Cornell Physicists Show Uranium Ditelluride Hosts Two Superconducting 'Flavors' at Once

Physicists at Cornell University have shown that a uranium compound can carry two different kinds of superconductivity at the same time. That makes it the leading candidate for a topological superconductor, a material that has been sought for years because it could enable major advances in quantum computing.

"No one has yet convincingly demonstrated a topological superconductor, but now we know that uranium ditelluride at least has the right ingredients to make it happen," said Brad Ramshaw, associate professor of physics and director of Cornell's Laboratory of Atomic and Solid State Physics. The paper, "Thermodynamic Discovery of Tetracriticality and Emergent Multicomponent Superconductivity in UTe2," was published in Nature Physics on Oct. 30. Doctoral student Sahas Kamat is the first author.

In a superconductor, electrons pair up and flow without resistance. Ramshaw explained that different "flavors" of superconductivity are defined by the amount and orientation of angular momentum the two paired electrons have. About 6,000 superconductors have been cataloged, he said, but most of them superconduct in basically the same way.

In uranium ditelluride, one type of superconductivity appears in extreme cold. Another shows up only in a very strong magnetic field or under squeezing pressure. Kamat set out to see whether pressure would bring out the second type and found fundamental problems with earlier published work. "The map people had in their heads about what the superconductor does as a function of temperature and pressure, it turns out it's not consistent with thermodynamics," he said. "That bugged me quite a bit."

So the group redrew the phase diagram, the map of where superconductivity appears as temperature and pressure change, on its own. The experiment squeezed the sample with about 18,000 pounds of force focused on five millimeters, "like nine cars balanced on your pinky," Kamat said. The pressure had to be locked in at room temperature, and the team then cooled the sample to between 2 and 1 kelvin.

The new diagram shows a region of superconductivity caused by deep cold, a separate region caused by pressure, and a region where the two coexist and interact. "We showed that one type of this orbital state exists on one part of the map and another type of this orbital state exists on another part of the map, and they intersect," Kamat said. "In that intersecting region, you have both happening at the same time."

The ultrasound measurements also caught a first. The two types of superconductivity compete, and one of them disappears as the temperature drops, the reverse of the usual pattern. Ramshaw said this is theoretically possible but had never been demonstrated in an experiment. "It can happen here because another superconductor comes in and pushes it out of the way," Kamat said.

The result does not prove that uranium ditelluride is a topological superconductor. It establishes that the ingredients are there. Ramshaw cautioned that any use in quantum computing is several steps away, and he noted that at least one technology company has spent billions of dollars trying to engineer such a material without success. "The parallel approach is that nature has at least 6,000 superconductors we've found. One of them might be that superconductor," he said. "We've established that this is the most likely one we have." Researchers from the University of Maryland, Karlsruhe Institute of Technology and the Canadian Institute for Advanced Research also contributed.