Reviewed by Lexie CornerSep 4 2024
Researchers from Lawrence Berkeley National Laboratory have successfully created chiral edge states in stacked graphene layers. These states act as one-way pathways for electrons, allowing for highly efficient, resistance-free electrical conduction. By adjusting the electron density within the graphene layers, the team was able to control both the formation and location of these chiral channels, presenting opportunities for the development of innovative electronic devices. The study was published in Nature Physics.
Scanning tunneling microscope map of a device made of three atomic layers of graphene. The contrast reveals the wavefunction of a chiral interface state (bright stripe) between two insulating regions having opposite chirality (dark color sides). Image Credit: Lawrence Berkeley National Laboratory
Chiral edge states in two-dimensional quantum materials are one-dimensional conducting channels that electrons can only move through in one direction, much like traffic on a one-way street. This behavior arises from chirality, a unique kind of asymmetry.
A chiral edge state is analogous to the flow of electrons along the edge of a material, such as a sheet of paper. In a chiral system, if electrons move from left to right along one edge, they will flow in the opposite direction, from right to left, along the other edge. In this scenario, electrons are unable to reverse direction, significantly reducing collisions.
These chiral channels enable highly efficient, resistance-free conduction. Researchers have successfully created chiral edge states in atomically thin devices made of three stacked graphene layers. They used Scanning Tunneling Microscopy (STM) to visualize the chiral channels at atomic resolution.
The Impact
Electrons in chiral states travel in a fixed path along resistance-free electrical channels. Devices using these channels could eliminate power loss due to resistance, potentially leading to future low-power magnetic memory devices and energy-efficient microelectronics. Researchers' ability to create these chiral channels on demand is a key step toward the development of such technologies.
Additionally, visualizing these channels will help researchers determine the limits of device miniaturization. The methods used in this research will also be used to investigate even more unusual quantum phenomena, which hold potential for quantum computing applications.
Summary
Chiral edge states arise when a material exhibits non-trivial topology, meaning its electron wavefunction is twisted or knotted, similar to a Möbius strip. This characteristic is quantified by the Chern number, and a material with a non-zero Chern number has non-trivial topology. When two materials with different Chern numbers meet, the electrons' wavefunctions adjust, creating a chiral channel at their interface. Despite the materials being insulators, the interface acts as a perfect conductor due to this chiral channel.
Two-dimensional quantum materials often host non-trivial topology due to the quantum properties of their electrons. At the boundary of such materials or where two different materials meet, one-dimensional chiral edge states emerge.
In this study, researchers created a device consisting of two graphene layers stacked with a third layer, precisely rotated to generate these chiral interfaces on demand. By utilizing existing charge inhomogeneity and adjusting electron density through a gate electrode, they formed neighboring regions with opposite Chern numbers in the same material. Using STM, they visualized the wavefunction of the chiral interface state.
The team found that they could manipulate the chiral interface state by altering the gate voltage, allowing it to shift across the sample. They also demonstrated the ability to "write" or "delete" these chiral states using voltage pulses applied to the STM probe's tip.
Funding
The study was funded by the Department of Energy Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering, and the National Science Foundation. Individual researchers were supported by JSPS KAKENHI, World Premier International Research Center Initiative (WPI), MEXT, Japana Kavli ENSI Philomathia Graduate Student Fellowship, and the Masason Foundation.
Journal Reference:
Zhang, C., et al. (2024) Manipulation of chiral interface states in a moiré quantum anomalous Hall insulator. Nature Physics. doi.org/10.1038/s41567-024-02444-w.