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.

中文翻译：

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二维 III2-VI3 材料中的墨西哥帽势能面和熵势垒在超快可逆铁电相变中的重要性

第一性原理计算揭示了二维 (2D) In ₂ Se ₃的墨西哥帽势能面 (PES) 。这种独特的 PES 导致了伪中心对称顺电β相，解决了当前理论和实验之间的争议。我们进一步表明，虽然α到β（铁电到顺电）相变快速且连贯，但在面内剪切声子模式的帮助下，PES 槽中原子的随机分布充当了逆β到α跃迁的熵势垒。这将是电流 In ₂ Se ₃限速的由来铁电器件。然而，如果对β相进行排序（由于面内铁电畴的形成），则在垂直电场存在的情况下，可以在数十皮秒内发生反向跃迁。最后，墨西哥帽 PES 是整个 2D III ₂ -VI ₃材料系列的通用特征。我们的发现为控制 2D In ₂ Se ₃和其他 III ₂ -VI ₃材料中的超快可逆相变提供了关键的物理图片，这将有利于他们对先进铁电器件的实际工业发展。