Weakly coupled ferroelectric/dielectric superlattice thin-film heterostructures exhibit complex nanoscale polarization configurations that arise from a balance of competing electrostatic, elastic, and domain-wall contributions to the free energy. A key feature of these configurations is that the polarization can locally have a significant component that is along the thin-film surface normal direction with an overall configuration maintaining zero net in-plane polarization. ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ thin-film superlattice heterostructures on a conducting ${\mathrm{SrRuO}}_3$ bottom electrode on ${\mathrm{SrTiO}}_3$ have a room-temperature stripe nanodomain pattern with a nanometer-scale lateral period. Ultrafast time-resolved x-ray free electron laser diffraction and scattering experiments reveal that above-bandgap optical pulses induce propagating acoustic pulses and a perturbation of the domain diffuse scattering intensity arising from the nanoscale stripe domain configuration. With 400-nm optical excitation, two separate acoustic pulses are observed: a high-amplitude pulse resulting from strong optical absorption in the bottom electrode and a weaker pulse arising from the depolarization-field-screening effect due to absorption directly within the superlattice. The picosecond scale variation of the nanodomain diffuse scattering intensity is consistent with a larger polarization change than would be expected due to the polarization-tetragonality coupling of uniformly polarized ferroelectrics. The polarization change is consistent, instead, with polarization rotation facilitated by the reorientation of the in-plane component of the polarization at the domain boundaries of the striped polarization structure. The complex steady-state configuration within these ferroelectric heterostructures leads to ultrafast polarization rotation phenomena that have previously been available only through the selection of bulk crystal composition.

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铁电/介电超晶格纳米域中超快极化旋转的结构证据

弱耦合铁电/介电超晶格薄膜异质结构表现出复杂的纳米级极化配置，这是由于静电、弹性和畴壁对自由能的竞争贡献的平衡而产生的。这些配置的一个关键特征是极化可以局部具有沿薄膜表面法线方向的重要分量，而整体配置保持零净平面内极化。${\mathrm{钛酸铅}}_3/{\mathrm{钛酸锶}}_3$ 导电体上的薄膜超晶格异质结构 ${\mathrm{氧化锶}}_3$ 底部电极 ${\mathrm{钛酸锶}}_3$具有纳米级横向周期的室温条纹纳米域图案。超快时间分辨 X 射线自由电子激光衍射和散射实验表明，带隙以上的光脉冲会引起传播的声脉冲和由纳米条纹域配置引起的域漫散射强度的扰动。使用 400 nm 光激发，观察到两个独立的声脉冲：由底部电极中的强光吸收产生的高振幅脉冲和由超晶格内直接吸收的去极化场屏蔽效应产生的较弱脉冲。由于均匀极化铁电体的极化-四方性耦合，纳米域漫散射强度的皮秒级变化与比预期更大的极化变化一致。相反，偏振变化与偏振旋转是一致的，偏振旋转是通过在条纹偏振结构的域边界处重新定向偏振的面内分量来促进的。这些铁电异质结构中复杂的稳态配置导致超快极化旋转现象，以前只能通过选择块状晶体成分才能获得。通过在条纹偏振结构的域边界处重新定向偏振的面内分量来促进偏振旋转。这些铁电异质结构中复杂的稳态配置导致超快极化旋转现象，以前只能通过选择块状晶体成分才能获得。通过在条纹偏振结构的域边界处重新定向偏振的面内分量来促进偏振旋转。这些铁电异质结构中复杂的稳态配置导致超快极化旋转现象，以前只能通过选择块状晶体成分才能获得。