d-Wave Polarization-Spin Locking in Two-Dimensional Altermagnets

We report the emergence of an uncharted phenomenon, termed d-wave polarization-spin locking (PSL), in two-dimensional (2D) altermagnets. This phenomenon arises from nontrivial Berry connections, resulting in perpendicular electronic polarizations in the spin-up and spin-down channels. Symmetry-protected d-wave PSL occurs exclusively in d-wave altermagnets with tetragonal layer groups. To identify 2D altermagnets capable of exhibiting this phenomenon, we propose a symmetry-eigenvalue-based criterion and a rapid method by observing the spin-momentum locking. Using first-principles calculations, monolayer Cr₂X₂O (X = Se, Te) characterizes promising candidates for d-wave PSL, driven by the unusual charge order in these monolayers. This unique polarization-spin interplay leads to spin-up and spin-down electrons accumulating at orthogonal edges, enabling potential applications as spin filters or splitters in spintronics. Furthermore, d-wave PSL introduces an unexpected spin-driven ferroelectricity in conventional antiferromagnets. Such magnetoelectric coupling positions the d-wave PSL as an ideal platform for fast antiferromagnetic memory devices.