In the rigid world of quantum physics, there is a long-standing rule: magnetic fields are the enemy of superconductors. For most materials capable of conducting electricity with zero resistance, a sufficiently strong magnetic field acts as a kill switch, disrupting the delicate alignment of electrons and ending the superconducting state entirely.
However, a team of physicists has documented a rare exception in uranium ditelluride (UTe2), a material where superconductivity dies then comes back to life under extreme conditions. This phenomenon, which researchers have dubbed the “Lazarus phase,” challenges the traditional understanding of how magnetism and superconductivity interact, suggesting that some materials can actually thrive in environments that would destroy others.
The study, published in the journal Science, describes a paradoxical relationship where superconductivity is first suppressed by a magnetic field, only to re-emerge once the field reaches an intensity far beyond the usual breaking point. This “resurrection” does not happen randomly; it occurs only when the magnetic field is applied at specific angles relative to the material’s crystal structure.
The Paradox of the Lazarus Phase
For typical superconductors, there is a critical limit—a threshold of magnetic strength—beyond which the material reverts to a normal, resistive state. Uranium ditelluride initially follows this pattern. At a field strength of 10 Tesla, which is already immensely powerful, the superconductivity in UTe2 vanishes.
But as the intensity continues to climb, the material does something unexpected. Once the field exceeds 40 Tesla, the superconducting state returns. This gap between 10 and 40 Tesla creates a “dead zone” where the material behaves normally, followed by a sudden revival.
“When I first saw the experimental data, I was stunned,” said Andriy Nevidomskyy, a physicist with the Rice University 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 puzzling behavior.”
Mapping the Superconducting Halo
To understand why this revival happens, researchers from Rice University, the University of Maryland (UMD), and the National Institute of Standards and Technology (NIST) mapped the material’s response to magnetic fields from various directions. They discovered that the Lazarus phase is highly directional.
Rather than returning uniformly throughout the material, the superconductivity forms a three-dimensional, toroidal shape—essentially a doughnut-like halo—that wraps around the “hard b-axis” of the crystal. Which means the material only regains its zero-resistance properties if the magnetic field is oriented correctly relative to its internal geometry.
“Our measurements revealed a three-dimensional superconducting halo that wraps around the hard b-axis of the crystal,” said Sylvia Lewin of NIST, a co-lead author on the study. “This was a surprising and beautiful result.”
Decoding the Quantum Mechanism
The key to this behavior lies in the nature of Cooper pairs—the pairs of electrons that allow superconductivity to happen. In most materials, these pairs are fragile and easily ripped apart by magnetic fields. In UTe2, however, the Cooper pairs behave as if they possess angular momentum, similar to a spinning object.
Nevidomskyy developed a theoretical model using a phenomenological approach, focusing on the overall behavior of the system rather than the minute microscopic details. His model suggests that when a massive magnetic field is applied, it interacts with the angular momentum of these spinning Cooper pairs. At the right angle and intensity, this interaction stabilizes the pairs rather than destroying them, allowing the superconducting state to return.
This discovery provides a new framework for understanding how magnetism and superconductivity can coexist. Usually, these two forces are mutually exclusive, but UTe2 demonstrates that under the right directional constraints, they can work in tandem.
The Metamagnetic Mystery
While the “halo” has been mapped, one piece of the puzzle remains: the metamagnetic transition. Here’s a sudden, sharp increase in the sample’s magnetization that occurs just as the superconductivity returns.
| Field Strength | State of Material | Observation |
|---|---|---|
| Below 10 Tesla | Superconducting | Standard superconducting state |
| 10 to 40 Tesla | Normal/Resistive | Superconductivity is suppressed |
| Above 40 Tesla | Superconducting | Lazarus phase (re-emergent) |
According to Peter Czajka, a co-lead author from NIST, this transition is the trigger for the revival. “The high-field superconductivity only appears once the field magnitude has reached this value, itself highly angle-dependent,” Czajka said.
Physicists are still debating the exact cause of this metamagnetic transition. However, Nevidomskyy believes that identifying the magnetic moment of the Cooper pairs is a critical step forward. He noted that while the “pairing glue”—the force that holds the electrons together—is still not fully understood, this insight should guide all future investigations into the material.
The research was supported by the National Science Foundation and the U.S. Department of Energy, involving a broad collaboration between NIST, UMD, and Los Alamos National Laboratory.
The next phase of research will focus on isolating the specific “pairing glue” responsible for these spin-triplet Cooper pairs, a discovery that could eventually lead to more robust quantum materials capable of operating in extreme environments.
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