Electron–Hole Crossover in Gate-Controlled Bilayer Graphene Quantum Dots

Abstract

Electron and hole Bloch states in bilayer graphene exhibit topological orbital magnetic moments with opposite signs, which allows for tunable valley-polarization in an out-of-plane magnetic field. This 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 opposite signs of the magnetic moments associated with the Berry curvature. Using three layers of top gates, we independently control the tunneling barriers 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 and 5 meV. The extracted valley g -factor is around 17 and leads to opposite valley polarization for electrons and holes at moderate B -fields. Our measurements agree well with tight-binding calculations for our device.

Spin-valley coupling in single-electron bilayer graphene quantum dots

“…4 c, d). The values of g v are similar to those reported in previous studies of similar BLG QDs and compatible with theoretical calculations 6 , 23 . Considering the same geometry of both QDs and the similar voltages applied to GL and GR, it is reasonable to assume that both QDs have very similar valley g -factors.…”

“…2 b and the band edge diagram in Fig. 2 c, highlighting the potential landscape along the narrow channel) 6 . By applying V GC = −4 V to the central FG between GL and GR, the interdot tunnel coupling is reduced in order to enhance the energy resolution of the bias spectroscopy measurements.…”

“…Details on the fabrication process can be found in ref. 6 . Figure 2 a shows a scanning electron micrograph of the gate structure, where the gates used as plunger gates are color coded.…”

“…QDs are created using three layers of top gates, following previous studies of gate-defined BLG QDs 5 , 6 , 9 , 10 , 22 , 23 . A band gap is opened by applying an out-of-plane displacement field 24 , 25 with the help of the SG ( V SG = 1.73 V) and BG ( V BG = −1.56 V), while the Fermi energy ( E F ) is tuned into the band gap.…”

“…This pseudospin arises from the orbital degree of freedom of the independent energy valleys located at the inequivalent vertices ( and ) of the hexagonal Brillouin zone 3 . In analogy to the real spin, the valley pseudospin exhibits also a valley Zeeman effect 4 – 6 , where the valley Zeeman splitting – varying linearly with (out-of-plane) magnetic field – is a result of the orbital magnetic moments originating from the non-vanishing Berry curvature, Ω, at the K-points of gapped BLG (see Fig. 1 a).…”