December 11, 2019
Electrons are often described as ‘flowing’ through materials, but in fact they do not move like a fluid. However this hydrodynamic electron flow had been predicted and now with the help of specially-produced, nanoscopic ‘pipes’, Weizmann Institute of Science researchers have imaged electrons flowing like the water through household pipes to a shower or sink.
This is the first time such ‘liquid electron flow’ has been visualised, and it has vital implications for future electronic devices.
Electron movement though materials – mainly conductors like metals – usually looks more like a gas than a fluid. Meaning they do not collide with one another, but tend to bounce off impurities and imperfections in the material. In contrast, a fluid flow takes it shape – be it waves or whirlpools – from frequent collisions between the particles in the fluid.
To make electrons flow like a fluid a different kind of conductor is needed. The Weizmann team turned to graphene, a one-atom-thick sheet of carbon which can be made exceptionally clean.
“Theories suggest that liquid electrons can perform cool feats that their 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 Professor Shahal Ilani head of the team in the Institute’s Condensed Matter Physics Department.
Even using graphene as a conductor, the team needed to develop a technique that would be both powerful enough to peer inside a material, yet gentle enough to avoid disrupting the electron flow. They succeeded in this task, which was recently reported in Nature Nanotechnology.
The technique consists of a nanoscale detector built from a carbon-nanotube transistor which the team found 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,” said Staff Scientist, 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 Professor Andre Geim at the University of Manchester. These devices were microscopic ‘pipes’ – tiny 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 centre of the channel and slowed down at the walls.
This demonstration – that given the right conditions, electrons can mimic the patterns of a conventional fluid – may prove beneficial for creating new types of electronic devices, including low-power ones in which hydrodynamic flow lowers the electrical resistance.
“Computing centres 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 team at Weizmann also included Asaf Rozen and Debarghya Dutta. The graphene devices were produced by John Birkbeck, David Perello, and Dr Moshe Ben-Shalom from the group of Professor 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 Professor Ady Stern.
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