A new study suggests that the Wiedemann-Franz law, which links electronic conductivity and thermal conductivity, remains valid for copper oxide superconductors. Research shows that differences in quantum materials arise from non-electronic factors such as lattice vibrations. This discovery has important implications for understanding unconventional superconductors and may advance the field. This surprising result is important for understanding unconventional superconductors and other materials in which electrons come together to act collectively.
Long before researchers discovered electrons and their role in producing electric current, they had understood electricity and were exploring its potential. They learned early on that metals are the best conductors of electricity and heat.
In 1853, two scientists discovered that there was a connection between these two amazing properties of metals: At any given temperature, the ratio of electronic conductivity to thermal conductivity was roughly the same for any metal they tested. This so-called Widmann-Franz law still applies today - except in quantum materials, where electrons no longer behave like individual particles but clump together, forming a kind of electron soup. Experimental measurements show that this 170-year-old law is broken in these quantum materials.
Illustration: Strongly interacting electrons carry heat and charge from hotter regions of a quantum material to cooler regions. A theoretical study by SLAC, Stanford University, and the University of Illinois found that in quantum materials like cuprates—where electrons clump together and act cooperatively—the ratio of heat transfer to charge transfer should be similar to the ratio in ordinary metals where the electrons act as individuals. This surprising result overturns the 170-year-old idea that the Widmann-Franz law does not apply to quantum materials. Source: Greg Stewart/SLAC National Accelerator Lab
New insights into quantum materials
Now, a theoretical argument by physicists at the U.S. Department of Energy's SLAC National Accelerator Laboratory, Stanford University, and the University of Illinois suggests that the law actually applies roughly to a type of quantum material—copper oxide superconductors, or cuprates—that conduct electricity at relatively high temperatures without loss.
They proposed in a paper published in the journal Science on November 30 that if only electrons in cuprate are considered, the Wiedemann-Franz law should still roughly hold. They suggested that other factors, such as vibrations in the material's atomic lattice, must explain why the experimental results did not appear to apply to the law.
Learn about unconventional superconductors
Wang Wen (transliteration), the first author of the paper and a doctoral student at SLAC's Stanford Institute for Materials and Energy Sciences (SIMES), said that this surprising result is very important for understanding unconventional superconductors and other quantum materials.
"The original law was proposed for materials in which electrons interact weakly and behave like small balls bouncing off defects in the material's crystal lattice," Wang said. "We wanted to theoretically test the law in a system where neither situation exists."
Superconducting materials were discovered in 1911 and can carry electrical current without resistance. But they operate at extremely low temperatures and have very limited uses.
This changed in 1986, when the first so-called family of high-temperature or unconventional superconductors - copper oxides - was discovered. Although cuprates still need extremely cold conditions to work their wonders, their discovery raises hope that superconductors could one day operate closer to room temperature, making revolutionary technologies like lossless power lines possible.
After nearly four decades of research, this goal remains elusive, although great progress has been made in understanding the conditions that bring superconducting states in and out.
Theoretical Research and the Role of Hubbard’s Model
Theoretical studies with the help of powerful supercomputers are crucial to interpret experimental results on these materials and to understand and predict phenomena that are not possible experimentally.
In this study, the SIMES team performed simulations based on the so-called Hubbard model, which has become an important tool for simulating and describing systems in which electrons stop acting independently and join forces to produce unexpected phenomena.
The results show that if only electron transport is considered, the ratio of electronic conductivity to thermal conductivity is close to the value predicted by the Widmann-Franz law, so the differences seen in the experiment should come from other aspects, such as phonons or lattice vibrations, which are not in Hubbard's model.
future research directions
While the study didn't investigate how the vibrations caused the difference, "somehow the system still knew that there was this correspondence between charge and heat transfer between electrons," said SIMES scientist and paper co-author Brian Moritz. "That was the most surprising result."
Reference "Wiedemann-Franz law in doped Mott insulators without quasiparticles", author: Wang Wennao, Ding Jixun, Yoni Schattner, Edwin W. Huang, Brian Moritz and Thomas P. Devereaux, November 30, 2023, "Science".
DOI:10.1126/science.ade3232
Compiled source: scitechdaily