Ferroelectricity on the nanoscale has been the subject of much fascination in condensed-matter physics for over half a century. In recent years, multiple reports claiming ferroelectricity in ultrathin ferroelectric films based on the formation of remnant polarization states, local electromechanical hysteresis loops, and pressure-induced switching were made. However, similar phenomena were reported for traditionally non-ferroelectric materials, creating a significant level of uncertainty in the field. Here we show that in nanoscale systems the ferroelectric state is fundamentally inseparable from the electrochemical state of the surface, leading to the emergence of a mixed electrochemical–ferroelectric state. We explore the nature, thermodynamics, and thickness evolution of such states, and demonstrate the experimental pathway to establish its presence. This analysis reconciles multiple prior studies, provides guidelines for studies of ferroelectric materials on the nanoscale, and establishes the design paradigm for new generations of ferroelectric-based devices.

中文翻译：

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纳米铁电体中的电化学-铁电混合态

在过去的半个多世纪里，纳米级的铁电一直是凝聚态物理学中非常着迷的主题。近年来，根据残余极化态的形成，局部机电磁滞回线和压力引起的转换，发表了多篇声称超薄铁电膜中具有铁电的报道。然而，据报道，传统的非铁电材料也有类似现象，这在该领域造成了很大的不确定性。在这里，我们表明，在纳米级系统中，铁电状态与表面的电化学状态基本上是分不开的，从而导致了电化学-铁电混合状态的出现。我们探索了这种状态的性质，热力学和厚度演变，并演示建立其存在的实验途径。该分析调和了多项先前的研究，为纳米级铁电材料的研究提供了指南，并为新一代基于铁电的设备建立了设计范式。