Dynamical hybrid improper ferroelectricity in incipient ferroelectric SrTiO₃

SrTiO₃ (STO) has been intensively studied, in part, because of its ‘incipient’ ferroelectricity. A small perturbation, such as strain, stress, chemical variation, or surface truncation, can induce a paraelectric to ferroelectric phase transition. Recent optical pump-probe experiments found that even THz light excitations can trigger the transition in STO. The underlying mechanism for the light-mediated ferroelectricity remains controversial; explaining it is crucial for both ultrafast manipulation and control. Here, we explain this process as a form of dynamical hybrid improper ferroelectricity. The inversion symmetry is lifted by the trilinear coupling between a polar mode, a zone-center antiferrodistortive mode, and a zone-boundary in-phase rotation mode activated under THz pumping. With density functional theory calculations, we compute the free energy profiles of the coupled modes. With the fitted coupling coefficients, we estimate the time evolution of the inversion symmetry lifting and compare our model with the experimental results. Our understanding extends the scope of improper ferroelectricity, and may open a new route to manipulate inversion symmetry of other materials in the time domain.