The study of superconductivity is littered with disappointments, dead ends and serendipitous findings, according to Antia Botana, professor of physics at Arizona State University.
“As theorists, we usually fail to predict new superconductors,” she said.
Yet in 2021, she experienced the high point of her early career. Together with experimenter Julia Mundy from Harvard University, she discovered a new superconducting material – a five-layer nickelate. They reported their findings in Nature Materials in September 2021.
“It was one of the best times of my life,” Botana recalls. “I was coming back from Spain and I received a message from my colleague Julia Mundy during my stopover. When I saw the resistivity drop to zero, there’s nothing better than that.
Botana was chosen as a 2022 Sloan Research Fellow. His research is supported by a National Science Foundation (NSF) CAREER Award.
“Prof. Botana is one of the most influential theorists in the field of unconventional superconductivity, particularly in layered nickelates which have received considerable attention from the materials physics and condensed matter communities” “, said Serdar Ogut, program director in the Division of Materials Research at the National Science Foundation. “I expect his pioneering theoretical studies, in collaboration with leading experimentalists in the United States, to continue to push the boundaries, lead to the discovery of new superconducting materials and uncover fundamental mechanisms that could one day pave the way to room-temperature superconductivity.”
Superconductivity is a phenomenon that occurs when electrons form pairs rather than traveling in isolation, repelling any magnetism and allowing electrons to travel without losing energy. The development of room-temperature superconductors would allow lossless transmission of electricity and faster, cheaper quantum computers. The study of these materials comes under the theory of condensed matter.
“We’re trying to figure out what’s called quantum materials – materials where everything classical we learned in our undergraduate studies is falling apart and no one understands why they do the fun things they do,” joked Botana.
She began studying nickelates, largely to better understand cuprates – copper oxide superconductors first discovered in 1986. Thirty years later, the mechanism that produces superconductivity in these materials is still hotly contested.
Botana addresses the problem by examining materials that resemble cuprates. “Copper and nickel are side by side in the periodic table,” she said. “It was an obvious thing to do, so people had been looking at nickelates for a long time without success.”
But then, in 2019, a team at Stanford discovered superconductivity in a nickelate, although it had been “doped” or chemically altered to improve its electronic characteristics. “The material they found in 2019 is part of a larger family, which we want because it allows us to better make comparisons to cuprates,” she said.
Botana’s 2021 discovery built on this foundation, using an undoped form of nickelate with a unique, square, planar layered structure. She decided to study this specific form of nickelate – a rare earth, quintuple-layered, square-plane nickelate – based on intuition.
“Having played with many different materials for years, this is the kind of intuition that people who study electronic structure develop,” she said. “I’ve seen that over the years with my mentors.”
The identification of another form of nickelate superconductor allows researchers to unravel the similarities and differences between nickelates and between nickelates and cuprates. So far, the more nickelates are studied, the more they resemble cuprates.
“The phase diagram looks quite similar. The electron pairing mechanism appears to be the same,” Botana says, “but that’s an open question.”
Conventional superconductors exhibit s-wave pairing – electrons can pair up in any direction and can sit on top of each other, so the wave is a sphere. Nickelates, on the other hand, probably display d-wave pairing, meaning that the cloud-like quantum wave that describes the paired electrons is shaped like a four-leaf clover. Another key difference is the strength with which oxygen and transition metals overlap in these materials. Cuprates exhibit large “super-exchange” – the material exchanges electrons in copper atoms via a pathway that contains oxygen, rather than directly.
“We think this may be one of the factors that governs superconductivity and causes the lower critical temperature of nickelates,” she said. “We can look for ways to optimize this characteristic.”
Botana and colleagues Kwan-Woo Lee, Michael R. Norman, Victor Pardo, Warren E. Pickett described some of these differences in a review article for Frontiers in Physics in February 2022.
Searching for the root causes of superconductivity
Writing in Physical Review X in March 2022, Botana and collaborators at Brookhaven National Laboratory and Argonne National Laboratories further investigated the role of oxygen states in the low-valence nickelate, La4Ni3O8. Using computational and experimental methods, they compared the material to a prototype cuprate with similar electronic filling. The work was unique in that it directly measured the energy of Nickel-Oxygen hybrid states.
They found that despite requiring more energy to transfer charge, the nickelates retained considerable superexchange capacity. They conclude that “Coulomb interactions” (the attraction or repulsion of particles or objects due to their electric charge) and charge transfer processes must be taken into account when interpreting the properties of nickelates. .
The quantum phenomena studied by Botana occur at the smallest known scales and can only be probed obliquely by physical experience (as in the article Physical Review X). Botana uses computer simulations to make predictions, help interpret experiments, and infer the behavior and dynamics of materials like infinite-layered nickelate.
His research uses Density Functional Theory, or DFT – a means of computer solving the Schrödinger equation that describes the wave function of a quantum mechanical system – as well as a later and more accurate known branching. under the name of dynamical mean field theory which can deal with electrons which are highly correlated.
To conduct its research, Botana uses the Stampede2 supercomputer from the Texas Advanced Computing Center (TACC) – the second fastest of any US university – as well as machines from Arizona State University. Even on the world’s fastest supercomputers, studying quantum materials is no small feat.
“If I see a problem with too many atoms, I say, ‘I can’t study that,'” Botana said. “Twenty years ago, perhaps a few atoms were too much alike.” But more powerful supercomputers allow physicists to study larger, more complex systems — like nickelates — and add tools, like dynamic mean-field theory, that can better capture quantum behavior.
Despite living in a golden age of discovery, the field of condensed matter physics still doesn’t have the reputation it deserves, says Botana.
“Your phone or computer wouldn’t be possible without condensed matter physics research – from the screen to the battery to the tiny camera. It’s important for the public to understand that even though it’s fundamental research, and even if the researchers do not know how it will be used later, this type of materials research is essential.
- Y. Shen, J. Sears, G. Fabbris, J. Li, J. Pelliciari, I. Jarrige, Xi He, I. Božović, M. Mitrano, Junjie Zhang, JF Mitchell, AS Botana, V. Bisogni, MR Norman , S. Johnston, MPM Dean. Role of oxygen states in the low-valent nickelate La4Ni3O8. Physical examination X, 2022; 12 (1) DOI: 10.1103/PhysRevX.12.011055
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