Ferroelastic twin walls in nonpolar materials can give rise to a spontaneous polarization due to symmetry breaking. Nevertheless, the bistable polarity of twin walls and its reversal have not yet been demonstrated. Here, we report that the polarity of ${\text{SrTiO}}_3$ twin walls can be switched by an ultralow strain gradient. Using first-principles-based machine-learning potential, we demonstrate that the twin walls can be deterministically rotated and realigned in specific directions under the strain gradient, which breaks the inversion symmetry of a sequence of walls and leads to a macroscopic polarization. The system can maintain polarity even after the constraint is removed. As a result, the polarization of twin walls can exhibit a ferroelectriclike hysteresis loop upon cyclic bending, namely flexoferroelectricity. Finally, we propose a scheme to experimentally detect the polarity of the twin wall by measuring the bulk photovoltaic responses. Our findings suggest a twin-wall-mediated flexoferroelectricity in ${\text{SrTiO}}_3$, which could be potentially exploited as functional elements in nanoelectronic devices design.

中文翻译：

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SrTiO3 中铁弹性双壁介导的类铁电行为和体光伏效应

非极性材料中的铁弹性孪生壁会因对称性破缺而产生自发极化。然而，双壁的双稳态极性及其逆转尚未得到证实。在这里，我们报告极性${\text{钛酸锶}}_3$双壁可以通过超低应变梯度切换。利用基于第一性原理的机器学习潜力，我们证明了双壁可以在应变梯度下确定性地旋转并在特定方向上重新排列，这打破了一系列壁的反演对称性并导致宏观极化。即使在去除约束后，系统也可以保持极性。因此，双壁的极化在循环弯曲时可以表现出类似铁电的磁滞回线，即挠曲铁电性。最后，我们提出了一种通过测量体光伏响应来实验检测双壁极性的方案。我们的研究结果表明双壁介导的弯曲铁电${\text{钛酸锶}}_3$，它有可能被用作纳米电子设备设计中的功能元件。