Look-alike Landau levels in locally biased twisted bilayer graphene

TL;DR

This paper investigates how localized bias voltages in twisted bilayer graphene affect Landau level formation and Hofstadter butterfly spectra, revealing robustness of Landau levels despite broken translational symmetry.

Contribution

It demonstrates that Landau levels can persist and split into self-similar structures even with spatial potential fluctuations in locally biased twisted bilayer graphene.

Findings

Landau levels can condense outside the biased central region.

A second energy branch appears under same-layer biasing.

Distinct localized states emerge depending on bias configuration.

Abstract

The large lattice constant of Moire superlattice formed in twisted bilayer graphene for small twist enables observing the Landau levels splitting into Hofstadter butterflies in energy spectra for moderate magnetic field. This is expected for generic system under homogeneous bias conditions but its robustness against spatial potential fluctuations is left open question. We study the energy structure of twisted bilayer system in dependence of both, the homogeneous magnetic field and the bias voltage applied exclusively in its central part. Although the translational symmetry is broken, the energy states mainly localized outside the central region may still condense on Landau levels and these would split revealing self-similarity feature. Moreover, besides the generic branch of energy states with zero-mode Landau level at charge neutrality point, when both layers are biased with the same voltage, the second look-alike energy branch shifted upwards can be developed by states largely localized in central region. Otherwise, for counter-biasing of layers, only generic branch exists but with lowest Landau layers flanked by either, hole-like and electron-like states localized at the top or at the bottom layer of central biased part of twisted bilayer system.