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News | PlasCarb

An exception to Ohm’s Law? Graphene makes electrons act like a viscous liquid with ‘negative resistance’

Opposites attract. It’s one of the fundamental rules that explain why electricity and magnetism do what they do. Magnets and charge have two “flavors” we call poles, like charges repel, and the electrical polarity of the system determines which way current will flow. Except — apparently graphene puts an asterisk after Maxwell’s equations.

Two teams of physicists just found evidence that graphene makes electrons act less like charge carriers moving at relativistic speeds and more like a viscous liquid, flowing against the electrical current in eddies and whirlpools like those at a river’s edge. That property makes electrons flow against electrical polarity in a phenomenon that physicists are calling “negative resistance,” and as Nature Physics and Science report, we’ve finally seen it happen at room temperature.

Negative resistance

Graphene has a strange cross-section of electromagnetic properties, including high conductance and low resistance, like metals. Unlike a metal, though, graphene does some truly bizarre things to the electrons going through it. When you run current through a wire, it mostly moves in smooth, laminar flow, in a way we usually call “ballistic.” Imperfections in the material are the dominant deflecting force introducing turbulence into the system, and that’s a minor effect.

We thought graphene acted the same way, with the only electron deflections occurring at junctions in the sheet. When you apply a voltage to a graphene ribbon, though, only some of the current moves in predictable, laminar Ohmic flow. Professor Leonid Lebitov of MIT and Professor Gregory Falkovich of Israel’s Weizmann Institute of Science demonstrated that some of it displays negative resistance: Instead of slowing electrons and dissipating their energy through heat as with an Ohmic resistor, the electrons are deflected and bound up in little eddies that move against the electrical polarity of the system, like a viscous liquid.

Andre Geim, a professor of condensed matter physics at the University of Manchester, ran with that idea and measured the viscosity observed in the eddies. He and his team “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.”

Graphene’s electrodynamic properties lead to viscous current flows, creating tiny whirlpools that cause electrons to travel against electrical polarity. White lines show current streamlines, colors show electrical potential, and green arrows show the direction of current, for viscous (top panel) and normal (ohmic, bottom panel) flows. Image: Nature Physics

This research is so new that we’re not even sure how to apply the findings yet. One of the potential implications is that heat transfer is strongly coupled to charge transfer, so there will probably be related thermal conductance phenomena to be uncovered here. As noted in Nature, “Viscous [electron] flow results in a highly complex heating pattern with intense hot spots near contacts and cold arc-shaped patches at vortices surrounded by warmer regions.”

White arrows show current direction. Viscous flow shows a highly complex heating pattern.  Ohmic flow (bottom) shows an essentially featureless heat production rate decaying monotonously away from contacts.  Image: Nature Physics

These experiments mark the first time we’ve ever been able to directly observe these long-predicted effects of graphene chemistry. What’s more, they offer a window into the macroscale implications of quantum physics. While quantum effects are normally insignificant at scales larger than individual particles, in the graphene environment they play a dominant role, says Professor Levitov via MIT news. In this setting, “we show that [the way charge carriers move] has collective behavior similar to other strongly interacting fluids, like water.” Given how difficult graphene still is to produce in quantity, though, it may be that we won’t know how to use it until we can make enough to use.

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