After two years of effort, researchers have successfully measured the collective mass of ‘massless’ electrons in motion in graphene.


“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.”


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.

" 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).


“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.”




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.