Researchers at ETH Zurich have demonstrated for the first time how electrons form spins in a material at room temperature. Their experiment used quantum sensing microscopy with very high resolution.
When an ordinary electrical conductor – such as a metal wire – is connected to a battery, the electrons in the conductor are accelerated by the electric field created by the battery. As the electrons move, they frequently collide with impurity atoms or vacancies in the crystal lattice of the wire, and convert part of their kinetic energy into lattice vibrations. The energy lost in this process is converted into heat, which can be felt by touching an incandescent light bulb, for example.
While collisions with lattice impurities are frequent, collisions between electrons are extremely rare. However, the situation changes when graphene, a single layer of carbon atoms arranged in a honeycomb lattice, is used instead of a common iron or copper wire. In graphene, impurity collisions are rare and collisions between electrons play an important role. In this case, the electrons behave like a viscous fluid. Therefore, well-known flow phenomena such as vorticity must occur in the graphene layer.
Reporting in the scientific journal Science, researchers at ETH Zurich in Christian Teigen’s group have now been able to directly detect electron spins in graphene for the first time using a high-resolution magnetic field sensor.
Highly Sensitive Quantum Sensing Microscopy
During the fabrication process, Degen and his colleagues developed spins on small circular disks attached to a micrometer-wide strip of conductive graphene. The discs have different diameters between 1.2 and 3 micrometers. Theoretical calculations suggested that electron spins should form in small, but not large disks.
To see the spins, the researchers measured tiny magnetic fields produced by electrons flowing through the graphene. To this end, they used a quantum magnetic field sensor called a nitrogen-vacancy (NV) center embedded in the tip of a diamond needle. Being an atomic defect, the NV center behaves like a quantum object whose energy levels depend on the external magnetic field. Using laser beams and microwave pulses, quantum states of the core can be made to be maximally sensitive to magnetic fields. By studying quantum states with a laser, researchers can determine the strength of those fields with great precision.
„With the small dimensions of the diamond needle and the small distance from the graphene layer – only about 70 nanometers – we were able to see the electron currents with a resolution of less than a hundred nanometers,” says former Marius Baum. PhD student in Degen’s group. This resolution is sufficient to see loops.
Reverse flow direction
In their measurements, the researchers observed a characteristic symptom of vortices expected in small disks: a reversal of flow direction. While in normal (diffusive) electron transport, the electrons in the strip and in the disk flow in the same direction, in the case of a spiral, the direction of flow inside the disk is reversed. As predicted by calculations, spins are not observed in large disks.
„Thanks to our highly sensitive sensor and high spatial resolution, we didn’t need to cool the graphene and were able to conduct experiments at room temperature,” says Baum. Moreover, he and his colleagues not only detected electron spins, but also spins created by hole carriers. By applying an electric voltage from below the graphene, the number of free electrons changed so that the current flow was no longer carried by electrons, but instead by missing electrons, also known as holes. Only at the point of charge neutrality, where there is a small and uniform concentration of both electrons and holes, do spins disappear completely.
„At the moment, detecting electron spins is basic research, and there are still a lot of open questions,” Baum says. For example, researchers still need to figure out how collisions of electrons with graphene boundaries affect the flow pattern, and what the effects are on smaller structures. The new detection method used by ETH researchers allows close observation of many fascinating electron transport effects in mesoscopic structures – phenomena that occur on length scales from several tens of nanometers to a few micrometers.