Bismuth's mask uncovered: Implications for quantum computing and spintronics materials

There is a class of materials that are insulators in their bulk, but robustly conductive at their surface. As this conductivity does not suffer from defects or impurities, such "topological materials," as they are called, are expected to be highly suitable for use in quantum computers, spintronics and other advanced electronic applications. However, whether bismuth is a topological material or not has been under scientific debate for the past almost 20 years, with many calculations showing that it shouldn't be, but certain measurements indicating differently. Kobe University quantum solid state physicist FUSEYA Yuki says: "I have been fascinated by bismuth and have been conducting research with the desire to know everything there is to know about the element. As a bismuth lover, I could not overlook such a situation and delved into the debate, hoping to solve the mystery."

Fuseya's dedication to the material allowed him to consider phenomena others haven't. He explains: "Among the many properties of bismuth I have studied, I was the first to discover that the crystal structure spontaneously changes due to relaxation near the surface of a crystal. This made me wonder whether this surface relaxation might affect the material's topological nature." Thus, the Kobe University researcher and his team took to computer models of the behavior of electrons in the material and incorporated this change in crystal structure to investigate whether they can so contribute to the debate.

The team now published their results in a letter in the journal Physical Review B. Their calculations could prove that the relaxation of the surface of bismuth crystals leads to the material appearing to be topological at the surface, masking that its bulk is non-topological. "Until now, the topology of a material has been determined based on the principle of 'bulk-edge correspondence,' which holds that the characteristics at the surface represent those in the bulk. However, our study shows that this guiding principle can be broken," explains Fuseya.

"Our proposal that surface relaxation can lead to the breaking of bulk-edge correspondence is not limited to bismuth but can be broadly applied to other systems," the Kobe University team writes in their paper. Thus, the effect the researchers call "topological blocking" might be discovered in other materials, too. "The most important thing in topological materials science is to get the topology of matter right," comments Fuseya, hinting at the wide implications his team's work has for the whole field.

For bismuth lover Fuseya, the discovery is personal, too. He explains: "Bismuth has provided the setting for many discoveries and history has taught us that once a phenomenon is discovered there, similar phenomena are discovered in other substances one after another. I am very happy to know that another phenomenon first discovered in bismuth has been added to that list."

This research was funded by the Japan Society for the Promotion of Science (grants 23H00268, 23H04862 and 22K18318). It was conducted in collaboration with researchers from the University of Electro-Communications.