In altermagnets, the concept of spin-momentum locking was extended to the case of weak spin-orbit coupling. As a result, the small net magnetization is accompanied by alternating spin splitting in the k-space. Consequently, an altermagnet sometimes behaves as an antiferromagnet, and sometimes as a ferromagnet, depending on the crystal-field or interface-crystal relative orientations.

Spin-momentum locking is a key feature not only of altermagnets, but also of a large class of topological materials, where it is responsible for the spin polarization of the topological surface states. In proximity topological devices, spin-polarized surface states lead to the different realizations of the Josephson diode effect.

Experimental investigations of the Josephson current asymmetry can be conveniently performed for CrSb, which reveals both altermagnetic and topological features. Here, we experimentally investigate charge transport in In-CrSb and In-CrSb-In proximity devices, which are formed as junctions between superconducting indium leads and thick single crystal flakes of altermagnet CrSb. For double In-CrSb-In junctions, dV /dI(B) curves are mirrored in respect to zero field for two magnetic field sweep directions, which is characteristic behavior of a Josephson spin valve. Also, we demonstrate Josephson diode effect by direct measurement of the critical current for two opposite directions in external magnetic field. We interpret these observations as a joint effect of the spin-polarized topological surface states and the altermagnetic spin splitting of the bulk bands in CrSb. For a single In-CrSb interface, the superconducting gap oscillates in magnetic field for both field orientations, which strongly resembles the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) behavior. FFLO is based on finite-momentum Cooper pairing, therefore, it is fully compatible with the requirements for the Josephson diode effect.

(a) Josephson spin-valve effect as the dV /dI(B) curves reversal for two opposite magnetic field sweep directions at 30 mk temperature. The zero-resistance region is not only shifted to finite magnetic fields, but the dV /dI(B) curves are mirrored in respect to zero field,
including regions with finite resistance. (b) dV /dI(I) curves after magnetization of the sample. Differential resistance is always finite at zero field now, but there is wide zero-resistance region at 13 mT magnetic field.

Esin V.D. et.al,
JETP Letters 123, issue 8 (2026)