Although theorists have previously predicted the cause of the unusual properties of the material known as kagome metal, the phenomenon underlying these properties has been observed in the laboratory for the first time.
“We hope that our new understanding of the electronic structure of kagome metal will help us create a rich platform for the discovery of other quantum materials. This, in turn, could lead to a new class of superconductors, new approaches to quantum computing and other quantum technologies,” said Riccardo Comin, assistant professor of physics at MIT, whose group led the study.
Kagome metal is a new quantum material that exhibits the exotic properties of quantum mechanics on a macroscopic scale. In 2018, Komin and Joseph Chekelsky, Mitsui Associate Professor of Physics at MIT, led the first study on the electronic structure of kagome metals, sparking interest in this family of materials. Members of the kagome family of metals are composed of layers of atoms arranged in repeating units, similar to the Star of David or the American Sheriff’s badge. This pattern is also common in Japanese culture, especially as a basket weaving motif.
“This new family of materials has attracted a lot of attention as a rich new playground for quantum matter, which can exhibit exotic properties such as unconventional superconductivity, nematicity, and charge density waves,” Komin noted.
Superconductivity and hints of wave order of charge density in the new family of kagome metals studied by Komin and colleagues were discovered in the laboratory of Professor Steven Wilson at the University of California at Santa Barbara, where single crystals were also synthesized. The kagome material studied in the work consists of only three elements – cesium, vanadium and antimony – and has the chemical formula CsV3Sb5.
The researchers focused on two exotic properties that kagome metal exhibits when cooled below room temperature. One of these is superconductivity, which allows a material to conduct electricity extremely efficiently. In ordinary metal, electrons behave in the same way as people dancing alone in a room. In a kagome superconductor, when the material is cooled to 3 Kelvin (-270°C), the electrons begin to move in pairs, like pairs dancing.
“And all these couples move in unison, as if they were part of a quantum choreography,” Komin explained.
At 100 kelvins (-173 °C), kagome exhibits another strange behavior known as charge density waves. In this case, the electrons line up in ripples, much like a sand dune.
“Charge density waves are very different from superconductors, but they are still a state of matter in which electrons must be arranged in a collective, highly organized manner. They form a choreography again, but they don’t dance anymore. Now they form a static pattern, ”said the scientist.
“What makes electrons “talk” to each other, influence each other? This is the key question. By examining the electronic structure of this material, we found that the electrons exhibit an intriguing behavior known as an electron singularity,” said Mingu Kang, a graduate student in the Department of Physics at the Massachusetts Institute of Technology.
The team of researchers found that in kagome metal, all electrons moving at different speeds have the same energy. It is known that when many electrons with the same energy exist simultaneously in a material, they interact much more strongly. As a result of these interactions, electrons can pair and become superconductive or otherwise form charge density waves. The presence of this singularity is, according to scientists, the very “secret sauce” that ensures the quantum behavior of kagome metals.
A new understanding of the relationship between energy and velocities in the kagome material “is also important because it will allow us to establish new design principles for the development of new quantum materials,” Komin said. According to him, scientists now know how to find a similar singularity in other systems.