Scientists in Austria have finally uncovered the reason why a strange uranium-based superconductor appears to come back to life under crushing magnetic fields, seemingly defying the conventional laws of physics.

The material, known as uranium ditelluride (or UTe2), is an unconventional heavy-fermion superconductor. It is widely believed to be a spin-tripled superconductor, and it has puzzled physicists since its accidental discovery in 2019.

The reason lies in its unusual behavior compared to most superconductors, which allow electrical current to flow with zero resistance, particularly at extremely low temperatures. In most cases, strong magnetic fields destroy superconductivity.

However, UTe2 first loses its superconducting state at magnetic fields of roughly 10 Tesla (more than three times stronger than those used in most MRI scanners), only to mysteriously regain zero resistance again under vastly stronger magnetic fields between 40 and 70 Tesla. A single Tesla can lift a car in a scrapyard.

Defying quantum physics

Scientists at the Institute of Science and Technology Austria (ISTA), located in the town of Klosterneuburg, carried out extensive studies on the UTe2 material. They revealed they may have finally understood what causes this so-called “reentrant superconductivity.”

Kimberly Modic, PhD, an assistant professor at the institute, said researchers have long believed magnetism plays a huge role in unconventional superconductivity. “But the catch is that UTe2 is not magnetic,” she noted. “So, at first glance, it’s not obvious why this material exhibits such a special superconducting state.”

To understand how this unusual superconductivity emerges in UTe2, the scientists studied the material before reentrant superconductivity appeared under extreme magnetic fields. Using pulsed-field facilities, they exposed samples to rapid bursts of magnetism reaching up to 60 Tesla within a tenth of a second.

Valeska Zambra, a PhD student and lead author of the study, said the team aimed to determine whether the magnetic fluctuations inside the material could explain the high-field superconductivity. They developed a method to probe the sample under extreme magnetic fields using a controlled mechanical “wiggle.”

The method in practice

Zambra said they placed the sample on a cantilever-like stick to manipulate and shake it inside the magnetic field. “From the crystal’s point of view, the shaking makes it look like the direction of the magnetic field oscillates in time, allowing for a fast check of the magnetization under that changing field.”

This method allowed the researchers to access transverse magnetic susceptibility in previously unreachable conditions. This is how they found a region of strong transverse magnetic susceptibility in UTe2 that acts as the glue binding electrons together. It helped explain the high-field superconductivity.

The researchers said the work is not only important for understanding UTe2 itself, but also for opening new ways to study exotic quantum materials. Using samples smaller than a grain of salt, they were able to measure nearly defect-free pieces of UTe2.

They also developed techniques to fabricate and precisely integrate these tiny samples into the experiment. “Measuring small samples roughly as large as the thickness of a human hair is especially challenging, but this is precisely what our group specializes in,” Modic highlighted.

High-field laboratories have already contacted the ISTA group to adopt the new measurement technique for their own experiments. Modic noted that UTe2 may exhibit a previously unknown type of superconductivity.

“Although other unconventional superconductors exist, UTe2 makes the word ‘unconventional’ almost sound like an understatement,” she concluded in a press release.

The study has been published in the journal Nature Communications.