The CrSb crystal behaves like an intersection where electrons with different spins choose different lanes. In a conventional magnet, all electrons align in one direction, but in an altermagnet, spins alternate like squares on a chessboard. This creates a complex map of velocities that is hard to see directly.
The researchers used the method of quantum oscillations: they placed the crystal in a strong magnetic field and monitored how its resistance and torque changed. The magnetic field forces electrons to move in circles, and when the circle fits exactly into an integer number of revolutions, the resistance oscillates. From the frequency of these oscillations, one can reconstruct the shape of the Fermi surface — the boundary separating occupied electron states from empty ones.
It turned out that in CrSb, the Fermi surface consists of closed pockets concentrated at a special point in the Brillouin zone called the A point. Previously, it was assumed that there were tubes open along one axis, or pockets at the zone center. The new data indicate that the pockets are closed and sit at the A point. This is important because the shape of the Fermi surface determines how the material conducts electricity and responds to magnetic fields.
Now that the topology of CrSb's Fermi surface is established, one can look for new effects, such as anomalous current without an external field. This is a step toward electronics where spin is controlled without magnets.
Altermagnets do not produce an external magnetic field, but inside them, electron spins alternate like a chessboard.