Continuously Tunable Berry Phase and Valley-Polarized Energy Spectra in Bilayer Graphene Quantum Dots

TL;DR

This paper demonstrates how a continuously tunable Berry phase in bilayer graphene quantum dots induces large, controllable valley-polarized energy spectra, advancing the understanding of valleytronics applications.

Contribution

It reports the experimental realization of tunable Berry phase and valley polarization in bilayer graphene quantum dots, a novel control mechanism for valleytronic devices.

Findings

Berry phase tuned from pi to 2pi by magnetic field

Valley splitting of ~10 meV at 1 T

Large tunable valley polarization observed

Abstract

Berry phase plays an important role in determining many physical properties of quantum systems. However, a Berry phase altering energy spectrum of a quantum system is comparatively rare. Here, we report an unusual tunable valley polarized energy spectra induced by continuously tunable Berry phase in Bernal-stacked bilayer graphene quantum dots. In our experiment, the Berry phase of electron orbital states is continuously tuned from about pi to 2pi by perpendicular magnetic fields. When the Berry phase equals pi or 2pi, the electron states in the two inequivalent valleys are energetically degenerate. By altering the Berry phase to noninteger multiples of pi, large and continuously tunable valley polarized energy spectra are detected in our experiment. The observed Berry phase-induced valley splitting, on the order of 10 meV at a magnetic field of 1 T, is about 100 times larger than Zeeman splitting for spin, shedding light on graphene-based valleytronics.