Charge-transfer insulation in twisted bilayer graphene

TL;DR

This paper investigates the electronic structure and interactions in twisted bilayer graphene at the magic angle, revealing charge transfer effects, magnetic properties, and potential superconducting pairing symmetries.

Contribution

It introduces an effective model including more than four flat bands and analyzes the impact of Coulomb interactions on charge transfer and pairing symmetries.

Findings

Charge transfer from 'center' to 'ring' orbitals at half-filling.

Estimated Heisenberg coupling J = 3.3 K, matching experiments.

Dopant-dependent pairing symmetries: p+ip for holes, d+id for electrons.

Abstract

We studied the real space structure of states in twisted bilayer graphene at the `magic angle' $\theta = 1.08^\circ$. The flat bands close to charge neutrality are composed of a mix of `ring' and `center' orbitals around the AA stacking region. An effective model with localized orbitals is constructed, which necessarily includes more than just the four flat bands. Long-range Coulomb interaction causes a charge-transfer at half-filling of the flat bands from the `center' to the `ring' orbitals. Consequently, the Mott phase is a featureless spin-singlet paramagnet. We estimate the effective Heisenberg coupling that favors the singlet coupling to be $J = 3.3$ K, consistent with experimental values. The superconducting state depends on the nature of the dopants: hole-doping yields $p+ip$-wave whereas electron-doping yields $d+id$-wave pairing symmetry.