Finding superconductivity in nickelates

The study of superconductivity is riddled with disappointments, dead ends and accidental discoveries, according to Antia Botanaprofessor of physics at Arizona State University.

“As theorists, we generally fail to predict new superconductors,” she said.

However, in 2021 she experienced the pinnacle of her early career. In collaboration with experimenter Julia Mundy at Harvard University, she discovered a new superconducting material: a five-layer nickelate. They reported their findings in Natural materials in September 2021.

“It was one of the best moments of my life,” recalls Botana. “I flew back from Spain and received a message from my co-worker Julia Mundy during my layover. When I saw the resistivity drop to zero, there’s nothing better than that.”

Botana was chosen as a 2022 Sloan Research Fellow† Her research is supported by a CARRIAGE price from the National Science Foundation (NSF).

“Prof. Botana is one of the most influential theorists on unconventional superconductivity, particularly in layered nickelates that have received tremendous attention from the materials and condensed matter communities,” said Serdar Ogut, program director in the National Science Department of Materials Research. Foundation. “I expect that her groundbreaking theoretical studies, in collaboration with leading experimenters in the US, will continue to push the boundaries, result in the discovery of new superconducting materials and uncover fundamental mechanisms that could one day pave the way for room temperature superconductivity. †

Superconductivity is a phenomenon that occurs when electrons pair up instead of traveling in isolation, repelling all magnetism and allowing electrons to travel without losing energy. Developing superconductors at room temperature would enable lossless electricity transmission and faster, cheaper quantum computers. Studying these materials is the domain of condensed matter theory.

“We’re trying to understand what are called quantum materials — materials where all the classics we learned in our undergraduate studies are falling apart and nobody understands why they do the fun things they do,” Botana joked.

She started to investigate nickelateslargely, to better understand cuprates — copper oxide-based superconductors that were first discovered in 1986. Thirty years later, the mechanism that produces superconductivity in these materials is still hotly contested.

Botana approaches the problem by looking at materials that resemble cuprates. “Copper and nickel are next to each other on the periodic table,” she said. “This was an obvious case, so people had been looking at nickelates for a long time with no success.”

But then, in 2019, a team from Stanford discovered superconductivity in a nickelate, albeit one that had been ‘dipped’ or chemically modified to improve its electronic properties. “The material they found in 2019 is part of a bigger family, and that’s what we want because it allows us to make comparisons to cuprates in a better way,” she said.

Botana’s 2021 discovery built on that foundation, using a form of undoped nickelate with a unique, square-planar, layered structure. She decided to investigate this particular form of nickelate – a rare-earth, five-layer, square-plane nickelate – based on intuition.

“After playing 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 with my mentors over the years.”

By identifying another form of superconducting nickelate, researchers can discover similarities and differences between nickelates and between nickelates and cuprates. So far, the more nickelates studied, the more they resemble cuprates.

“The phase diagram is very similar. The electron pairing mechanism appears to be the same,” Botana says, “but this is a question that remains to be resolved.”

Conventional superconductors exhibit s wave pairs – electrons can pair in any direction and sit on top of each other, so the wave is a sphere. Nickelates, on the other hand, likely exhibit d-wave pairs, meaning that the cloud-like quantum wave describing the paired electrons is shaped like a four-leaf clover. Another important difference is how strongly oxygen and transition metals overlap in these materials. Cuprates exhibit a large ‘super exchange’ – the material trades electrons in copper atoms through a pathway containing oxygen, rather than directly.

“We think this is one of the factors that regulates superconductivity and causes the lower critical temperature of the nickelates,” she said. “We can look for ways to optimize that property.”

Botana and colleagues Kwan-Woo Lee, Michael R. Norman, Victor Pardo, Warren E. Pickett described some of these differences in a review article for Limits in physics in February 2022.

SEARCHING FOR CAUSES OF SUPER SENSITIVITY

Signing up Physical Assessment X in March 2022, Botana and collaborators at Brookhaven National Laboratory and Argonne National Labs took a closer look at the role of oxygen states in the low-grade nickelate, La4Ni3O8. Using computational and experimental methods, they compared the material with a prototypical cuprate with a similar electron filling. The work was unique in that it directly measures the energy of the hybridized nickel-oxygen states.

They found that, despite requiring more energy to transfer charges, nickelates retained significant superexchange capacity. They conclude that both the “Coulomb interactions” (the attraction or repulsion of particles or objects due to their electrical charge) and charge transfer processes should be considered when interpreting the properties of nickelates.

The quantum phenomena Botana studies occur on the smallest known scale and can only be examined obliquely through physical experiments (as in the Physical Review X paper). Botana uses computer simulations to make predictions, help interpret experiments and infer the behavior and dynamics of materials such as infinite-layer nickelate.

Her research uses Density functional theoryor DFT — a way to computationally solve the Schrödinger equation describing the wavefunction of a quantum mechanical system — as well as a newer, more precise offshoot known as dynamic mean field theory that can handle electrons that are highly correlated.

To conduct her research, Botana uses the Stampede2 supercomputer at the Texas Advanced Computing Center (TACC) — the second fastest at any university in the U.S. — and Arizona State University machines. Even on the fastest supercomputers in the world, studying quantum materials is no easy matter.

“If I see a problem with too many atoms, I say, ‘I can’t study that,'” Botana said. “Twenty years ago, a few atoms may have seemed too many.” But more powerful supercomputers allow physicists to study larger, more complicated systems — such as nickelates — and add tools, such as 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, Botana says.

“Your phone or computer wouldn’t be possible without research in condensed matter physics – from the screen, to the battery, to the tiny camera. It’s important for the public to understand that even when it comes to basic research, and even if the researchers don’t know how it will be used later, this kind of materials research is critical.”

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