Two-dimensional (2D) ferroelectrics are a special class of materials that fit the need of miniaturization of devices, which can lead to a plethora of applications. Up to now, albeit the breakthrough in the discovery of a handful of 2D ferroelectrics, the mechanism of which still requires better understanding. In this work, we clarify that the ferroelectric ground state in monolayer (ML) ${\mathrm{In}}_2{\mathrm{Se}}_3$ is strongly affected by in-plane strain, and the strain-induced phase transition is of a first-order transition. We further propose that strain is an effective approach to tune the ferroelectric ground state in 2D materials, which can even intrinsically introduce out-of-plane polarizations. By performing first-principles calculations, we show that the epitaxial strain is able to tune the structural transition between polar and nonpolar layer groups. According to this, the epitaxial strain can not only fully suppress the polarization in ML-${\mathrm{In}}_2{\mathrm{Se}}_3$, but also cause out-of-plane polarizations in a group of 2D nonpolar materials, i.e., ML-${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ML-${\mathrm{Bi}}_2{\mathrm{Te}}_3$, ML-${\mathrm{Sb}}_2{\mathrm{Te}}_3$, and ML-${\mathrm{Sb}}_2{\mathrm{Se}}_3$. The coupling mechanism between strain and polarization in these compounds is further illustrated.

中文翻译：

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二维材料中的应变可调面外极化

二维（2D）铁电体是一类特殊的材料，可满足设备小型化的需要，这可能导致大量的应用。到目前为止，尽管在发现少数2D铁电体方面取得了突破，但其机理仍需要更好地理解。在这项工作中，我们阐明了单层（ML）中的铁电基态${在}_2{硒}_3$受到平面内应变的强烈影响，应变诱发的相变属于一阶过渡。我们进一步提出，应变是一种在2D材料中调整铁电基态的有效方法，甚至可以固有地引入面外极化。通过执行第一性原理计算，我们表明外延应变能够调节极性和非极性层组之间的结构转变。据此，外延应变不仅可以完全抑制ML-${在}_2{硒}_3$，但也会在一组二维非极性材料（例如ML-${双}_2{硒}_3$，ML-${双}_2{特}_3$，ML-${锑}_2{特}_3$和ML-${锑}_2{硒}_3$。这些化合物中应变和极化之间的耦合机理得到进一步说明。