Researchers at the U.S. Department of Energy have announced the manipulation of quantum properties of graphene, the single-atom-thick layer of carbon that has captivated researchers for about two decades.
In their research, scientists constructed quantum electronic devices using a bespoke configuration of graphene layers: a twisted monolayer–bilayer graphene structure. This setup consists of a single layer of graphene placed on top of two layers of graphene (bilayer), with a slight rotational misalignment. This created what is known as a moiré superlattice, which exhibits chiral edge states — one-way paths for electrons along the edges of the material.
Scanning tunneling microscope map of a device made of three atomic layers of graphene, showing the wave function of a chiral interface state (bright stripe) between two insulating regions with opposite chirality (dark sides). [Credit: Lawrence Berkeley National Laboratory]
Tuning electron density
By tuning electron density with a gate electrode, the scientists created neighboring regions with opposing topological properties, enabling the control of the direction of electron flow. The unique properties of these chiral interface also suppress electron collisions, potentially supporting near-zero resistance electron transport.
Using a scanning tunneling microscope, researchers could visualize these chiral states with atomic resolution, move them like pieces on a quantum chessboard. They could also even ‘write’ or ‘delete’ them at will. This allowed them to characterize the channels’ properties and spatial distribution. This research contributes significantly to our understanding of topological phenomena in two-dimensional materials, a field that explores how the “shape” or topology of a material can influence its electronic properties.
The research was the subject of a paper from Zhang et al. published in Nature Physics.
How it works in a nutshell
In their research, the scientists created a moiré system using twisted monolayer–bilayer graphene, which hosts correlated and topological states that support electrical controlled via applied gate voltages. .
By tuning the carrier concentration, the team stabilized neighboring domains with opposite Chern numbers. This created topological interfaces without any structural boundaries, allowing for direct imaging of the chiral interface states using a scanning tunneling microscope.
As the research Nature Physics abstract explained:
Moiré systems made from stacked two-dimensional materials host correlated and topological states that can be electrically controlled with applied gate voltages. One prevalent form of topological state that can occur are Chern insulators that display a quantum anomalous Hall effect. Here we manipulate Chern domains in an interaction-driven quantum anomalous Hall insulator made from twisted monolayer–bilayer graphene and observe chiral interface states at the boundary between different domains. By tuning the carrier concentration, we stabilize neighbouring domains of opposite Chern number that then provide topological interfaces devoid of any structural boundaries. This allows the wavefunction of chiral interface states to be directly imaged using a scanning tunnelling microscope. Our theoretical analysis confirms the chiral nature of observed interface states and allows us to determine the characteristic length scale of valley polarization reversal across neighbouring Chern domains.
The Department of Energy Office of Science, the National Science Foundation, and various international organizations and foundations backed the research.
