Excitonic condensation and metal-semiconductor transition in AA bilayer graphene in the external magnetic field

TL;DR

This study investigates how an external magnetic field influences excitonic properties and phase transitions in AA-stacked bilayer graphene, revealing a metal-semiconductor transition, excitonic effects, and spin-dependent transport phenomena.

Contribution

The paper provides a detailed analysis of magnetic field effects on excitonic pairing, electronic structure, and phase transitions in AA-stacked bilayer graphene using the bilayer Hubbard model.

Findings

Magnetic field induces a metal-semiconductor transition in AA-BLG.

Excitonic order parameters depend on spin orientation and magnetic field.

High magnetic fields lead to band gaps similar to conventional semiconductors.

Abstract

In this paper, the effects of the external transverse magnetic field $B$ (perpendicular to the surface of the layers) on the electronic and excitonic properties are studied in the AA-stacked bilayer graphene (BLG). The effects of the Coulomb interactions and excitonic pairing have been taken into account and analyzed in detail within the bilayer Hubbard model. Both half-filling and partial filling regimes have been taken into account and the magnetic field dependence of a series of physical parameters was found. It is shown that the difference between the average electron concentrations in the layers vanishes at some critical value of magnetic field $B_{c}$ and the chemical potential is calculated numerically above and below that value. The role of the Coulomb interactions on the average carrier concentrations in the layers has been analyzed, and the excitonic order parameters have been calculated for different spin orientations. We found a possibility for the particle population inversion between the layers when varying the external magnetic field. The calculated electronic band structure in the AA-BLG shows the presence of metal-semiconductor transition, governed by the strength of the applied magnetic field or the interlayer interaction potential. We show that for high magnetic fields the band-gap is approaching the typical values of the gaps in the usual semiconductors. It is demonstrated that, at some parameter regimes, the AA-BLG behaves like a spin-valve device, by permitting the electron transport with only one spin direction. All calculations have been performed at the zero-temperature limit.