A flower-like pattern exposes chiral superconductivity's long-sought fingerprint

 

With a carefully designed experiment and a handful of tin atoms, University of Tennessee, Knoxville's physicists have found a long-sought form of superconductivity, taking one more step toward creating custom quantum materials.

Scientists have known about superconductivity for more than a century. At low temperatures, resistance in certain materials vanishes and they carry electrical current without losing any energy. Superconductors are part of particle accelerators and magnetic resonance imaging machines. While they need extremely cool environments to work, the mechanism that drives them is quite well understood: electrons, which normally repel each other, form pairs and carry the current.

Twists and turns

Chiral superconductivity is another story. Here, electron pairs reject the typical symmetry and twist into a signature left or right "handedness." Scientists have searched for this phase for decades because it has promise for quantum technologies.

In 2023, Chancellor's Professor Hanno Weitering and Bains Professor Steve Johnston published findings in Nature Physics reporting that strategically scattering tin atoms across a silicon base could give rise to a superconductor. They proposed the system could also be a potential chiral superconductor. This latest work, outlined in Physical Review X, provides the evidence.

The research team carefully deposited one-third layer of tin atoms on a silicon substrate, then used sophisticated imaging to pick up distinctive chiral superconductivity patterns.

Simply beautiful interference

Weitering said the structural and electronic simplicity of the tin-silicon material is the key to seeing chirality. More complex materials have overlapping states and multiple interactions that can mask the telltale patterns.

In this system, one-third layer of tin means a controlled deposition of atoms placed relatively far apart on a silicon layer. Those atoms spontaneously organize into a nicely ordered triangular lattice. The geometry is important.

"Chirality is non-existent in high-temperature cuprate superconductors because they have a square lattice," Weitering said, which gives you a different phase. "But in the tin layer, it exists because the lattice isn't square. It's a triangle."

To see chiral superconductivity's distinctive fingerprint, he and his colleagues turned to quasiparticle interference imaging, or QPI.

"In condensed matter, these particles are always moving in a surrounding that affects their behavior, so they're not really a single entity anymore," Weitering said. "They're all under the influence of their surroundings. That's why we call them quasiparticles."

If you picture electrons behaving like waves, he explained, and think about throwing stones in a pond, one after another in different spots, you'll see waves start to run into each other.

"We call those interference patterns," he said. "This is where the interference comes from: quasiparticle interference. With the scanning tunneling microscope (STM) we can see those waves. Quasiparticles interfere and give these beautiful patterns."

The calling card for chirality is encoded in these patterns surrounding a point defect; a single atom defect to be precise.

"That could be a missing tin atom; it could be a tin atom that has been replaced by a silicon atom because there's a big reservoir of silicon underneath and sometimes atoms do that," Weitering said. "You can never get a crystal perfect. There are always some defects in there. One crucial point in this system is that with the STM we can see each and every point defect in the tin layer."

This research could serve as a template for using QPI to find other forms of unconventional superconductivity. "Most superconductors are discovered through serendipity," Weitering said. "This was all by design."

Customizing materials is an important step on the long journey to possible applications. "Quantum materials are usually not very useful unless you can make devices out of them," he explained. "To make devices you usually need to make thin films and interfaces."

He said that chiral superconductors are topological, and "people are excited about topological systems because we could use topological superconductors to build, for instance, qubits for quantum computing."

Qubits store or process information that's highly sensitive to outside influences like temperature, everyday radiation, etc.

"Topological systems are interesting in a sense that topological property is not something that's local," Weitering said. "It's global. By making it global it's much less sensitive to perturbations."

Showing off for friends

Though the tin-silicon system was designed with intention, recognizing the chirality pattern actually did involve a little bit of serendipity.

Weitering's former postdoctoral research associate, Fangfei Ming, is now a professor in China. He sent QPI images of the material taken by his group, showing how the detail has improved over time—specifically patterns that resemble flowers.

Weitering was impressed and showed the results to Assistant Professor Ruixing Zhang. "Ruixing looked at this pattern and saw something very striking that was at the center of the page," he said.

Zhang went back to his office and had his graduate students (Zhuo Chen and Yuchang Cai) do some numerical and analytical calculations, proving that only chiral superconductivity will result in the flower-like QPI pattern with an atomic-size hole at the center—chiral fingerprints.

"Ruixing saw that and his intuition was right," Weitering said, admitting that, at first, he was more focused on the sharpness of the images when he shared them with Zhang. "I was just showing off," he joked.

Having friends to work with (and impress) is a critical element to moving fundamental research forward. "If you do science by yourself, you're never going to get there," Weitering said.

He, Zhang, Johnston, Chen, and Cai all contributed to the PRX research, along with colleagues from China. He and Zhang are also reviewing patterns from QPI images and plan to create a database. They hope to train a machine learning program to recognize the intricate details of those patterns.

"It's really part of the MRSEC (the university's Materials Research Science and Engineering Center)," Weitering said. "It's using AI to analyze data. It's curating data. You need to train AI with data. It's not just theory; it's also experimental data. Then there's the validation component of theory predictions."

Verifying where the theory leads next is a big part of his work. First there was the Nature Physics research; now the PRX findings.

"We found superconductivity," Weitering said. "Then we found chiral superconductivity. Every time we get a step further."