• Graphene experiment reveals major breakdown of Wiedemann-Franz law
• Heat and electrical conduction unexpectedly move in opposite directions
• Deviation from classical law exceeds two hundredfold under conditions

For decades, the Wiedemann-Franz law stood as a reliable rule in condensed matter physics.

This principle holds that a material’s ability to conduct electricity should rise and fall in lockstep with 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 Reversal at the Dirac Point

Their experiments on 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 atomic defects and impurities.

They then carefully measured both electrical and thermal conduction across a range of conditions. What emerged was a striking contradiction to established physics.

As electrical conductivity increased, thermal conductivity dropped, and the reverse 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 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 conduction appear to obey a universal constant that does not depend on the specific properties of the material.

This constant connects directly to the quantum of conductance, a basic quantity that governs how electrons move at the smallest conceivable scales.

The researchers reached this unusual state by tuning the electron density to a special condition known as the Dirac point, where graphene hovers exactly between being a metal and an insulator.

At this critical point, electrons stop acting 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 fluid — 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 transforms graphene into a tabletop window into extreme physics.

Scientists can now investigate phenomena usually associated with black hole thermodynamics and high-energy particle collisions without leaving their labs.

The Dirac fluid might enable highly sensitive quantum sensors capable of detecting faint 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 collective electron behavior collide.

Via Science Daily

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