Uranium ditelluride shows two superconductivity types at once

Cornell physicists have shown that uranium ditelluride can host both flavours of superconductivity at the same time, according to Cornell University. The result, published in Nature Physics on 30 October, redraws the phase map of the compound and settles years of uncertainty about how it behaves under pressure. It also leaves the material as the leading natural candidate for a topological superconductor, a form of superconductivity that has never been convincingly demonstrated and that could matter for quantum computing.

Superconductors carry electric current without resistance, but they do not all work in the same way. The difference lies in how electrons pair up inside the material, and in the amount and orientation of angular momentum each pair carries. In the compound studied here, a familiar kind of superconductivity appears under extreme cold. A distinct kind appears only under a strong magnetic field or under squeezing pressure. The Cornell team, led by doctoral student Sahas Kamat with Brad Ramshaw as corresponding author, found a region of the redrawn map where those kinds overlap and interact.

What the team found

The work began with a problem in the existing literature. Kamat, investigating whether pressure could produce the distinct flavour previously seen under a magnetic field, found that earlier published accounts of the material under pressure did not square with thermodynamics. Ramshaw described thermodynamics as about as solid a theory as science offers, so the team set out to build the map independently rather than adjust its predecessor. Their tool was precise pulse ultrasound, which tracks changes in a sample as sound pulses pass through it.

The experiments were demanding. Pressure had to be locked in at room temperature before cooling began, and the load involved was comparable to nine cars balanced on a pinky finger, spread across a sample measured in millimetres. Temperatures then fell to between 2 and 1 kelvin, close to absolute zero. Under those conditions the team mapped a cold-driven superconducting region, a separate pressure-driven region, and an overlapping region where both kinds were present together. In that overlapping region the kinds compete, and the contest produces an unusual reversal: a kind of superconductivity vanishes as the temperature falls, pushed out by the other kind.

Ramshaw said the point that matters most is simultaneity. Other compounds can switch between kinds as conditions change, but this material sustains both kinds together in the same region of its phase map. That simultaneity is what a topological superconductor would require, because such a material would combine distinct flavours into a single state. The researchers are clear that they have not yet demonstrated a topological superconductor. What they have shown is that the ingredients for such a state are present in a real material, which narrows a search that has run for years across thousands of catalogued superconductors, reported by Phys.org.

Why the result matters for quantum computing

A topological superconductor is prized because its combined state could protect quantum information in ways that ordinary superconductors cannot. Quantum computers are fragile machines, and errors creep in when their basic units are disturbed by heat, vibration or stray electromagnetic signals. A material whose superconducting state is protected by its own topology could, in principle, hold information more steadily, easing the burden on error correction. That promise explains why a major technology company has spent billions of dollars trying to engineer such a material without success so far, according to the researchers.

The Cornell result offers a different route. Instead of building a topological superconductor from scratch, researchers can study a natural material that already combines the required flavours and ask what else it would take to reach the combined state. The catalogue of known superconductors runs to 6,000 entries, and most of them perform superconductivity in the same basic way. Sorting through that catalogue for a material with the right combination has been slow, and the new phase map gives the search a concrete starting point. Readers following quantum computing coverage will recognise the stakes: steadier quantum hardware would change what such machines can attempt, from modelling molecules to solving optimisation problems that defeat ordinary computers.

Caution is warranted all the same. The overlapping region demonstrates potential, not proof, and the combined state itself remains unobserved. The experiments also took place under conditions far removed from everyday use, with crushing pressure and temperatures near absolute zero. Any practical device would need to bridge an enormous gap between laboratory conditions and workable hardware, and the researchers do not claim to have closed it. Their claim is narrower and, for that reason, stronger: the map is now consistent with thermodynamics, and the material belongs at the head of the queue.

What comes next

The next step is to probe the overlapping region more closely and to test whether the combined state can be coaxed out of it. That work will involve collaborators at the University of Maryland, the Karlsruhe Institute of Technology and the Canadian Institute for Advanced Research, where Ramshaw co-directs the Quantum Materials programme. Support for the research came from the United States Department of Energy, the Gordon and Betty Moore Foundation, the NIST Center for Neutron Research, the Maryland Quantum Materials Center and the German Research Foundation. The full paper is available through Nature Physics.

For science coverage more broadly, the finding is a reminder of how much basic physics still depends on careful measurement rather than new apparatus alone. The team did not discover a new element or build a larger machine. They remeasured a known material, found the old map wanting, and drew a better map. If the combined state eventually appears in this material, the advance will owe as much to that patient redrawing as to any single dramatic experiment.