Trinity-led study makes discovery that could shock you in reverse

19 Apr 2022

Image: © chevanon wonganuchitmetha/EyeEm/

Currents were found to flow from cold to hot temperatures in some materials, described as ‘almost like watching a movie in reverse’.

Scientists in Ireland and Spain have made a peculiar discovery that seems to go against one of the fundamental laws of physics – the second law of thermodynamics.

While energy always moves from hotter regions to colder regions, in accordance with the law, the scientists found a potential exception to this rule in so-called topological materials, where current can appear to flow from colder to hotter regions.

The research, published in the journal Physical Review Letters last month, looks at how a quantum effect forces current passing through a piece of matter to flow around its edges – and sometimes against the typical direction of heat transfer.

These ‘robust edge currents’ have been known for a long time to exist in topologically non-trivial materials. A trivial topology is a simple one with no splits or loops, such a ball. An example of a non-trivial topology would be a doughnut or a teacup, both of which have holes in their shape.

“The existence of edge currents in topologically non-trivial materials has been known and understood for decades,” said Prof Mark Mitchison, assistant professor at Trinity College Dublin’s School of Physics and lead author of the study.

“But we didn’t expect to see robust edge currents appear in topologically trivial systems as well.”

Potential applications

Mitchison, along with his colleagues Prof Ángel Rivas and Prof Miguel-Ángel Martin Delgado from the Complutense University of Madrid, found that the counterintuitive edge current is remarkably robust and arises in a wider class of materials than was previously believed.

They also found that the robust edge currents can happen in systems with a temperature gradient, meaning that different parts of the material are at different temperatures.

Researchers said the finding makes it easier to observe the phenomenon in experiments and could potentially lead to the development of new methods to control the flow of energy through nanoscale structures – leading to advancements in materials science and powerful, sustainable computing.

For example, further study in more geometrically complex structures used in real electronic devices could help scientists design more energy efficient processors or circuit elements that can recycle waste heat.

No rules broken

While current was found to flow in the ‘wrong’ direction in certain instances, the net transfer of heat from one part of a system to another is always in compliance with the laws of physics.

“The overall, net transfer of heat is always from the hot to the cold reservoir. The second law of thermodynamics is never violated,” clarified Mitchison. “But locally, on one edge, the current flows in the other direction so a being that lives on that surface would observe very strange physics!” he added.

“The current would be flowing the wrong way from their perspective, almost like watching a movie in reverse.”

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Vish Gain is a journalist with Silicon Republic