Altermagnetic spin textures coupled to superconductors: Domain wall spin-triplet superconductivity and supercurrent-induced torques

Abstract

Motivated by the absence of sizable stray fields and the recently discovered highly non-trivial impact of altermagnetic textures on itinerant electrons, we here study the form of Cooper pairs in spatially varying altermagnets coupled to conventional $s$-wave superconductors. As a consequence of the detrimental impact of altermagnetism on spin-singlet pairing and the local symmetry reduction caused by textures in the magnetic order parameter, we show that superconductivity predominantly impacts the regions between altermagnetic domains. Focusing on a planar radial domain wall for concreteness, we show that emergent Zeeman and spin-orbit fields create spatially separated triplet hotspots and transitions between nodal and fully gapped superconducting regions, whose structure is set by both the domain wall and the altermagnetic order parameter. We also identify a reciprocal effect, where a supercurrent generates a quasiparticle-mediated quadrupolar torque that inherits the symmetry of the altermagnetic order. Our results show that accounting for spatial inhomogeneities in the altermagnetic order parameter is essential for an understanding of the superconducting proximity effect and suggest that hybrid systems of altermagnetic textures and superconductors offer unique opportunities for local engineering of Cooper pairs and for detecting altermagnetic order.