Phonon-Programmable Hidden Unconventional Magnetism in Two-Dimensional Spin-Degenerate Antiferromagnets

Abstract

Spin-degenerate antiferromagnets can host hidden unconventional magnetism in their lattice degrees of freedom. We show that coherent phonons activate this magnetism by removing the spin-layer operations that enforce equilibrium band degeneracy without changing the collinear Néel order. The frequency and polarization of a pump electric field select a resonant \(Γ\)-point optical phonon and, within a doublet, its coordinate direction. This choice fixes the residual spin-layer symmetry. In monolayer MnPSe$_3$, an \(A_{2u}\) mode produces \(i\)-wave splitting odd in the mass-weighted phonon coordinate $Q$, reversing sign under $Q\to-Q$, whereas two orthogonal directions of the same doubly degenerate \(E_u\) doublet produce \(d\)- and \(s\)-wave splitting at a common resonance. Rotating the in-plane pump field \(\bm E_\parallel\) therefore programs both the spin-splitting texture and the thermoelectric spin current, continuously tuning the response between transverse pure-spin and longitudinal spin-polarized currents. A complete classification of two-dimensional collinear spin layer groups identifies the \(Γ\)-point coordinates that remove the degeneracy-enforcing operations and activate such unconventional magnetism.