- The graphene experiment reveals an important break in the Wiedemann-Franz law
- Heat and electrical conduction move unexpectedly in opposite directions.
- The deviation from the classical law exceeds two hundred times under conditions
For decades, the Wiedemann-Franz law was a reliable rule in condensed matter physics.
This principle holds that a material’s ability to conduct electricity must increase and decrease at the same time as its ability to conduct heat.
A team of researchers from the Indian Institute of Science and Japan’s National Institute of Materials Science has documented a dramatic violation of this long-standing principle.
Article continues below.
An unexpected change in 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 rather than together.
The scientists created exceptionally clean graphene samples to eliminate interference from impurities and atomic defects.
They then carefully measured electrical and thermal conduction under a variety of conditions. What emerged was a striking contradiction with established physics.
As the electrical conductivity increased, the thermal conductivity decreased and the opposite also occurred.
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 break in a rule that has guided physicists for more than a century.
Despite this apparent anarchy, behavior is not random. Both types of conduction seem 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 basic quantity that governs how electrons move on the smallest scales imaginable.
The researchers achieved this unusual state by tuning the electron density to a special condition known as the Dirac point, where graphene oscillates exactly between being a metal and an insulator.
At this critical point, the electrons stop acting as independent particles. Instead, they move collectively, forming a flowing fluid with remarkably low resistance.
“Since this water-like behavior is found near the Dirac point, it is called Dirac fluid, an exotic state of matter that mimics quark-gluon plasma, a soup of highly energetic subatomic particles observed in CERN’s particle accelerators,” explains Aniket Majumdar, first author and PhD student in 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 embodiments of a perfect fluid ever observed in a laboratory.
This discovery transforms graphene into a tabletop window in extreme physics.
Scientists can now investigate phenomena usually associated with the thermodynamics of black holes and high-energy particle collisions without leaving their laboratories.
Dirac fluid could enable highly sensitive quantum sensors capable of detecting weak magnetic fields or amplifying extremely weak electrical signals.
Although the experiment does not overturn all of physics, it does show that even fundamental laws have limits when quantum mechanics and the collective behavior of electrons collide.
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