Ultra-Clean Graphene and Dirac Fluids

Scientists exploring the many novel and curious properties of graphene have discovered what could be a means of experimentally investigating the physics of black holes and other extreme astrophysical phenomena.

It may also have certain practical and important applications in industry and technology.

In a paper published in the February 11 issue of Science, researchers with Harvard and Raytheon BBN Technologies discuss a so-called “Dirac fluid” which they teased into life on an atom-thin film of ultra-clean graphene.

What that means, in layman’s terms, is that they observed electrons behaving like water on a sheet of pure carbon.  The ultra-clean graphene was created by slipping it between ten layers of electrically insulating carbon crystal, which effectively buffered the graphene sheet against environmental disturbances—something such atom-thin materials are highly susceptible to.

Then the researchers introduced charged particles to the graphene sheet—and that’s when things got really weird.

In your basic 3D metal, electrons behave in their usual antisocial manner: they barely Tnteract with one another.  But force them onto a 2D metal like the graphene sheet, and the electrons are suddenly jostled out of their complacency, and compelled to move together like lanes of traffic on a highway.

Predictably, the electrons react with characteristic ill-humor and bad manners.

Their speed increases fantastically (1/300th the speed of light) and they slam into each other ten trillion times a second.  In other words, they begin to approximate the behavior of massless, relativistic particles—photons, by another name.

And that’s how you get a Dirac fluid—or, in slightly less abstruse terms, a “strongly-interacting, quasi-relativistic electron-hole plasma.”

Representation of the new research: electrons behaving like a fluid on a sheet of graphene. Credit: Peter Allen/Harvard SEAS
Tabletop Black Holes and Supernovae

The gist of the new discovery is that scientists have created an experimental platform where they can observe a class of objects—which are ordinarily governed by quantum mechanics—that is now verging into the domain of relativistic physics.

Theoretically, having access to relativistic particles that can be described by classical theories of hydrodynamics (the behavior of fluids) means that physicists could experiment with simulations of black holes, supernovae, and other high-energy astrophysical phenomena.

“This is the first model system of relativistic hydrodynamics in a metal,” said Andrew Lucas, a coauthor of the study.

And all this can be done on an atom-thin sheet of ultra-pure graphene.

Moving outside of the theoretical realm, there are industrial and technological applications as well.  The team was able to precisely measure the heat conduction of the electrons, which has implications for controlling the transference of heat energy into electricity.  This is important for energy and sensing systems, but has been devilishly hard to achieve with the usual materials available to researchers.

But, according to Lucas, “[W]ith a clean sample of graphene there may be no limit to how good a device you could make.”

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