Physicists are uncovering materials so strange, they seem to break the very laws of nature. From "weird metals" where electrons appear to split into separate charges and spins, to twisted bilayer graphene that becomes superconductive with a rotation of just 1.1°, these discoveries are defying long-held beliefs in solid-state physics. One class of materials, known as kagome metals, exhibits behavior that doesn’t fit into existing quantum models where particles flow with no resistance or behave as if mass and charge are decoupled. These phenomena challenge the Standard Model and could unlock a new era in quantum computing, energy transmission, and material science. As researchers push deeper, one thing is clear: we may be standing on the edge of a physics revolution.
Unusual properties
Superconductivity and hints of charge density wave order in the new family of kagome metals studied by Comin and colleagues were discovered in the laboratory of Professor Stephen Wilson at the University of California at Santa Barbara, where single crystals were also synthesized (Wilson is a coauthor of the Nature Physics paper). The specific kagome material explored in the current work is made of only three elements (cesium, vanadium, and antimony) and has the chemical formula CsV3Sb5.
The researchers focused on two of the exotic properties that a kagome metal shows when cooled below room temperatures. At those temperatures, electrons in the material begin to exhibit collective behavior. “They talk to each other, as opposed to moving independently,” says Comin.
One of the resulting properties is superconductivity, which allows a material to conduct electricity extremely efficiently. In a regular metal, electrons behave much like people dancing individually in a room. In a kagome superconductor, when the material is cooled to 3 kelvins (about -454 degrees Fahrenheit) the electrons begin to move in pairs, like couples at a dance. “And all these pairs are moving in unison, as if they were part of a quantum choreography,” says Comin.
At 100 K, the kagome material studied by Comin and collaborators exhibits yet another strange kind of behavior known as charge density waves. In this case, the electrons arrange themselves in the shape of ripples, much like those in a sand dune. “They’re not going anywhere; they’re stuck in place,” Comin says. A peak in the ripple represents a region that is rich in electrons. A valley is electron-poor. “Charge density waves are very different from a superconductor, but they’re still a state of matter where the electrons have to arrange in a collective, highly organized fashion. They form, again, a choreography, but they’re not dancing anymore. Now they form a static pattern.”
Comin notes that kagome metals are of great interest to physicists in part because they can exhibit both superconductivity and charge density waves. “These two exotic phenomena are often in competition with one another, therefore it is unusual for a material to host both of them.”
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