First Time:Electrons Caught Flowing like Water

We often speak of electrons “flowing” through materials, but in fact, they do not normally move like a liquid. Such “hydrodynamic” electron flow had been predicted, though, and Weizmann Institute of Science researchers recently managed, with the help of a unique technique to image electrons flowing like the water flowing in a pipe. This is the first time such “liquid electron flow” has been visualized, and it has vital implications for future electronic devices.

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 wavefront

Electrons usually move through conductors more like a gas thana liquid. That is, they do not collide with one another, but rather, they tend to bounce off impurities and imperfections in the material. A fluid flow, in contrast, takes it shape – be it waves or whirlpools – from frequent collisions between the particles in liquid.  

To make electrons flow like a liquid, one needs a different kind of conductor, and 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 Condensed Matter Physics Department.

To image the electron flow in the graphene, 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 created such a technique, as they recently reported in Nature Nanotechnology. This method uses a nanoscale detector built from a carbon-nanotube transistor, and the team found that can image the properties of flowing electrons with unprecedented sensitivity. “Our technique is at least 1000 times more sensitive than alternate methods; this enables us to image phenomena that previously could only be studied indirectly,” says Dr. Joseph Sulpizio, in Ilani’s lab.

In a new study published in Nature, the Weizmann researchers applied their novel imaging technique to state-of-the-art graphene devices produced in the group of Prof. Andre Geim at the University of Manchester. These devices were nanoscale channels designed to guide the flowing electrons. 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 channel and slowed down at 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, also of Ilani’s group.

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 from 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.