Researchers at ETH Zurich, led by Christian Degen, have successfully visualized electron vortices in graphene at room temperature for the first time. Using a high-resolution quantum sensing microscope, the team observed the vortices in small circular disks attached to a conducting graphene strip. The discovery could provide insights into exotic electron transport effects in mesoscopic structures. The team also detected vortices formed by hole carriers, or missing electrons, by applying an electric voltage below the graphene. The research was published in the scientific journal Science.
Discovery of Electron Vortices in Graphene
Researchers at ETH Zurich have successfully visualized the formation of electron vortices in graphene at room temperature, a feat previously unseen. This breakthrough was achieved using a quantum sensing microscope with an extremely high resolution.
Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, behaves differently from common conductors like iron or copper wire. In graphene, collisions between electrons are more frequent than collisions with lattice impurities, causing the electrons to behave more like a viscous liquid. This unique behavior leads to the formation of vortices within the graphene layer.
Quantum Sensing Microscope: A Tool for High-Resolution Imaging
The ETH Zurich team, led by Christian Degen, used a high-resolution magnetic field sensor to detect these electron vortices. The sensor was part of a quantum sensing microscope, a highly sensitive device capable of detecting minute magnetic fields.
The vortices were formed in small circular disks attached to a conducting graphene strip during the fabrication process. The disks varied in diameter from 1.2 to 3 micrometres. Theoretical calculations suggested that electron vortices would form in the smaller disks, but not in the larger ones.
The Role of Nitrogen-Vacancy Centres in Quantum Sensing
The quantum magnetic field sensor used in this experiment consisted of a nitrogen-vacancy (NV) centre embedded in the tip of a diamond needle. The NV centre, an atomic defect, behaves like a quantum object whose energy levels depend on an external magnetic field.
Using laser beams and microwave pulses, the quantum states of the centre can be prepared to be maximally sensitive to magnetic fields. By reading out the quantum states with a laser, the researchers could determine the strength of those fields very precisely.
Observations and Findings
The researchers observed a characteristic sign of the expected vortices in the smaller discs: a reversal of the flow direction. While in normal (diffusive) electron transport, the electrons in strip and disc flow in the same direction, in the case of a vortex, the flow direction inside the disc is inverted. As predicted by the calculations, no vortices could be observed in the larger discs.
The researchers also detected vortices formed by hole carriers. By applying an electric voltage from below the graphene, they changed the number of free electrons in such a way that the current flow was no longer carried by electrons, but rather by missing electrons, also called holes. Only at the charge neutrality point, where there is a small and balanced concentration of both electrons and holes, the vortices disappeared completely.
Future Implications and Research Directions
The detection of electron vortices in graphene is a significant step in basic research, but many questions remain. Researchers still need to understand how collisions of the electrons with the graphene’s borders influence the flow pattern, and what effects occur in even smaller structures.
The new detection method used by the ETH researchers also allows for a closer examination of many other exotic electron transport effects in mesoscopic structures – phenomena that occur on length scales from several tens of nanometres up to a few micrometres. This could open up new avenues for research and potential applications in the field of nanotechnology and quantum computing.
External Link: Click Here For More
