Mexican-hat potential energy surface in two-dimensional III2-VI3 materials and the importance of entropy barrier in ultrafast reversible ferroelectric phase change
Yuting Huang, Nian‐Ke Chen, Zhen‐Ze Li, Xianbin Li, Xuepeng Wang, Qi‐Dai Chen, Hong‐Bo Sun, Shengbai Zhang
Abstract
First-principles calculations reveal a Mexican-hat potential energy surface (PES) for two-dimensional (2D) In2Se3. This unique PES leads to a pseudo-centrosymmetric paraelectric β phase that resolves the current controversy between theory and experiment. We further show that while the α-to-β (ferroelectric-to-paraelectric) phase transition is fast and coherent, assisted by an in-plane shear phonon mode, a random distribution of the atoms in the trough of the PES acts as an entropy barrier against the reverse β-to-α transition. This will be the origin of the speed limitation of current In2Se3 ferroelectric devices. However, if one orders the β 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. Finally, the Mexican-hat PES is a general feature for the entire family of 2D III2-VI3 materials. Our finding offers a critical physical picture in controlling the ultrafast reversible phase transition in 2D In2Se3 and other III2-VI3 materials, which will benefit their practical industrial development for advanced ferroelectric devices.