February 26, 2024

Researchers show that an ancient law still holds true for peculiar quantum materials

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An illustration shows electrons interacting strongly, transporting heat and charge from hotter to cooler regions of a quantum material. A theoretical study carried out by SLAC, Stanford and the University of Illinois found that the ratio of heat transport to charge transport in cuprates – quantum materials like this where electrons cluster together and act cooperatively – should be similar to the ratio in normal metals, where electrons behave as individuals. This surprising result overturns the idea that the 170-year-old Wiedemann-Franz law does not apply to quantum materials. Credit: Greg Stewart/SLAC National. Accelerator Laboratory

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An illustration shows electrons interacting strongly, transporting heat and charge from hotter to cooler regions of a quantum material. A theoretical study carried out by SLAC, Stanford and the University of Illinois found that the ratio of heat transport to charge transport in cuprates – quantum materials like this where electrons cluster together and act cooperatively – should be similar to the ratio in normal metals, where electrons behave as individuals. This surprising result overturns the idea that the 170-year-old Wiedemann-Franz law does not apply to quantum materials. Credit: Greg Stewart/SLAC National. Accelerator Laboratory

Long before researchers discovered the electron and its role in generating electric current, they already knew about electricity and were already exploring its potential. One thing they learned early on was that metals were excellent conductors of electricity and heat.

In 1853, two scientists showed that these two admirable properties of metals were somehow related: at any temperature, the relationship between electronic conductivity and thermal conductivity was approximately the same in any metal tested. This so-called Wiedemann-Franz law has been valid ever since — except in quantum materials, where electrons stop behaving like individual particles and clump together in a kind of electron soup. Experimental measurements have indicated that the 170-year-old law breaks down in these quantum materials, by a lot.

Now, a theoretical argument presented by physicists at the Department of Energy’s SLAC National Accelerator Laboratory, Stanford University and the University of Illinois suggests that the law should, in fact, hold roughly for one type of quantum material – oxide superconductors. copper, or cuprates, which conduct electricity without loss at relatively high temperatures.

In an article published in Science today, they propose that the Wiedemann-Franz law should still be valid if we consider only the electrons in cuprates. They suggest that other factors, such as vibrations in the material’s atomic lattice, must be responsible for experimental results that make it appear that the law does not apply.

This surprising result is important for understanding unconventional superconductors and other quantum materials, said Wen Wang, lead author of the paper and a Ph.D. student at the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC.

“The original law was developed for materials where electrons interact weakly with each other and behave like small balls that bounce off defects in the material’s lattice,” Wang said. “We wanted to test the law theoretically in systems where none of these things were true.”

Peeling a Quantum Onion

Superconducting materials, which carry electrical current without resistance, were discovered in 1911. But they operated at such extremely low temperatures that their usefulness was quite limited.

This changed in 1986, when the first family of so-called high-temperature or unconventional superconductors – the cuprates – were discovered. Although cuprates still require extremely cold conditions to work their magic, their discovery has raised hopes that superconductors could someday operate at temperatures much closer to room temperature – making revolutionary technologies such as lossless power lines possible. .

After nearly four decades of research, that goal is still elusive, although much progress has been made in understanding the conditions under which superconducting states enter and disappear.

Theoretical studies, carried out with the aid of powerful supercomputers, have been essential for interpreting the results of experiments with these materials and for understanding and predicting phenomena that are beyond experimental reach.

For this study, the SIMES team carried out simulations based on the so-called Hubbard model, which has become an essential tool for simulating and describing systems where electrons stop acting independently and join forces to produce unexpected phenomena.

The results show that when only electron transport is taken into account, the relationship between electronic conductivity and thermal conductivity approaches what the Wiedemann-Franz law predicts, Wang said. “So the discrepancies that were observed in experiments would have to come from other things, like phonons, or lattice vibrations, which are not in Hubbard’s model,” she said.

SIMES staff scientist and paper co-author Brian Moritz said that although the study did not investigate how vibrations cause the discrepancies, “somehow the system still knows that there is this correspondence between charge and heat transport between the electrons.” That was the most surprising result.”

From here, he added, “maybe we can peel the onion to understand a little more.”

More information:
Wen O. Wang et al, The Wiedemann-Franz law on doped Mott insulators without quasiparticles, Science (2023). DOI: 10.1126/science.ade3232. www.science.org/doi/10.1126/science.ade3232

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