Josephson diode and spin-valve effects on the surface of altermagnet CrSb

Abstract: 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, the obtained $dV/dI(B)$ 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 transition into the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The latter is based on finite-momentum Cooper pairing against a background of the Zeeman splitting, so it is fully compatible with the requirements for the Josephson diode effect.