Mexican-Hat Potential Energy Surface in Two-Dimensional III₂-VI₃ Materials and the Role of Entropy Barrier in Their Ultrafast Reversible Ferroelectric Phase Change

Because of the spontaneous-aligned electric dipole, which can be switched reversibly by an external electric field, ferroelectric materials have been utilized for applications in nonvolatile memories and neuromorphic computing. However, the miniaturization of conventional ABO₃ ferroelectrics is hindered by the depolarization field. Recently, a new kind of two-dimensional (2D) van der Waals layered material, In₂Se₃, has emerged as a potential solution for low-dimensional ferroelectrics due to its intrinsic ferroelectric polarization at the nanoscale, even down to the limit of a monolayer. At present, the atomic structure and ferroelectricity of the α phase have been firmly identified. However, the study of the paraelectric β phase is still lacking and even its atomic structure is controversial, which limits the speed of the ferroelectric phase transition. Here, using first-principles calculations and molecular dynamics studies, a unique Mexican-hat potential energy surface (PES) is identified for the family of 2D III₂-VI₃ materials for the first time.^[1] Taking In₂Se₃ as an example, the PES results in an unexpected pseudo-centrosymmetric (paraelectric) β phase where the presumed centrosymmetry is in fact broken at every moment, which resolves the current structural controversy between theory and experiment. We further show that while the <i>α</i>-to-<i>β</i> (ferroelectric-to-paraelectric) phase transition is fast and coherent, assisted by an in-plane shear phonon mode. However, a random distribution of the Se atoms in the trough of the Mexican-hat PES sets up a sizable effective entropy barrier to slow down the β-to-α transition considerably, which will be the origin of the speed limitation of current In₂Se₃-based ferroelectric devices. If one orders the <i>β</i> phase (due to the formation of in-plane ferroelectric domains), the reverse transition can take place within tens of picoseconds in the presence of a perpendicular electric field. This study unfolds new physics in 2D III₂-VI₃ materials with important technological implications. Not only it unifies theory with experiment, but also it points to new directions for achieving ultrafast reversable ferroelectric phase change, whereby improving the performance of existing ferroelectric devices, such as memory and neural computing hardware, especially in terms of their integrated storage density and speed.<br/>[1] 1. Huang Y-T, Chen N-K, Li Z-Z, Li X-B, et al. Mexican-hat potential energy surface in two-dimensional III₂-VI₃ materials and the importance of entropy barrier in ultrafast reversible ferroelectric phase change. <i>Appl. Phys. Rev.</i> 2021;8(3):031413.