Externally controlled band gap in twisted bilayer graphene

TL;DR

This paper theoretically demonstrates that applying a transverse electric field to twisted bilayer graphene with a certain twist angle induces an exciton band gap, controllable by voltage, with potential for nanoscale device applications.

Contribution

It reveals how electron-electron interactions and external electric fields can tune the band gap in twisted bilayer graphene, introducing a new method for electronic control.

Findings

Bias voltage induces a tunable exciton band gap.

Band structure exhibits perfect nesting leading to instability.

Gap depends on twist angle and electric field.

Abstract

We theoretically study the effects of electron-electron interaction in twisted bilayer graphene in applied transverse dc electric field. When the twist angle is not very small, the electronic spectrum of the bilayer consists of four Dirac cones inherited from each graphene layer. Applied bias voltage leads to the appearance of two hole-like and two electron-like Fermi surface sheets with perfect nesting among electron and hole components. Such a band structure is unstable with respect to exciton band gap opening due to the screened Coulomb interaction. The exciton order parameter is accompanied by the spin-density-wave order. The value of the gap depends on the twist angle. More importantly, it can be controlled by applied bias voltage which opens new directions in manufacturing of different nanoscale devices.