Researchers detect electron vortices in graphene at room temperature

Researchers at ETH Zurich have made a breakthrough by showing how electrons form vortices in a material at room temperature. This is the first time that such electron vortices have been detected at this temperature. The experiment used an extremely high resolution quantum sensing microscope.

In graphene, electrons behave like a liquid, allowing the formation of vortices. These vortices have now been made visible using quantum magnetic field sensors with high spatial resolution.

Generally, transport events can be more easily detected at lower temperatures. However, thanks to their highly sensitive sensors, ETH researchers can observe the vortices even at room temperature.

The researchers used tiny circular disks attached to a conducting graphene strip that was only one micrometer wide during the fabrication process. The discs had varying diameters, between 1.2 and 3 micrometres.

Theoretical calculations suggested that electron vortices should form in smaller disks, but not in larger disks. To make the vortices visible, the researchers measured tiny magnetic fields generated by electrons flowing inside the graphene.

For this purpose, they used a quantum magnetic field sensor that contained a so-called nitrogen-vacancy (NV) center in the tip of a diamond needle.

Using laser beams and microwave pulses, the quantum state of the center can be tailored in such a way that it is maximally sensitive to the magnetic field. By reading quantum states with lasers, researchers can determine the strength of those fields very precisely.

The researchers observed a typical sign of vortices expected in the small disk: a reversal of flow direction. While in normal (extended) electron transport, electrons in the strip and disk flow in the same direction, in the case of the vortex, the direction of flow inside the disk is reversed. As predicted by calculations, no vortices could be observed in the larger disk.

“Thanks to our extremely sensitive sensor and high spatial resolution, we did not even need to cool the graphene and were able to conduct the experiment at room temperature,” says Marius Palm, a former PhD student in the Dagen group. Furthermore, he and his colleagues detected not only electron vortices, but also vortices produced by hole carriers.

“At this time, detecting electron vortices is basic research, and there are still a lot of open questions,” Pam says. For example, researchers still need to figure out how the collisions of electrons with graphene's boundaries affect the flow patterns, and what effects are happening even in smaller structures.

The new detection method used by the ETH researchers also allows a closer look at many other exotic electron transport effects in mesoscopic structures – phenomena that occur on length scales ranging from several tens of nanometers to a few micrometers.

In conclusion, the discovery of electron vortices in graphene at room temperature is an important milestone in the field of physics and materials science. This achievement opens up new avenues for researchers to explore the behavior of electrons at the microscopic level. This could have exciting implications for the development of new technologies.

journal reference

  1. Palm M, Ding C, Huxter W, Taniguchi T, Watanabe K, Degen C: Observation of eddy current in graphene at room temperature. Science25. April 2024, DOI:10.1126/science.adj2167

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