A team of researchers has made a breakthrough by detecting well-known flow phenomena expected to occur in graphene.
Electronic Transport in a Conductor
When an ordinary conducting material, like a metal wire, connects to a battery, the electrons in the conductor are accelerated by the electric field created by the battery. As they move, the electrons frequently hit the impurity atoms or vacancies in the wire's crystal lattice. This converts some parts of their kinetic energy into lattice vibrations. In this process, the energy lost is converted into heat, which can be felt like an incandescent light bulb is touched.
Although these collisions with lattice impurities happen frequently, the collisions between electrons are much rarer. However, the situation dramatically changes when a single layer of carbon atoms arranged in a honeycomb lattice, called graphene, is used instead of a standard copper or iron wire.
In this type of carbon arrangement, impurity collisions are not common, and the interaction between electrons plays an important role. In this case, the behavior of the electrons resembles that of a viscous liquid. Therefore, it is expected that flow phenomena like vortices should take place in the graphene layer.
Direct Detection of Electron Vortices in Graphene
At ETH Zurich, a team of experts has demonstrated the direct detection of electron vortices in graphene. This breakthrough was made possible using a high-resolution magnetic field sensor. The details of the study are discussed in the paper "Observation of current whirlpools in graphene at room temperature."
Led by Christian Degen, the team attached small circular disks to a conducting graphene strip that measures only one micrometer wide during the fabrication process. This led to the formation of electron vortices. The diameters of the disks range from 1.2 to 3 micrometers, and theoretical calculations suggest that the vortices should form in the smaller disks.
Degen and colleagues quantified the tiny magnetic fields produced by the electrons that flow inside the graphene to make the electron vortices visible to the observer. The researchers used a quantum magnetic field sensor composed of a nitrogen-vacancy (NV) center embedded in the tip of a diamond needle.
As an atomic defect, the nitrogen vacancy center behaves like a quantum material with energy levels that depend on an external magnetic field. With the help of microwave pulses and laser beams, the quantum states of the NV center can be prepared in such a way that it would be maximally sensitive to magnetic fields. Meanwhile, reading out the quantum states with a laser allows the scientists to precisely determine the strength of those fields.
The diamond needle and the small distance from the graphene layer is only about 70 nanometers. According to researcher Marius Palm, this feature makes the electron currents visible with a resolution of less than 100 nanometers, enough to see the vortices.
The research team observed a reversal of the flow direction in their measurements, a characteristic feature of the expected vortices in the smaller disks. In regular electron transport, the electrons in the disk and strip flow in the same direction. In the case of an electron vortex, an inverted flow direction is observed inside the disk, and, as predicted by the calculations, no vortices were found in the largest disks.
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