Liquid-like graphene could be the key to understanding black holes

Scientists at Harvard and Raytheon BBN Technology have made a breakthrough in our understanding of graphene’s basic properties, observing for the first time electrons in a metal behaving like a fluid(Credit: Peter Allen/Harvard SEAS)

Researchers at Harvard University and Raytheon BBN Technology have discovered that the charged particles inside high-purity graphene behave as a fluid with relativistic properties. This find could lead to devices that efficiently convert heat into electricity, as well as graphene-based chips that can accurately model the behavior of faraway celestial objects like supernovas and black holes.

Graphene is extremely light and strong, a great conductor of heat and electricity, and both very stiff and very ductile. This unique set of features suggests it could replace silicon in electronics and lithium in high-density batteries – or, rolled into carbon nanotubes, perhaps even do something as foolishly ambitious as help build a space elevator.

Making graphene is simple enough, all that's needed is a piece of adhesive tape to peel graphite crystals over and over down to a single layer. But because the end product is only one atom thick, studying the properties of graphene in isolation has not been nearly as easy.

Researchers led by Prof. Philip Kim have now found a way to isolate high-purity graphene and have used it to discover yet another remarkable property of this wonder-material. For the first time in a metal, scientists have found that the charge-carrying particles in graphene behave as a fluid, where, rather than avoiding each other, particles collide trillions of times a second.

Kim and colleagues first isolated a sample of pure graphene by protecting it between layers of hexagonal boron nitride, an insulating, transparent crystal also known as "white graphene" for its similar properties and atomic structure. The scientists then covered the (still exposed) ends of the graphene sheet with charged particles and observed how charge flowed as they applied both thermal and electric currents.

When most materials are subjected to an electric field, their negatively charged electrons and positively charged "electron holes" are driven in opposite directions; by contrast, a difference in temperature causes both types of charges to move in the same direction. In either case, the charged particles hardly ever interact with each other.

As Kim and colleagues found out, however, things are very different inside high-purity graphene. The two-dimensional nature and honeycomb structure of the material forces the charged particles to travel along the same paths and collide very often, forming a strongly interacting, quasi-relativistic plasma known as a Dirac fluid.

"Physics we discovered by studying black holes and string theory, we're seeing in graphene," said Andrew Lucas, co-author of the study. "This is the first model system of relativistic hydrodynamics in a metal."

This could mean that graphene-based chips, already held as promising candidates for the next generation of ultra-thin electronics, could not only bring us much faster number crunching but also help scientists understand the complex quantum phenomena that take place inside celestial objects at the other end of our universe.

In terms of consumer applications, high-purity graphene could also be a great option to build efficient thermoelectric devices that convert heat into electric current (and vice versa) with little energy loss – for instance, creating lightweight circuitry woven into clothes that turns body heat into charge for our smartphones.

A paper further detailing the study appears in the latest edition of the journal Science.

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