Symmetry in light-driven magnons and chiral phonons

We theoretically study the impact of light-driven structural changes on the magnetism of a bilayer honeycomb lattice. High-intensity light can induce large-amplitude phonon displacements and place the system in the non-linear phononics regime. Using symmetry group theory, we analyze the vibrational modes and nonlinear couplings between them. We find that the phonon-induced structural changes generate changes in the magnetic exchange constants of local moments situated on lattice sites. We observe that changes in the lattice symmetry can generate nonzero interlayer Dzyaloshinskii–Moriya interactions, which themselves give rise to new magnetic ground states with canted spins. When the light intensity is increased, we observe a phase transition from a gapped magnon phase to a gapless phase (at finite frequency) characterized by magnon nodal lines. Our work suggests experimental strategies for engineering magnetic ground states and manipulating magnon dispersion in broad classes of layered van der Waals materials. Time permitting, we will also discuss related symmetry analysis in the group theory of chiral phonons.