Filled circles (●) and open squares (□) indicate superconducting transition points. The color scale represents an indicator that reflects changes in entropy. Therefore, it changes markedly at the points where the state switches, allowing the phase boundaries to be identified. In the low-field superconducting phase (Low-field SC), the spin orientation of superconducting pairs is not aligned along a specific direction. Upon increasing the magnetic field, a phase transition occurs at the boundary shown by the solid green line, into a high-field superconducting phase (High-field SC) where the spins become aligned along the field direction.

In conventional superconductors, two electrons forming the superconducting pair have antiparallel spins, yielding a total spin of S = 0 (Fig. 1a). As a result, the net magnetization is zero and the spin does not constitute an active degree of freedom. In contrast, in a spin-triplet superconductor, the two electrons in a pair have parallel spins and the pair has a total spin of S = 1 (Fig. 1b). Consequently, the spin orientation becomes a relevant degree of freedom, and the spin configuration can in principle change in response to an external magnetic field. This property is generally expected to make superconductivity more robust against strong magnetic fields, and thus spin-triplet superconductors are considered promising for applications such as superconducting magnets that generate very high fields.

In this study, we investigated the high-field properties of the spin-triplet superconductor, UTe₂. We found that, whereas the low-field superconducting phase exhibits no preferred alignment of the spin orientation, increasing the magnetic field induces a phase transition into a high-field superconducting phase where the spins become aligned along the field direction (Fig. 2). As a consequence, we show that the upper critical field at which superconductivity is destroyed reaches twice the theoretical expectation for conventional superconductors.

This achievement not only provides design guidelines for developing high-field superconducting magnet wires, but also demonstrates that unconventional superconductors possess the flexibility to adapt to external environments—such as magnetic fields—through a reconstruction of their spin and orbital degrees of freedom, much like a chameleon blending into its surroundings. This is an important insight that will contribute to establishing a general framework common to unconventional superconductivity.

This work was supported by JSPS KAKENHI Grants Number JP16KK0106, JP17K05522, JP17K05529, JP20K03852, JP24K00590, JP24KK0062, JP23H04871, JP23H01132, JP23K03332.

Paper URL: https://doi.org/10.1103/z8yx-yzdh

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