Altermagnets are the third class of collinear magnets — neither ferromagnetic (net magnetization) nor antiferromagnetic (zero net magnetization, degenerate spin bands). They have zero net magnetization but non-degenerate spin bands: the spin-up and spin-down Fermi surfaces are split in momentum space, related by a rotation rather than by simple translation. The symmetry is lower than antiferromagnetism but higher than ferromagnetism. CrSb is the prototypical example, with g-wave symmetry — the spin splitting has the angular dependence of a g-wave (l=4) harmonic.
Cadez, Sunanta, and Kim (arXiv 2602.22736, February 2026) show that when a g-wave altermagnet becomes superconducting, the pairing symmetry inherits the altermagnet's exotic spin structure. The g-wave exchange field distorts the Fermi surfaces in a way that favors chiral superconducting states — Cooper pairing with a definite handedness.
The mechanism is Fermi surface geometry. In a conventional s-wave superconductor, the pairing gap is uniform around the Fermi surface. In a g-wave altermagnet, the spin-split Fermi surfaces have non-trivial topology in momentum space — lobes and nodes set by the g-wave angular dependence. Cooper pairs form between electrons on these distorted Fermi surfaces, and the pairing symmetry that maximizes the condensation energy depends on the shape of the surface.
At strong altermagnetic fields and high electron densities, chiral p-wave pairing wins. This is the most sought-after unconventional pairing symmetry — it breaks time-reversal symmetry, hosts Majorana edge modes, and has been proposed as the pairing in strontium ruthenate but never conclusively demonstrated. At weak fields and intermediate densities, chiral d-wave states dominate instead — also time-reversal breaking, also topological. At weak fields with typical electron densities, the system falls back to conventional non-chiral states: s-wave, extended s-wave, or f-wave.
The g-wave symmetry is the key. Lower-order altermagnets (d-wave) produce less dramatic Fermi surface distortions and less exotic pairing. The g-wave's higher angular momentum creates more complex Fermi surface topology, which supports more complex pairing channels. The pairing symmetry descends from the magnetic symmetry — the Cooper pairs inherit the angular structure of the exchange field that shapes the Fermi surface they condense on.
CrSb is a real material that can be synthesized. The prediction is specific: measure the quasiparticle dispersion and the density of states, and the pairing symmetry reveals itself through the gap structure. Chiral p-wave has point nodes; chiral d-wave has line nodes; the non-chiral states are fully gapped. The altermagnet provides the stage; the superconductor reveals which act is playing.