Visualization and Manipulation of Bilayer Graphene Quantum Dots with Broken Rotational Symmetry and Nontrivial Topology

Abstract

Electrostatically defined quantum dots (QDs) in Bernal stacked bilayer graphene (BLG) are a promising quantum information platform because of their long spin decoherence times, high sample quality, and tunability. Importantly, the shape of QD states determine the electron energy spectrum, the interactions between electrons, and the coupling of electrons to their environment, all of which are relevant for quantum information processing. Despite its importance, the shape of BLG QD states remains experimentally unexamined. Here we report direct visualization of BLG QD states by using a scanning tunneling microscope. Strikingly, we find these states exhibit a robust broken rotational symmetry. By using a numerical tight-binding model, we determine that the observed broken rotational symmetry can be attributed to low energy anisotropic bands. We then compare confined holes and electrons and demonstrate the influence of BLG's nontrivial band topology. Our study distinguishes BLG QDs from prior QD platforms with trivial band topology.

Recent progresses of quantum confinement in graphene quantum dots

“…Researchers have detected the valley and Zeeman splittings with a spin g-factor g s ≈ 2 in single electron charging of gate-defined bilayer GQDs [156], and the valley splitting linearly depends on the external perpendicular magnetic field [156,157]. Ge et al [159] have directly visualized the wave function shape of bilayer GQD states with a robust broken rotational symmetry, which is contributed to the low energy anisotropic bands. And they also demonstrated that the nontrivial band topology of bilayer graphene could be manifested and manipulated by imaging quantum interference patterns in bilayer GQDs.…”

Theory of tunneling spectra for a few-electron bilayer graphene quantum dot

“…In bilayer graphene, recent experiments achieve confinement of charge carriers in one-and zero-dimensional structures by electrostatic gating [4][5][6][7][8][9][10] . To electrostatically define a nanostructure in bilayer graphene multiple gates locally modulate the bilayer graphene band gap and charge carrier density, cf.…”

Electronic properties of bilayer graphene with magnetic quantum structures studied using the Dirac equation

“…The properties of two or more coupled quantum dots in BLG have also been studied experimentally and theoretically [25][26][27]. Recently, the electronic density within BLG quantum dots has been visualized using the tip of a scanning tunneling microscope (STM) [28,29]. The valley properties of an electrostatically confined BLG quantum dot have been investigated experimentally [30,31], and a valley filter device based on these properties has been proposed [32].…”