Imagine pouring a glass of water that flows more smoothly than anything you have ever seen — and which, while it's at it, quietly tears up a page of the physics textbook. That, more or less, is what a team of physicists in Bangalore say they have just done with electrons.
Researchers at the Indian Institute of Science (IISc), working with collaborators at Japan's National Institute for Materials Science, have reported that electrons inside exceptionally clean sheets of graphene stop behaving like ordinary particles and instead flow together as a near-frictionless liquid. The work, published in Nature Physics, appears to break one of the oldest rules in the physics rulebook.
The law in question
The rule is the Wiedemann-Franz law, first set down in 1853. In plain English, it says that any decent metal that is good at conducting electricity should be roughly equally good at conducting heat. Copper wires get warm for a reason: charge and heat travel together.
In the IISc experiment, they didn't. As the graphene's electrical conductivity went up, its thermal conductivity went down — and vice versa. At low temperatures the team measured a deviation from Wiedemann-Franz of more than 200 times. Charge and heat, in other words, had been prised apart.
A liquid made of electrons
The trick was cleanliness. Real-world materials are riddled with tiny imperfections that smother delicate quantum effects. By engineering graphene — a single layer of carbon atoms arranged in a honeycomb — almost free of defects, the researchers were able to nudge it to a sweet spot known as the "Dirac point", where the material is neither quite a metal nor quite an insulator.
At that point, the electrons appear to forget they are individuals. They move collectively, sloshing through the carbon lattice in what physicists call a Dirac fluid.
"Since this water-like behaviour is found near the Dirac point, it is called a Dirac fluid — an exotic state of matter which mimics the quark-gluon plasma, a soup of highly energetic subatomic particles observed in particle accelerators at CERN," said Aniket Majumdar, the paper's first author and a PhD student at IISc.
The team measured the fluid's viscosity and found it to be astonishingly low — about as close to a "perfect" liquid as anything yet seen in a laboratory.
A tabletop window on extreme physics
The implications go well beyond a footnote in a textbook. Quark-gluon plasma is the searingly hot soup that filled the universe a fraction of a second after the Big Bang, normally only glimpsed in the world's biggest particle colliders. If a flake of graphene on a benchtop can mimic some of its behaviour, exotic physics suddenly becomes a great deal more accessible.
The researchers suggest the work also opens doors to studying ideas borrowed from astrophysics, including black-hole thermodynamics and the strange mathematics of quantum entanglement, without leaving the lab.
"It is amazing that there is so much to do on just a single layer of graphene even after 20 years of discovery," said Professor Arindam Ghosh of the IISc Department of Physics, one of the study's corresponding authors.
What it could mean for quantum tech
There is a practical pay-off in view, too. A material in which charge and heat can be controlled independently is a gift to engineers building quantum sensors — the kind of devices that detect vanishingly faint electrical or magnetic signals, and which underpin the next generation of medical imaging, navigation and quantum computers.
For now, the headline is simpler: a 170-year-old law has been bent rather firmly out of shape, by a sheet of carbon one atom thick. Not bad for a Monday.
