zero flat band modes converge into coherent Landau states with a dispersion $\sigma^2=1/3\alpha$, where $\alpha$ is a coupling parameter that incorporates the twist angle and energetic scales. Then it is proved that the square of the hamiltonian, which is a $2\times 2$ matrix operator, turns out to be equivalent to a two-dimensional quantum harmonic oscillator. The interlayer currents between graphene's bipartite lattices are identified with the angular momentum term while the confinement potential is an effective quadratic potential. From there it is proved a limiting quantization rule for high-order magic angles, i.e., $\alpha_{m+1}-\alpha_{m}=3/2$ where $m$ is the order of the angle. All these results are in very good agreement with numerical calculations.