Nonlinear phononic control and emergent magnetism in Mott insulating titanates

Optical control of structure-driven magnetic order offers a platform for magneto-optical terahertz devices. We control the magnetic phases of $d^1$ Mott insulating titanates using nonlinear phononics to transiently perturb the atomic structure based on density functional theory (DFT) simulations and solutions to a lattice Hamiltonian including nonlinear multimode interactions. We show that magnetism is tuned by indirect excitation of a Raman-active phonon mode, which affects the amplitude of the ${\mathrm{TiO}}_6$ octahedral rotations that couple to static Ti-O Jahn-Teller distortions, through driven infrared-active modes of ${\mathrm{LaTiO}}_3$ and ${\mathrm{YTiO}}_3$. The mode excitation reduces the rotational angle, driving a magnetic phase transition from a ferromagnetic (FM) to $G$-type antiferromagnetic (AFM) state. A novel $A$-type AFM state hidden in the bulk equilibrium phase diagram emerges as a dynamically accessible optically induced phase under multimode excitations. Our work shows that nonlinear phononics can stabilize phases inaccessible to static chemical substitutions or lattice strains.