After two years of effort, researchers have successfully measured the collective mass of ‘massless’ electrons in motion in graphene.
By shedding light on the fundamental kinetic properties of electrons in graphene, this research may also provide a basis for the creation of miniaturized circuits with tiny, graphene-based components.
“Graphene is a unique material because, effectively, individual graphene electrons act as though they have no mass. What that means is that the individual electrons always move at a constant velocity,” explains one of the researchers. “But suppose we apply a force, like an electric field. The velocity of the individual electrons still remains constant, but collectively, they accelerate and their total energy increases—just like entities with mass. It’s quite interesting.”
Without this mass, the field of graphene plasmonics cannot work, so Ham’s team knew it had to be there—but until now, no one had accurately measured it.
As Newton’s second law dictates, a force applied to a mass must generate acceleration. The research team knew that if they could apply an electric field to a graphene sample and measure the electrons’ resulting collective acceleration, they could then use that data to calculate the collective mass.
"...it was like a ‘through darkness comes light’ moment.”Hosang Yoon, Ph.D., Electrical Engineering and Applied Physics at the Harvard School of Engineering and Applied Sciences (SEAS).
But the graphene samples used in past experiments were replete with imperfections and impurities—places where a carbon atom was missing or had been replaced by something different. In those past experiments, electrons would accelerate but very quickly scatter as they collided with the impurities and imperfections.
“The scattering time was so short in those studies that you could never see the acceleration directly,” says a researcher.
To overcome the scattering problem, several smart changes were necessary.
The team was able to reduce the number of impurities and imperfections by sandwiching the graphene between layers of hexagonal boron nitride, an insulating material with a similar atomic structure. They designed a better way to connect electrical signal lines to the sandwiched graphene and applied an electric field at a microwave frequency. This allowed for the direct measurement of the electrons’ collective acceleration in the form of a phase delay in the current.
“By doing all this, we translated the situation from completely impossible to being at the verge of either seeing the acceleration or not,” says the researcher. “However, the difficulty was still very daunting, and Hosang [Yoon] made it all possible by performing very fine and subtle microwave engineering and measurements—a formidable piece of experimentation.”
“To me, it was a victorious moment that finally justified a long-term effort, going through multiple trials and errors,” says a researcher. “Until then, I wasn’t even sure if the experiment would really be possible, so it was like a ‘through darkness comes light’ moment.”
Collective mass is a key aspect of explaining plasmonic behaviors in graphene. By demonstrating that graphene electrons exhibit a collective mass and by measuring its value accurately, the researcher says, “We think it will help people to understand and design more sophisticated plasmonic devices with graphene.”
The team’s experiments also revealed that, in graphene, kinetic inductance (the electrical manifestation of collective mass) is several orders of magnitude larger than another, far more commonly exploited property called magnetic inductance. This is important in the push toward smaller and smaller electronic circuitry—the main theme of modern integrated circuits—because it means the same level of inductance can be achieved in a far smaller area.
Furthermore, the team says that this miniature graphene-based kinetic inductor could enable the creation of a solid-state voltage-controlled inductor, complementary to the widely used voltage-controlled capacitor. It could be used to substantially increase the frequency tuning range of electronic circuits, which is an important function in communication applications.
For now, the challenge remains to improve the quality of graphene samples so that the detrimental effects of electron scattering can be further reduced.
This story is reprinted from material from Harvard School of Engineering and Applied Sciences (SEAS), with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.