In the fascinating world of quantum materials, scientists have uncovered a surprising truth: a 170-year-old law about electricity and heat in metals still holds true, even in the realm of superconductors.
Back in 1853, scientists discovered the Wiedemann-Franz law.
This law states that in metals, the ability to conduct electricity (electronic conductivity) and heat (thermal conductivity) are closely linked. The ratio between these two types of conductivity is almost the same for all metals, at any given temperature.
However, this law faced a challenge with the discovery of quantum materials. In these materials, electrons don’t act as individual particles.
Instead, they merge into a sort of ‘electron soup,’ changing the game in how we understand electrical and heat conductance.
For a long time, experiments suggested that this old law didn’t apply to quantum materials. But now, physicists from SLAC National Accelerator Laboratory, Stanford University, and the University of Illinois have a new theory.
They propose that this law does indeed apply to a specific type of quantum material: copper oxide superconductors, or cuprates.
These are special because they conduct electricity without any loss, even at relatively high temperatures.
In their study, published in the journal Science, they argue that if we focus only on the electrons in cuprates, the Wiedemann-Franz law still makes sense.
They think other factors, like the way atoms vibrate in these materials, might explain why experiments seemed to contradict the law.
Wen Wang, the lead author of the study and a Ph.D. student at the Stanford Institute for Materials and Energy Sciences (SIMES), explains that the original law was based on materials where electrons don’t interact much with each other. In quantum materials like cuprates, electrons behave in more complex ways.
Superconductors, discovered in 1911, can carry electric current without any resistance. This property is hugely beneficial, potentially leading to technologies like power lines that don’t lose any electricity.
The discovery of cuprates in 1986, which work at higher temperatures, brought us closer to this dream. These materials still need very cold conditions to function, but they opened up new possibilities for superconductors operating closer to room temperature.
Understanding these materials has been a complex journey. Scientists use powerful supercomputers to simulate and interpret experiments, helping to reveal phenomena that we can’t directly observe.
The SIMES team used a tool called the Hubbard model for their simulations. This model is crucial for studying systems where electrons work together in unexpected ways.
Their simulations show that if you only consider how electrons transport in cuprates, the ratio of electronic to thermal conductivity aligns with the Wiedemann-Franz law. The differences seen in experiments might be due to other aspects, like phonons or lattice vibrations, which the Hubbard model doesn’t include.
Brian Moritz, a staff scientist at SIMES and co-author of the paper, notes that while the study doesn’t explore how vibrations cause these discrepancies, it’s intriguing that the relationship between charge and heat transport among electrons still exists in these systems.
This finding opens new doors to understanding the intricate world of superconductors.
As researchers continue to unravel these mysteries, they hope to get closer to making superconductors that can operate at even higher temperatures, paving the way for revolutionary technologies in the future.
