Unlocking the secrets of a superconductor
When superconductors were discovered in 1911, they astounded researchers with their ability to conduct electricity with no resistance. However, they could only do so at temperatures close to absolute zero. But 1986, scientists discovered that cuprates (a class of copper oxides) were superconductive at a relatively warm -225 degrees Fahrenheit (above liquid nitrogen) - a step toward the ultimate goal of a superconductor that could operate at close to room temperature. Applications of such a superconductor include compact and portable MRI machines, levitating trains, long-range electrical transmission without power loss, and more resilient quantum bits for quantum computers. Unfortunately, cuprates are a type of ceramic materials, which makes their application at industrial scales difficult - their brittleness, for example, would pose problems. However, if researchers could understand what makes them superconduct at such high temperatures, they could recreate such processes in other materials. Despite a great deal of research, though, there is still a lack of consensus on the microscopic mechanism leading to their unusual superconductivity, making it difficult to take advantage of their unusual properties.
“There’s a lot of excitement about them, but it's been very hard to figure out what makes them tick. As in, why are the critical temperatures for superconductivity so high?” said Sohrab Ismail-Beigi, the Strathcona Professor of Applied Physics. “There's no clear consensus on a detailed microscopic understanding of how the structure of these materials is related to the properties. There are still many unknowns about these materials, even though people have been studying them now for 30 years or so.”
In a new study, Zheting Jin, a senior graduate student with Ismail-Beigi, together used new methodologies to get a better understanding of some key structure-property connections in materials. The results are published in Physical Review X.
One reason it’s so difficult to figure out cuprates, Ismail-Beigi said, is because they’re so structurally complex. Researchers had created simplified models of these materials, but these simple models often failed to provide much in the way of valuable or reliable information.
To get a clearer picture of how cuprates work, Ismail-Beigi and Jin use what’s known as “density functional theory” to explore the significance of structural complexity in cuprates by accurately depicting key structural, electronic, and magnetic properties of these materials. In doing so, “we resolve several long-standing puzzles in this material.”
“Can you use this theory to predict many properties of cuprates? The answer is yes,” Ismail-Beigi said. “You have to just do two things. One of them is to use an up-to-date methodology for the calculation. The more important thing is to include the actual complicated structure of the material, because it matters.”
Cuprates are interesting both for basic and applied sciences. Engineers, he said, want to know how they can increase the superconducting temperatures of cuprates. Physicists want to know, from a basic science point of view, why cuprates are such good superconductors. A key to success, he said, is taking the complexity of the structures into account.
“What is the magic sauce? If you can describe the complex structure of the materials very well and see which structural motifs give rise to which physical properties, perhaps this will help us figure out what makes cuprates tick,” he said. “The structure is complicated and influences the properties in important ways.”
It's also critical for theorists and experimentalists to work together. Ismail-Beigi said his lab is currently working closely on the cuprate problem with multiple research teams including experimental groups in Yale’s Applied Physics department as well as with theorists who use complementary methods, at the University of California, Irvine.
“I think that's what the future will look like for the next decade. We will do detailed simulations of these complex materials from first principles and then do a good deal of hand-shaking with other theorists and experimentalists to slowly chip away at this problem.” Reference First-Principles Prediction of Structural Distortions in the Cuprates and Their Impact on the Electronic Structure
Zheting Jin and Sohrab Ismail-Beigi
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