Graphene gains nearly perfect liquid status

Scientists have found the electrons in a layer of carbon atoms can become a strongly interacting swirling soup

Some things always hold true — the amount of time it takes to find your keys, for example, depends on how late you are. Similarly, the rate that electrons collide in a given material is closely linked to its temperature. But graphene — a sheet of carbon that’s only one atom thick — doesn’t conform to such rules. Over a wide temperature range, graphene’s electrons should become a strongly interacting swirling soup, scientists report online July 6 in Physical Review Letters.

That finding suggests that graphene’s electrons are behaving like a nearly perfect liquid — highly turbulent with extremely low viscosity. Such properties emerge as graphene approaches the “quantum critical point,” a phase transition that breaks the rules of ordinary physics. While a block of ice melts into water only within a narrow temperature range, the transition to a perfect liquid is believed to happen at a wide range of temperatures above this quantum critical point.

To understand the dynamics of graphene’s interacting electrons, Markus Müller of the Abdus Salam International Center for Theoretical Physics in Trieste, Italy, and colleagues used quantum kinetic theory to calculate the ratio of graphene’s viscosity to its entropy — a measure of gloopyness to disorder in the system. Graphene’s ratio comes close to the theoretical lower bound that physicists have calculated for that ratio, and close to the low ratio observed in quark-gluon plasma, the superhot state of matter that existed just after the Big Bang. Instead of behaving like a gas, the quark-gluon plasma behaved more like a soup with extremely low viscosity, physicists learned in experiments at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory on Long Island.

The unusually low viscosity of graphene, with its strongly interacting electrons, suggests the possibility of some interesting nanoelectronic applications, Müller says. Typically the electric resistance of a material doesn’t change as voltage changes, but that independence doesn’t hold in graphene’s case, he says.

Interactions of charged particles like electrons, known as Coulomb interactions, are usually ignored when studying graphene, comments physicist Daniel Sheehy of Louisiana State University in Baton Rouge. But the new work reveals the importance of graphene’s Coulomb interactions, he says. “It shows the electrons are strongly interacting.”

The work is also intriguing because it provides a third example of a nearly perfect liquid. Previously the phenomenon has been observed only in quark-gluon plasma and ultracold lithium atoms (SN: 4/25/09, p. 26).

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