Nonadiabatic dynamics of the ferroelectric soft mode

Most microscopic descriptions of structural dynamics assume the Born-Oppenheimer separation, where electrons adjust adiabatically to ionic motion. When this separation breaks down, electronic and lattice degrees of freedom can evolve on different timescales, giving rise to new physical phenomena beyond the adiabatic limit. Here, we use time-resolved, phase-sensitive second-harmonic generation and pump-probe reflectivity to reshape the ferroelectric free-energy landscape of SnTe while separately tracking polar order and coherent lattice motion. When photoexcitation transiently suppresses the double-well barrier, polarization dynamics become strongly nonlinear, while the coherent phonon dynamics remain harmonic. This decoupling cannot be described by a single adiabatic coordinate for the electronic polarization and ionic positions. We provide a unifying physical description for the nonadiabatic dynamics of the ferroelectric mode and the mixed displacive/order-disorder nature of SnTe based on a separation of scales for the renormalization of the ferroelectric stiffness.