Floquet Engineering of Molecular Crystals via Vibrational Excitation with Mid-Infrared Pulses

We demonstrate a new approach to a Floquet engineering in a molecular Mott insulator by exploiting the coupling between intramolecular vibrations and electronic states. In the spin-Peierls phase of K-TCNQ, mid-infrared excitation of an ag vibrational mode induces periodic spin-density modulation through intermolecular charge transfer. This modulation hybridizes nonmagnetic states with the original spin singlet states, destabilizing the spin-Peierls phase. Our approach opens a new pathway for next-generation optical control of electronic phases.