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

Area of Science:

• Condensed Matter Physics
• Quantum Information Science
• Materials Science

Background:

• Bilayer graphene exhibits unique electronic properties, including topological orbital magnetic moments in electron and hole states.
• These properties are crucial for developing valley and spin-valley qubits, essential components in quantum computing.
• Quantum dots (QDs) in bilayer graphene offer a promising platform for manipulating these quantum states.

Purpose of the Study:

• To investigate the electron-hole crossover in a bilayer graphene quantum dot.
• To demonstrate and characterize the opposite signs of magnetic moments associated with Berry curvature.
• To explore the potential of bilayer graphene QDs for valley and spin-valley qubit applications.

Main Methods:

• Fabrication of a bilayer graphene quantum dot using three layers of top gates for independent control of tunneling barriers.
• Tuning quantum dot occupation from the few-hole to the few-electron regime, crossing the displacement-field-controlled band gap.
• Measurement of magnetic moments and valley g-factor using an out-of-plane magnetic field.

Main Results:

• Demonstrated opposite signs of magnetic moments for electrons and holes in the bilayer graphene quantum dot.
• Measured a band gap of approximately 25 meV and charging energies between 3-5 meV for electron and hole dots.
• Extracted a valley g-factor of approximately 17, confirming opposite valley polarization for electrons and holes at moderate magnetic fields.

Conclusions:

• The study confirms the presence of opposite topological orbital magnetic moments in bilayer graphene quantum dots.
• These findings support the use of bilayer graphene QDs for tunable valley-polarized states, crucial for advanced qubit design.
• Experimental results show good agreement with theoretical tight-binding calculations, validating the device model.