A “river” of electrons flowing in a graphene channel. The viscosity generated by the repulsion between electrons (red balls) causes them to flow with a parabolic current density, illustrated here as a white foam wave-front
REHOVOT, ISRAEL—December 10, 2019—We often speak of electrons “flowing” through materials, but in fact, they do not normally move like a liquid. However, such “hydrodynamic” electron flow had long been predicted – and now, Weizmann Institute of Science physicists have managed, with the help of a unique technique, to image electrons flowing similarly to how water moves through a pipe. This is the first time such “liquid electron flow” has been visualized, and it has vital implications for future electronic devices.
Electrons usually move through conductors more like a gas than a liquid. That is, they do not collide with one another but, rather, tend to bounce off impurities and imperfections in the conductor. The flow of a liquid, in contrast, takes its shape – be it waves or whirlpools – from frequent collisions between the particles in the liquid.
To make electrons flow like a liquid, one needs a different kind of conductor. The team turned to graphene, which is a one-atom-thick sheet of carbon, and which can be made exceptionally clean. “Theories suggest that liquid electrons can perform cool feats that their non-liquid counterparts cannot. But to get a clear-cut proof that electrons can, indeed, form a liquid state, we wanted to directly visualize their flow,” said Prof. Shahal Ilani, head of the team in the Institute’s Department of Condensed Matter Physics.
To image the electron flow, the researchers needed to develop a technique that would be both powerful enough to peer inside a material, yet gentle enough to avoid disrupting the flow. The Weizmann team successfully created such a technique, as reported recently in Nature Nanotechnology. Their method consists of a nanoscale detector built from a carbon nanotube transistor that can image the properties of flowing electrons with unprecedented sensitivity. “Our technique is at least a thousand times more sensitive than alternative methods; this enables us to image phenomena that previously could only be studied indirectly,” said Weizmann team member Dr. Joseph Sulpizio.
Prof. Shahal Ilani
In another new study, this one published in Nature, the Weizmann researchers applied their novel imaging technique to state-of-the-art graphene devices produced in the group of Sir Andre Geim at the University of Manchester, England. These devices were nanoscale “channels” designed to guide the flowing electrons. The result: the team observed the hallmark signature of hydrodynamic flow. Just like water in a pipe, the electrons in the graphene flowed faster in the center of the channels and slowed down by the walls.
This demonstration – that under the right conditions, electrons can mimic the patterns of a conventional liquid – may prove beneficial for creating new types of electronic devices, including low-power ones in which hydrodynamic flow lowers the electrical resistance. “Computing centers and consumer electronics are devouring an ever-increasing amount of energy, and it’s imperative to find ways to make electrons flow with less resistance,” said Dr. Lior Ella of the Weizmann team.
The experimental group at Weizmann also included Asaf Rozen and Debarghya Dutta. The graphene devices were produced by John Birkbeck, Dr. David Perello, and Dr. Moshe Ben-Shalom in the group of Prof. Andre Geim at the University of Manchester. Theoretical calculations and computer simulations to support the experiments were performed by Dr. Thomas Scaffidi, Dr. Tobias Holder, Dr. Raquel Queiroz, Dr. Alessandro Principi, and Prof. Ady Stern.
Prof. Shahal Ilani’s research is supported by the Sagol Weizmann-MIT Bridge Program; the André Deloro Prize for Scientific Research; the Leona M. and Harry B. Helmsley Charitable Trust; and the European Research Council.
