- Graphene experiment reveals major breakdown of Wiedemann-Franz law
- Heat and electrical wire move unexpectedly in opposite directions
- Deviation from classical law exceeds two hundred times under conditions
For decades, the Wiedemann-Franz law stood as a reliable rule in condensed matter physics.
This principle states that a material’s ability to conduct electricity should increase and decrease in proportion to its ability to conduct heat.
A team of researchers from the Indian Institute of Science and the National Institute for Materials Science in Japan has now documented a dramatic violation of this long-standing tenet.
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An unexpected twist at the Dirac point
Their experiments with graphene, a single layer of carbon atoms, show that electrical conductivity and thermal conductivity can move in opposite directions instead of together.
The researchers created exceptionally pure graphene samples to eliminate interference from atomic defects and impurities.
They then carefully measured both electrical and thermal conductivity across a range of conditions. What emerged was a striking contrast to established physics.
As electrical conductivity increased, thermal conductivity decreased and vice versa.
At low temperatures, the observed deviation from the Wiedemann–Franz law exceeded a factor of 200.
This separation between charge flow and heat flow is not a minor anomaly, but a fundamental breakdown of a rule that has guided physicists for more than a century.
Despite this apparent lawlessness, the behavior is not random. Both types of wire appear to obey a universal constant that does not depend on the specific properties of the material.
This constant connects directly to the conductance quantum, a fundamental quantity that governs how electrons move on the smallest imaginable scales.
The researchers achieved this unusual state by tuning the electron density to a special state known as the Dirac point, where graphene hovers precisely between being a metal and an insulator.
At this critical point, electrons cease to function as independent particles. Instead, they move collectively, forming a fluid that flows with remarkably low resistance.
“Since this water-like behavior is found near the Dirac point, it is called a Dirac liquid – an exotic state of matter that mimics the quark-gluon plasma, a soup of highly energetic subatomic particles observed in particle accelerators at CERN,” explains Aniket Majumdar, first author and PhD student at the Department of Physics.
The team measured the fluid’s viscosity and found it to be extremely low, making this system one of the closest realizations of a perfect fluid ever observed in a laboratory.
This discovery turns graphene into a tabletop window into extreme physics.
Scientists can now study phenomena normally associated with black hole thermodynamics and high-energy particle collisions without leaving their laboratories.
The Dirac liquid may enable highly sensitive quantum sensors capable of detecting weak magnetic fields or amplifying extremely weak electrical signals.
While the experiment doesn’t overturn all physics, it does show that even fundamental laws have limits when quantum mechanics and collective electron behavior collide.
Via ScienceDaily
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