Using the nonlinearity in 2D ferroelectrics to overcome voltage-time dilemma

Using first-principles calculations and group-theoretical methods, we study the origin and stabilization of ferrielectricity (FiE) in CuInP₂Se₆. We find that the polar distortions of metal atoms create most of the polarization in the FiE phase. Surprisingly, the stabilization of the FiE phase comes from an anharmonic coupling between the polar mode and a fully symmetric Raman-active mode comprising primarily of the Se atoms. This coupling is large, and the degree of anharmonicity is comparable to improper ferroelectrics. Thus, the origin of polarization is different from the factors that stabilizes the FiE phase in CuInP₂Se₆, unlike conventional ferroelectrics. Our results open up possibilities for dynamical control of the single-step ferroelectric switching barrier by directly tuning the Raman-active mode. These finding has important implications not only for designing next-generation microelectronic devices that can overcome the voltage-time dilemma but also in understanding the emergent responses in these materials.