Feb 23 2016
Electrical resistance is considered a simple concept, and is similar to friction. While friction slows down objects rolling over a surface, electrical resistance slows the transfer of electrons via a conductive material. Two physicists have discovered that electrons can assist in turning resistance on its head, resulting in the production of vortices and a backward flow of electricity.
New work shows that interactions of electrons in graphene lead to viscous current flows, creating tiny whirlpools that cause electrons to travel in the direction opposite to the applied voltage — in direct violation of standard electrical theory. White lines show current streamlines, colors show electrical potential, and green arrows show the direction of current, for viscous (top) and normal (ohmic) flows. Courtesy of the researchers
The prediction of "negative resistance" is one of a set of counterintuitive and abnormal fluid-like effects that are discovered under unusual circumstances, involving a sheet of graphene, and the interacting particles within it, which is a form of 2D carbon. Gregory Falkovich and Leonid Levitov have published the findings in Nature Physics. Falkovich is a professor at Israel’s Weizmann Institute of Science, and Levitov is a professor of physics at MIT.
The researchers stated (based on a description provided by Ohm's law) that electrons present in graphene move in an orderly manner, which is similar to the motion of viscous fluids passing via a tube where they are affected by vortices and turbulence. This occurs as a result of interactions forming a long-range, current-field response, which varies from the simple “individualist” behavior expected to occur under normal circumstances, where electrons move in straight lines just like pinballs bouncing up and down in the midst of ions.
The concept of electron viscosity had been earlier suggested in theory, but testing this concept has been difficult because there hasn't been a method developed to directly examine such phenomena. Falkovich and Levitov have identified a set of signs that can be used as an indicator for the cooperative effects in the flow of electrons.
[This work is] a remarkable application of theoretical insight to the prediction of a new experimentally observable effect [This insight is] very significant and opens a new chapter in the study of electron flow in metals.
Subir Sachdev, Professor of Physics, Harvard University
“There was always a kind of dichotomy between what’s easy to do in theory and what’s easy to do in experiments,” Levitov says. “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.” Levitov also states that graphene is currently providing a number of the desired qualities of such a system.
[On a graphene surface] you have electrons behaving as relativistic particles coupled by interactions that are long-range and pretty strong. [With the exception of quark-gluon plasmas and other exotic fluids] graphene can be considered to be closer to the concept of a perfect strongly interacting fluid.
Leonid Levitov, Professor of Physics, MIT
This theoretical concept plays a significant role in quantum physics, more than any other currently available systems.
The collective behavior, delivered by the charge carriers, in such interacting systems is fairly uncommon. “In fact, it’s not so different from fluid mechanics,” Levitov says. The movement of the fluids can be determined “with very little knowledge of how individual atoms of the liquid interact. We don’t care that much” about specific individual motions. Levitov says that what matters is the collective behavior.
Quantum effects are generally not very important at scales greater than that of individual particles. However Levitov states that these effects play a major role in the graphene environment. With respect to this setting, “we show that [the way charge carriers move] has collective behavior similar to other strongly interacting fluids, like water.”
While that is true at the theoretical level, he says, “the question is, even if we have it” — that is, this fluid-like behavior — “how do we detect it? Unlike ordinary fluids, where you can directly track the flow by putting some beads in it, for example, in this system we don’t have a way to view the flow directly.” The 2D graphene structure allows electrons to travel through the material and because of this “we can get information from electrical measurements” ready from the outside, making it feasible to position probes anywhere on the sheet.
If you have a viscous flow, you expect the different parts of the liquid to drag on each other and produce whirlpools. They will create a flow that will drag on neighboring particles and will drive a vortex.
Leonid Levitov, Professor of Physics, MIT
A direct flow occurring in the center of a graphene ribbon will be followed by whirlpools forming along the sides. Electrons in these whirlpools are found to move in the opposite direction of the applied electric field, resulting in negative resistance.
It is not possible to directly examine the negative resistance, but it is possible to measure and compare the reverse movement of the electron flow in specific parts of the material with the theoretical predictions. Such experiments have not been personally executed by Levitov and Falkovich, and yet Levitov states that a few recent discoveries appeared to suit the predicted pattern.
[In a recently reported experiment] researchers saw something similar, where the voltage on the side turns negative. It’s very tempting to say that what they observed is a presentation of phenomena predicted by the study.
Leonid Levitov, Professor of Physics, MIT
Contrasting the behavior of electrons in graphene to fluid dynamics “is not just an analogy, but a direct correspondence,” Levitov says. But there is a number of significant differences, including the fact that this fluid contains electrical charge, and it does not behave just like water passing in a pipe, but behaves in a manner similar to certain plasmas, which are considered to be charged particle clouds.
Levitov states that this work is in its very early stages, and it is currently not possible to decide whether it might have any realistic applications. One unexpected implication of this study is that the transport of heat can be strongly coupled to transport of charge. In other words, heat can ride above the charge flow and spread in a wave-like pattern faster than under normal conditions. Heat can travel as much as 10 to 100 times faster. Levitov speculates that if this behavior is obtained, then it can be controlled at some point, maybe in sensing devices featuring rapid response times.
It is a brilliant piece of theory, which agrees very well with our recent experimental findings. [Those experiments] detected the vortices predicted by Levitov's group and showed that the electron liquid in graphene was 100 times more viscous than honey, contrary to the universal belief that electrons behave like a gas.
Andre Geim, Professor of Condensed Matter Physics, University of Manchester
Geim adds that the use of graphene has considerably increased in a wide range of applications.
Electronic engineers cannot really utilize the material without an understanding of its electronic properties. Whether your electrons move like bullets or swim in treacle creating whirlpools obviously makes a huge difference.
Andre Geim, Professor of Condensed Matter Physics, University of Manchester
This study was assisted by the Israeli Science Foundation, the National Science Foundation, the U.S. Army Research Laboratory, MISTI MIT-Israel Seed Fund, the Russian Science Foundation, and the U.S. Army Research Office.