A material made from yttrium, barium, and copper oxide (better known as YBCO) has intrigued scientists since its discovery in 1987, largely because it retains its superconductive properties at a higher-than-normal temperature. However, it’s extremely brittle, which makes it tricky to put it to any kind of practical use.
But researchers can still learn much from it. For instance, its unusual properties can provide insight into designing possible room-temperature superconductors—that is, materials that conduct electricity with no resistance at room temperature. Doing so would have a huge impact on power transmission, medical imaging, and fusion reactor magnets.
One thing in particular about YBCO that has mystified researchers in the field is that when you dope it with praseodymium, a rare earth element, it completely kills the material’s superconductive properties. That’s unusual since the addition of other rare earth elements to YBCO doesn’t have the same effect. It was not clear why praseodymium did this. Now, a team of researchers led by Professor Yu He in applied physics, in collaboration with the Maple group and Frano group at the University California San Diego, as well as the Xie group in Michigan State University, has figured it out. In doing so, they’ve overturned a decades-old theory and opened up a new line of research into YCBO and other cuprate superconductors based on the control of interlayer coupling. The results are published in The Proceedings of the National Academy of Sciences.
“Going back about 30 years ago, people have been trying to dope—basically substitute some atoms in this material—with the initial goal of making the superconductivity stronger so that we can make use of it at room temperature,” said Jinming Yang, a Ph.D. candidate who led the study. “But in this material, people found that when you substitute most atoms, the superconductivity was not changed, or slightly changed.”
But when scientists substituted yittrium with praseodymium in YBCO, something different happened: they found that its atoms killed YBCO’s superconductivity. Intrigued, researchers in the field began looking into why this happens. The theory that has previously gained the most traction was that praseodymium was directly involved in donating electrons to the copper oxide component of YBCO.
Using a technology known as angle-resolved photoemission spectroscopy (ARPES), however, He and his research team got a closer look at what was going on.
“We can directly measure how charge is redistributed in the praseodymium-doped material and the original YBCO,” Yang said. “And we found that praseodymium isn’t dragging the charge directly, but it’s actually preventing the interaction between the two layers of copper-oxygen planes. Our experimental observation is that it also changes the charge distribution between the two layers, but not in the previous model-predicted way.”
Co-authors Zheting Jin and Sohrab Ismail-Beigi performed complementary theoretical calculations and analysis.
“Our theoretical calculations reveal that praseodymium does not behave as the previous theories posited,” said Ismail-Beigi, the Strathcona Professor of Applied Physics at Yale.
This indicates interlayer coupling as an important new avenue for understanding, and potentially engineering, high-temperature superconductivity.
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Zheting Jin Applied Physics Ph.D. graduate
They note that its incorporation into the lattice reorganizes the copper-oxygen electronic structure and weakens the coupling between two copper-oxide layers in YBCO.
“This indicates interlayer coupling as an important new avenue for understanding, and potentially engineering, high-temperature superconductivity,” said Dr. Jin, a Ph.D. graduate from Yale’s Applied Physics department.
Having a clearer picture of the praseodymium-YBCO interactions could lead to new, critical breakthroughs in the field, and potentially superconductors that function at higher temperatures.
“Now that we understand how praseodymium kills superconductivity, it should help us understand the mechanism of superconductivity in this material,” Yang said.
Co-author M. Brian Maple, a professor at UC San Diego, noted that the study showed that even after decades of study, YBCO and other high-temperature superconductors “are still teaching very important lessons about quantum mechanics.”
“This work helps sharpen our understanding of what controls superconductivity in layered copper oxides,” he said. “Beyond the promise of immediate technological outcomes, we think its value is more fundamental: it tells us which interactions we need to pay attention to, and it gives us a better framework for discovering new phenomena in related materials. This is what fundamental science is all about!”
This story is a duplicate of the Yale Engineering news story of July 8, 2026. See below for a link to the original article and other related links.