Electrical resistance is akin to fluid viscosity broadly speaking. And like particles moving in a fluid there is also the possibility of vortices and reverse flow. In the electrical sense, the latter might give rise to the counter-intuitive notion of "negative resistance". Now, work published in Nature Physics by Leonid Levitov of Massachusetts Institute of Technology and Gregory Falkovich of the Weizmann Institute of Science in Israel suggests that negative resistance might be another property of the two-dimensional material graphene [Levitov and Falkovich, Nature Physics (2016) DOI: 10.1038/nphys3667].
Theoretically, electron movement in graphene might resemble the movement of a viscous fluid through a tube, in which turbulence and vortices lead to a bumpy ride because of the long-range current-field response. This is a quite different view of electron movement from the simple Ohmic perspective of electrons simply bouncing along from ion to ion like so many individual pinballs. Now, Levitov and Falkovich say they have found a way to observe this collective behavior of electron flow in graphene.
"There was always a kind of dichotomy between what's easy to do in theory and what's easy to do in experiments," Levitov explains. "There was a search for an ideal system that would be easy for experimentalists to work with and also be a benchmark system with strong interactions that would show strong interactive phenomena." Now, he adds, graphene is providing many of the sought-after qualities of such a system. He explains that on a graphene surface, electrons behave as relativistic particles behaving like a "strongly interacting fluid" an important theoretical concept in quantum physics. This collective behavior of charge carriers is more akin to fluid dynamics than the individualistic understanding of electron behavior under Ohm's Law.
In the graphene environment, quantum effects, which are ordinarily insignificant at scales larger than that of individual particles, play a dominant role, Levitov adds. In this setting, "we show that [the way charge carriers move] has collective behavior similar to other strongly interacting fluids, like water." However, because of the two-dimensional structure of graphene, it is necessary to place probes at strategic points on the graphene sheet to extract useful information about electron behavior and this is what the team has now allowed others to demonstrate with probes revealing the eddies and whirlpools of negative resistance across a graphene ribbon.
Because this is early-stage work, Levitov concedes, it's too early to tell whether it might ever have any practical applications. But, as with many surprising phenomena associated with graphene, it is likely that scientists will be searching for applications right now.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".