An ordinary superconductor is like a squad of soldiers marching in pairs: electrons pair up, and their spins (internal 'tops') point in opposite directions. A magnetic field easily flips such pairs, destroying superconductivity.
In UTe₂, things are different. There, electron pairs have spins pointing in the same direction, like dance partners holding hands and spinning in the same rhythm. When physicists applied a magnetic field along the c-axis, the spins barely changed up to 5 tesla (about 100,000 times stronger than Earth's magnetic field), and then quickly returned to their normal state. Superconductivity persisted even at fields above this threshold. The researchers noticed that when the field is oriented so that the pair spins align with it, superconductivity becomes more robust.
This behavior is reminiscent of superfluid helium-3—a liquid that flows without friction at ultra-low temperatures. Perhaps UTe₂ is a solid-state cousin of this exotic liquid. Understanding its properties could help create devices where spins can be manipulated without destroying the quantum state.
The stubborn tops in UTe₂ resist the magnetic field until the very end, then give in all at once, but superconductivity remains. This is a window into materials that could become the foundation of future quantum computers.
as written by the authors arXiv:2602.08358
Spin-triplet superconductivity is an intriguing quantum coherent state with both spin and orbital degrees of freedom, which holds significant potential for future applications in quantum technology. However, how the spin of the triplet pairs responds to an external magnetic field remains poorly understood. This is mainly due to the absence of suitable spin-triplet superconductors. Here, we report results of Knight-shift and ac-susceptibility measurements on UTe$$_2$$. We demonstrate that the spin susceptibility, which slightly decreases compared to the normal-state value below the superconducting (SC) transition temperature $$T_{\rm c}$$, is rapidly restored and nearly recovers to the normal-state values around 5 T, well below the SC upper critical field $$H_{c2}$$ when the magnetic field is applied along the $$c$$ axis ($$H \parallel c$$). In addition, we found that $$H_{\rm c2}$$ of superconductivity becomes larger when the SC spin aligns with the magnetic field. By considering the results on $$H \parallel b$$, our results suggest the presence of a close relationship between the spin configuration of the triplet pair and $$H_{\rm c2}$$, as well as the anisotropic pinning interaction acting on the triplet pairs. These phenomena, which have never been observed in spin-singlet superconductors, represent characteristic features unique to spin-triplet superconductors. We discuss the similarities between superconductivity in UTe$$_2$$ and superfluid $$^3$$He, focusing on their spin-triplet pairing states.
A magnetic field of 5 tesla, at which the spins in UTe₂ abruptly change behavior, is roughly a million times stronger than the field of a refrigerator magnet.
In the series 'The Expanse,' superconductors are used to create efficient engines and shields — exotic superconductors like UTe₂ could make such technology more real.