Quantum researchers are searching for a topological superconductor – a material that, if discovered, would allow major advances in quantum computing by combining two distinct “flavors” of superconductivity.
This potent material remains elusive, but Cornell researchers have demonstrated that uranium ditelluride can exhibit both flavors at once, setting it apart as the leading candidate.
“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 in the College of Arts and Sciences and director of the Laboratory of Atomic and Solid State Physics.
“Thermodynamic Discovery of Tetracriticality and Emergent Multicomponent Superconductivity in UTe2” published in Nature Physics on Oct. 30 with Ramshaw as the corresponding author. Doctoral student Sahas Kamat, M.S. ’25, is the first author.
In uranium ditelluride, one type of superconductivity emerges under regular conditions: extreme cold. Another shows up only in very strong magnetic fields or under squeezing pressure.
In experiments redrawing and correcting uranium ditelluride’s phase diagram – the map of where superconductivity emerges as a function of temperature and pressure or a magnetic field – researchers in the Ramshaw lab show the potential is there, although not proven yet, for both flavors to mix together to form a topological superconductor, settling years of uncertainty surrounding this much-studied material.
The lab’s precise pulse ultrasound experiments also measured a unique situation where one type of superconductivity is lost as temperature decreases, a reversal of the usual pattern.
The work stems from the Ramshaw group’s main focus: looking for new types of superconductors.
“We’re trying to find not just different compounds but different types of superconductivity that can be used to do different things,” he said. “There are new superconducting materials being discovered all the time. There have been 6,000 catalogued already. But most of those are basically the same kind of superconductor – they do superconductivity in the same way.”
Roughly speaking, electrons in a superconductor can orbit each other, Ramshaw said; different types – or “flavors” – of superconductivity are defined by the amount and orientation of angular momentum that two electrons have when paired together in a superconducting state.
There are other compounds that can show more than one type of superconductivity, Ramshaw said. For instance, one rare heavy-fermion superconductor changes what type it is as a function of magnetic field.
“The important thing about uranium ditelluride is that it can show two different types at the same time,” he said.
Previous research has shown that a very strong magnetic field will produce a new flavor of superconductivity in uranium ditelluride. Kamat was looking into whether pressure would bring it about, too. His initial search dug up more fundamental problems with previous published research on what uranium ditelluride does under pressure.
“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,” Kamat said. “That bugged me quite a bit.”
Thermodynamics is about as solid a theory as there is in all of science, Ramshaw said, so there had to be another answer.
“So, we went about making this map on our own, independent of what people had done in the past,” Kamat said. “That led us to finding another kind of superconductivity in this material under pressure.”
The pressure was extreme, about 18,000 pounds focused on five millimeters, “like nine cars balanced on your pinky,” Kamat said. The pressure had to be locked in at room temperature. Then the temperature went down and down, to between 2 and 1 kelvin.
The resulting phase diagram shows a section of superconductivity caused by deep cold, a separate section of superconducting caused by pressure (the “special flavor”), and then a section where the two coexisted and interacted during the experiment.
“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.”
Moreover, the two types of superconductivity compete as they interact, leading to one competing with the other and making it disappear as the temperature goes down. It’s remarkable, Ramshaw said, that one type of superconductivity actually goes away as the temperature drops. It’s theoretically possible, but this is the first time it has been demonstrated in an experiment.
“It can happen here because another superconductor comes in and pushes it out of the way,” Kamat said.
An application to quantum computing, although possible, is several steps away, Ramshaw said. At least one tech company has spent billions of dollars trying to engineer a topological superconductor, without success so far, the researchers said.
“The parallel approach is that nature has at least 6,000 superconductors we’ve found. One of them might be that superconductor,” Ramshaw said. “We’ve established that this is the most likely one we have.”
Contributing researchers to the paper are from the University of Maryland, Karlesruhe Institute of Technology and the Canadian Institute for Advanced Research, where Ramshaw is co-director of the Quantum Materials program.
The research was supported by the U.S. Department of Energy, the Gordon and Betty Moore Foundation, the NIST Center for Neutron Research and the Maryland Quantum Materials Center, the German Research Foundation and the Simons Foundation.
