Electron-hole crossover in gate-controlled bilayer graphene quantum dots

TL;DR

This paper demonstrates the tunable electron-hole crossover in bilayer graphene quantum dots, revealing opposite orbital magnetic moments and valley polarization, with implications for valley and spin-valley qubits.

Contribution

It provides experimental evidence of electron-hole crossover and orbital magnetic moments in bilayer graphene QDs, supported by tight-binding calculations.

Findings

Opposite sign of orbital magnetic moments for electrons and holes.

Controlled tunneling barriers and occupation crossing the band gap.

Valley g-factor around 17 indicating valley polarization.

Abstract

Electron and hole Bloch states in gapped bilayer graphene exhibit topological orbital magnetic moments with opposite signs near the band edges, which allows for tunable valley-polarization in an out-of-plane magnetic field. This intrinsic property makes electron and hole quantum dots (QDs) in bilayer graphene interesting for valley and spin-valley qubits. Here we show measurements of the electron-hole crossover in a bilayer graphene QD, demonstrating the opposite sign of the orbital magnetic moments associated with the Berry curvature. Using three layers of metallic top gates, we independently control the tunneling barriers of the QD while tuning the occupation from the few-hole regime to the few-electron regime, crossing the displacement-field controlled band gap. The band gap is around 25 meV, while the charging energies of the electron and hole dots are between 3-5 meV. The extracted valley g-factor is around 17 and leads to opposite valley polarization for electron and hole states at moderate B-fields. Our measurements agree well with tight-binding calculations for our device.