A spin valve filters electrons by spin — allowing one orientation to pass and blocking the other. Traditionally, this requires ferromagnetic electrodes with net magnetization. The magnetization provides the asymmetry between spin-up and spin-down. No magnetization, no valve.
Acharjee and colleagues (arXiv:2602.20838) show that altermagnets — materials with zero net magnetization but momentum-dependent spin splitting — can produce pronounced spin valve effects without any ferromagnet. The trick: altermagnets split spin bands not in real space (like ferromagnets) but in momentum space. Electrons moving in different directions see different spin potentials. Combined with Rashba spin-orbit coupling, which mixes spin orientations in a tunable way, the junction produces spin-selective transport.
The mechanism works because the altermagnet's anisotropic spin splitting creates direction-dependent filtering. An electron traveling along one crystal axis experiences a different spin environment than one traveling along another. The Rashba coupling acts as an electrically tunable knob — adjusting the degree of spin mixing without changing the material. Together, they produce large magnetoresistance and giant spin polarization.
For nodal p_x superconductor junctions, surface Andreev bound states produce strongly directional conductance and zero-bias spin polarization. For chiral p_x + ip_y junctions, topological edge modes generate smoother profiles with distinctive lobed patterns. Each superconductor type leaves a different fingerprint in the spin valve response.
The general point: functions that seem to require a specific symmetry breaking (net magnetization for spin valves) can sometimes be achieved through a different, subtler symmetry breaking (momentum-dependent spin splitting). The same functional outcome — spin filtering — from a fundamentally different physical mechanism.