In a new paper published in Science, researchers at the Harvard and Raytheon BBN Technology have advanced 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 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. This research could lead to novel thermoelectric devices as well as provide a model system to explore exotic phenomena like black holes and high-energy plasmas. In order to make this observation, the team improved methods to create ultra-clean graphene and developed a new way measure its thermal conductivity.
In ordinary, 3D metals, electrons hardly interact with each other. But graphene’s 2D honeycomb structure acts like an electron superhighway in which all the particles have to travel in the same lane. The electrons in graphene act like massless relativistic objects, some with positive charge and some with negative charge. They move at incredible speed – 1/300 of the speed of light – and have been predicted to collide with each other at 10 trillion times/s at room temp. These intense interactions between charge particles have never been observed in an ordinary metal before.
The team created an ultra-clean sample by sandwiching the 1-atom thick graphene sheet between tens of layers of an electrically insulating perfect transparent crystal with a similar atomic structure of graphene. “If you have a material that’s one atom thick, it’s going to be really affected by its environment,” said Jesse Crossno, Kim Lab. “If the graphene is on top of something that’s rough and disordered, it’s going to interfere with how the electrons move. It’s really important to create graphene with no interference from its environment.” Next, the team set up a kind of thermal soup of positively charged and negatively charged particles on the surface of the graphene, and observed how those particles flowed as thermal and electric currents.
What they observed flew in the face of everything they knew about metals. Most of our world – how water flows (hydrodynamics) or how a curve ball curves – is described by classical physics. Very small things, like electrons, are described by quantum mechanics while very large and very fast things, like galaxies, are described by relativistic physics, pioneered by Albert Einstein. Combining these laws of physics is notoriously difficult but there are extreme examples where they overlap. High-energy systems like supernovas and black holes can be described by linking classical theories of hydrodynamics with Einstein’s theories of relativity.
But it’s difficult to run an experiment on a black hole. Enter graphene. When the strongly interacting particles in graphene were driven by an electric field, they behaved not like individual particles but like a fluid that could be described by hydrodynamics. “Instead of watching how a single particle was affected by an electric or thermal force, we could see the conserved energy as it flowed across many particles, like a wave through water,” said Crossno. “Physics we discovered by studying black holes and string theory, we’re seeing in graphene,” said Andrew Lucas. “This is the first model system of relativistic hydrodynamics in a metal.” A small chip of graphene could be used to model the fluid-like behavior of other high-energy systems.
Industrial applications of graphene? To observe the hydrodynamic system, the team needed to develop a precise way to measure how well electrons in the system carry heat. “We needed to find a clever way to ignore the heat transfer from the lattice and focus only on how much heat is carried by the electrons,” Fong said. To do so, the team turned to noise. At finite temperature, the electrons move about randomly: the higher the temperature, the noisier the electrons. By measuring the temperature of the electrons to 3 decimal points, the team was able to precisely measure the thermal conductivity of the electrons. “Converting thermal energy into electric currents and vice versa is notoriously hard with ordinary materials,” said Lucas. “But in principle, with a clean sample of graphene there may be no limit to how good a device you could make.” http://science.sciencemag.org/content/early/2016/02/10/science.aad0343
https://www.seas.harvard.edu/news/2016/02/metal-that-behaves-like-water
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