Electron vortices in graphene have been detected by ETH Zurich researchers using a high-resolution magnetic field sensor, revealing new insights into graphene’s unique electronic properties.

This exciting discovery of electron vortices in graphene not only underscores the exceptional nature of graphene but also paves the way for further exploration and potential technological advancements in the field of quantum sensing and electronic materials.

Electron Vortices in Graphene

Electron vortices in graphene have been detected for the first time, thanks to researchers at ETH Zurich using a high-resolution magnetic field sensor. This breakthrough, published in the scientific journal Science, sheds light on the fascinating behavior of electrons in graphene, a single layer of carbon atoms arranged in a honeycomb lattice. The discovery reveals new possibilities for understanding and utilizing graphene’s unique electronic properties.

Understanding Electron Behavior in Conductors

In everyday electrical conductors, such as metal wires, electrons move when an electric field is applied by a battery. These moving electrons frequently collide with impurities or vacancies in the wire’s crystal lattice. These collisions convert some of the electrons’ motion energy into lattice vibrations, producing heat. This is why an incandescent lightbulb gets hot to the touch.

However, electron collisions in typical conductors mostly involve these impurities. Collisions between electrons themselves are much rarer. But things change dramatically when it comes to graphene.

Unique Properties of Graphene

Graphene, unlike regular conductors like iron or copper, has a unique structure that makes impurity collisions rare. Instead, electron-to-electron collisions are more common. This causes the electrons in graphene to behave like a viscous liquid, leading to phenomena like vortices, which are similar to the swirling motions seen in liquids.

The Groundbreaking Experiment

The team at ETH Zurich, led by Christian Degen, used a high-resolution magnetic field sensor to observe electron vortices in graphene. They attached small circular disks to a conducting graphene strip that was only one micrometer wide. These disks ranged in diameter from 1.2 to 3 micrometers. According to their theoretical calculations, electron vortices should form in the smaller disks.

To visualize these vortices, the researchers measured the tiny magnetic fields created by the flowing electrons using a quantum magnetic field sensor. This sensor had a nitrogen-vacancy (NV) center embedded in the tip of a diamond needle. The NV center is extremely sensitive to magnetic fields, allowing the researchers to detect very small changes in these fields with high precision.

Detecting Tiny Magnetic Fields

The NV center in the diamond needle exhibits quantum behavior, with its energy levels affected by external magnetic fields. By using laser beams and microwave pulses, the researchers made the NV center’s quantum states highly sensitive to these fields. This setup enabled them to measure the strength of the magnetic fields created by the electron flow in the graphene with a resolution of less than one hundred nanometers.

Observing Electron Vortices

During their measurements, the researchers observed a reversal in the flow direction of the electrons in the smaller disks. This reversal is a clear indication of the presence of electron vortices. In a vortex, the flow direction inside the disk is inverted compared to the normal electron transport, where electrons flow in the same direction throughout the strip and disk.

The larger disks, as predicted, did not show any vortices. The ability to detect these electron vortices at room temperature, without needing to cool down the graphene, is a significant achievement made possible by the highly sensitive sensor and high spatial resolution.

Discovering Hole Carriers

In addition to electron vortices, the researchers also discovered vortices formed by hole carriers (vacancies). By applying an electric voltage beneath the graphene layer, they could alter the number of free electrons, shifting the current flow from electron carriers to holes. The vortices disappeared completely at the charge neutrality point, where there is an equal and minimal concentration of both electrons and holes.

Implications for Future Research

While the detection of electron vortices in graphene is a major step forward, it represents basic research with many open questions remaining. For instance, scientists need to explore the effects of electron behavior in even smaller structures and understand how collisions with the edges of graphene influence the flow pattern.

This novel technique for detecting electron transport effects in mesoscopic structures will enable researchers to examine a variety of unusual electron transport phenomena on small length scales, from tens of nanometers to a few micrometers.

Conclusion

Electron vortices in graphene, as observed by the ETH Zurich team, provide a deeper understanding of the unique properties of graphene. This discovery highlights the material’s potential for future technological applications and opens up new avenues for research in electron transport phenomena.

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