Light-Induced Even-Parity Unidirectionally Polarized Spin Splitting in Coplanar Antiferromagnets

When a coplanar antiferromagnet (AFM) with xy-plane magnetic moments exhibits a spin-split band structure and unidirectional spin polarization along z, the spin polarization is forced to be an odd function of momentum by the fundamental symmetry [2z| T]. Coplanar AFMs displaying such odd-parity, unidirectionally polarized spin splittings are known as odd-parity magnets. In this work, we propose the realization of their missing even-parity counterparts. We begin by deriving the symmetry conditions required for an even-parity, out-of-plane spin splitting. We then show that irradiating a spin-degenerate coplanar AFM with circularly polarized light lifts the [2z| T] constraint, dynamically generating this even-parity state. Specifically, the light-induced unidirectionally polarized spin splitting exhibits a d-wave texture in momentum space, akin to that of a d-wave altermagnet. We prove this texture's robustness against spin canting and propose time-, spin- and angle-resolved photoemission photocurrent as a direct probe. Our work demonstrates that optical driving can generate novel spin-split phases in coplanar AFMs, thereby diversifying the landscape of materials exhibiting distinct spin splittings.