A puzzling form of superconductivity that arises only under strong magnetic fields has been mapped and explained by a research team including Andriy Nevidomskyy, professor of physics and astronomy at Rice University. Their findings, published in Science July 31, detail how uranium ditelluride (UTe2) develops a superconducting halo under strong magnetic fields.
Traditionally, scientists have regarded magnetic fields as detrimental to superconductors. Even moderate magnetic fields typically weaken superconductivity, while stronger ones can destroy it beyond a known critical threshold. However, UTe2 challenged these expectations when, in 2019, it was discovered to maintain superconductivity in critical fields hundreds of times stronger than those found in conventional materials.
“When I first saw the experimental data, I was stunned,” said Nevidomskyy, a member of the Rice Advanced Materials Institute and the Rice Center for Quantum Materials. “The superconductivity was first suppressed by the magnetic field as expected but then reemerged in higher fields and only for what appeared to be a narrow field direction. There was no immediate explanation for this behavior."
Superconducting resurrection in high fields
This phenomenon, initially identified by researchers at the University of Maryland (UMD) and the National Institute of Standards and Technology (NIST), has captivated physicists worldwide. In UTe2, superconductivity vanished below 10 Tesla, a field strength that is already immense by conventional standards, but surprisingly reemerged at field strengths exceeding 40 Tesla.
This unexpected revival has been dubbed the Lazarus phase. Researchers determined that this phase critically depends on the angle of the applied magnetic field in relation to the crystal structure.
In collaboration with experimental colleagues at UMD and NIST, Nevidomskyy decided to map out the angular dependence of this high-field superconducting state. Their precise measurements revealed that the phase formed a toroidal, or doughnutlike, halo surrounding a specific crystalline axis.
“Our measurements revealed that this superconducting phase exists for a halo of field directions around the hard magnetic b-axis of the crystal,” said Sylvia Lewin of NIST, a co-lead author on the study. “This was a surprising result for us; we haven’t seen superconductivity that behaves like this before.”
Building theory to fit halo
To explain these findings, Nevidomskyy developed a theoretical model that accounted for the data without relying heavily on debated microscopic mechanisms.
His approach employed an effective phenomenological framework with minimal assumptions about the underlying pairing forces that bind electrons into Cooper pairs.
The model successfully reproduced the nonmonotonic angular dependence observed in experiments, offering insights into how the orientation of the magnetic field influences superconductivity in UTe2.
Deeper understanding of interplay
The research team found that the theory, fitted with a few key parameters, aligned remarkably well with the experimental features, particularly the halo’s angular profile.
A key insight from the model is that Cooper pairs carry intrinsic angular momentum like a spinning top does in classical physics. The magnetic field interacts with this momentum, creating a directional dependence that matches the observed halo pattern.
This work lays the foundation for a deeper understanding of the interplay between magnetism and superconductivity in materials with strong crystal anisotropy like UTe2.
“One of the experimental observations is the sudden increase in the sample magnetization, what we call a metamagnetic transition,” said NIST’s Peter Czajka, co-lead author on the study. “The high-field superconductivity only appears once the field magnitude has reached this value, itself highly angle-dependent.”
The exact origin of this metamagnetic transition and its effect on superconductivity is hotly debated by scientists, and Nevidomskyy said he hopes this theory would help elucidate it.
“While the nature of the pairing glue in this material remains to be understood, knowing that the Cooper pairs carry a magnetic moment is a key outcome of this study and should help guide future investigations,” he said.
Co-authors of this study include Corey Frank and Nicholas Butch from NIST; Hyeok Yoon, Yun Suk Eo, Johnpierre Paglione and Gicela Saucedo Salas from UMD; and G. Timothy Noe and John Singleton from the Los Alamos National Laboratory. This research was supported by the U.S. Department of Energy and the National Science Foundation.