Spin Supercurrents in Superconducting Altermagnets
Superconductors are famous for transporting electric charge with zero resistance. This ability underpins technologies such as MRI scanners, quantum computers, and sensitive magnetometers known as superconducting quantum interference devices. However, in the field of spintronics—which seeks to process information using electron spin rather than charge—achieving a similar long-range dissipationless transport has remained elusive. In ordinary metals, electron spins are highly susceptible to scattering and spin-orbit coupling, both of which cause spin currents to decay over short distances. Although research in superconducting spintronics based on ferromagnets has made progress [1, 2], ferromagnets produce stray magnetic fields that interfere with external circuit elements, and their internal magnetic fields tend to destroy superconductivity. Now Kyle Monkman of the University of British Columbia in Canada and his colleagues have proposed a novel path to dissipationless spin transport [3]. They predict that a class of materials known as superconducting altermagnets can naturally generate and carry net spin supercurrents. Remarkably, these currents can propagate even in the presence of spin-orbit coupling and magnetic disorder.
To appreciate the significance of this work, one must first understand a major recent discovery in magnetism: altermagnets [4, 5] (see Viewpoint: Altermagnetism Then and Now). Neither ferromagnets nor antiferromagnets, altermagnets exhibit a new kind of magnetism that combines key features of its two predecessors. Like antiferromagnets, they have zero net magnetization and thus no internal fields that disrupt superconductivity. Like ferromagnets, they possess widely separated spin-split energy bands. This second property stems from their specific crystal symmetry, which lifts spin degeneracy in momentum space.
Shortly after this discovery, theorists began to explore the possibility that altermagnets could host a superconducting state. The core insight of Monkman and his collaborators is that when an altermagnet becomes a superconductor, something extraordinary happens. In conventional superconductors, spin-up and spin-down electrons pair up to form spin singlets, resulting in zero net spin. That’s not the case in altermagnets. Because the Fermi surface satisfies specific rotational symmetries (such as d wave or g wave), conventional singlet pairing is strongly suppressed. Instead, the system favors the formation of equal-spin triplet pairs [6]. Thanks to this pairing, the superconducting condensate is effectively composed of two independent superfluids: one made purely of spin-up electron pairs and the other of spin-down pairs. This two-superfluid structure is the key to generating persistent spin currents. Imagine the spin-up condensate flowing to the right, while the spin-down condensate flows to the left at the same speed. The charge currents cancel out (the net charge current is zero), but the spin currents add up. The result is a pure spin supercurrent that carries spin but not charge (Fig. 1).
In their calculations, Monkman and his collaborators revealed two striking properties of superconducting altermagnets. One is the spin-current dynamo effect, which occurs in certain crystal orientations, such as along the diagonal of a square lattice. Here, driving a standard charge current spontaneously generates a transverse pure spin supercurrent. The spin-current dynamo effect resembles a superconducting version of the spin Hall effect, and the magnitude of the spin flow can be precisely controlled by varying the charge current.
The second striking property that Monkman and his collaborators revealed is robustness. In normal materials, spin-orbit coupling usually kills spin currents, causing decoherence by torquing the spins. Using microscopic models and Ginzburg-Landau theory, the researchers proved that in superconducting altermagnets, spin-orbit coupling causes the spin current to spatially oscillate but not to decay. To explain their finding, they invoked the mechanical analogy of a frictionless pendulum. The position of the pendulum (representing the relative phase of the two spin currents) may swing back and forth, but because the system is in its lowest energy state, the motion will persist forever, unlike in normal metals where “friction” (resistance or spin relaxation) brings it to a halt. The team went on to envision a ring geometry. If a ring of a superconducting altermagnet is threaded with a half-integer magnetic-flux quantum, the system automatically enters a “charge counterflow” state, spontaneously generating an eternally flowing pure spin current. This state could be experimentally verified by detecting the disappearance of magnetization, without the need for complex spin-detection techniques.
This research serves as a compelling blueprint for superconducting spintronics. The proposed platform combines the strengths of ferromagnets (strong spin polarization) and of superconductors (dissipationless flow) while circumventing their weaknesses (stray fields and spin decay). Although a definitive superconducting altermagnet has yet to be confirmed, promising candidates such as MnTe [7] and RuO2 [8] have been identified. Fortunately, many altermagnets are good metals, making the existence of superconductivity at low temperatures a distinct possibility.
The work of Monkman and his collaborators suggests that searching for such materials is not only of fundamental physical interest but could provide a completely new hardware platform for future low-power, high-density information storage and processing. Detecting these predicted spin supercurrents in the lab will fundamentally reshape our understanding of the interplay between superconductivity and magnetism, marking a major milestone in the field.
References
- I. Žutić et al., “Spintronics: Fundamentals and applications,” Rev. Mod. Phys. 76, 323 (2004).
- J. Linder and J. W. A. Robinson, “Superconducting spintronics,” Nat. Phys. 11, 307 (2015).
- K. Monkman et al., “Persistent spin currents in superconducting altermagnets,” Phys. Rev. X 16, 011057 (2026).
- L. Šmejkal et al., “Beyond conventional ferromagnetism and antiferromagnetism: A phase with nonrelativistic spin and crystal rotation symmetry,” Phys. Rev. X 12, 031042 (2022).
- L. Šmejkal et al., “Emerging research landscape of altermagnetism,” Phys. Rev. X 12, 040501 (2022).
- C. Kallin and J. Berlinsky, “Chiral superconductors,” Rep. Prog. Phys. 79, 054502 (2016).
- C. Autieri, “New type of magnetism splits from convention,” Nature 626, 482 (2024).
- Y.-X. Li et al., “Exploration of altermagnetism in RuO2,” arXiv:2509.19932.