Chiral Asymmetry Creates a Path to High-Efficiency Future Electronics

The Science

In two-dimensional quantum materials, chiral edge states are one-dimensional conducting channels through which electrons travel in one direction only, like cars on a one-way road. This is because chirality is a special form of asymmetry. Imagine a material like a sheet of paper. A chiral edge state is like a flow of electrons along the edge of the paper. Chiral means that if the electrons flow from left to right on one edge, then they flow from right to left on the opposite edge. Under these conditions electron collisions are strongly suppressed because they cause electrons to change direction, which is not allowed on a one-way chiral edge “road.” These collisions are the primary cause of electrical resistance in wires, so chiral channels act like highly efficient resistance-free conductors. Researchers have created chiral edge states in atomically thin devices made of three graphene layers. They used a scanning tunneling microscope (STM) to image the chiral channels with atomic resolution.

The Impact

Chiral states are electricity channels without resistance, in which electrons move in a fixed direction. These channels hold promise for extremely energy-efficient electronics that lose no electricity to resistance. These electronics could form the building blocks of future energy-efficient microelectronics and low-power magnetic memory devices. The ability to create chiral channels on demand will help researchers build these devices. In addition, the ability to visualize these channels will help researchers determine the ultimate limits of device miniaturization. The techniques described here will ultimately be used to study even more exotic quantum phenomena that can be exploited for quantum computers.

Summary

Chiral edge states occur when a material has a property called non-trivial topology. This means that the wavefunction of electrons in the material is knotted or twisted, analogous to the twisting of a Möbius strip. Physicists have an index called the Chern number that measures this property. When a material’s Chern number is nonzero then it has nontrivial topology. If we join two materials with different Chern numbers, then the electrons’ wavefunctions on both sides must unwind to match in the middle. This causes a chiral channel to appear at the interface where they join and the chiral channel acts like a perfect conductor even if the materials are insulators. Two-dimensional quantum materials often host nontrivial topology due to the quantum behavior of their electrons. The chiral edge states are then one-dimensional channels that form at the boundaries where the material ends, or where different materials meet.

To create such interfaces on demand, the researchers used a device made from a stack of two atomically thin layers of graphene rotated precisely relative to a third layer of graphene. They were able to create neighboring regions having opposite Chern numbers in the same material by tuning the electron density in the device with a nearby gate electrode and leveraging a small amount of pre-existing charge inhomogeneity. By using STM, they were able to image the wavefunction of the resulting chiral interface state. They discovered that the chiral interface state could be moved across the sample by modulating the gate voltage. They also found that a voltage pulse from the tip of the STM probe can “write” or “delete” chiral interface states at will.

Funding

This research was supported 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.