Observation of polar vortices in oxide superlattices
Observation of polar vortices in oxide superlattices
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DOI：
10.1038/nature16463
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发表时间：
2016-02-11
期刊：
影响因子：
64.8
通讯作者：
Ramesh, R.
中科院分区：
文献类型：
--
作者：
Yadav, A. K.;Nelson, C. T.;Ramesh, R.
自旋、电荷、轨道和晶格自由度的复杂相互作用提供了过多的奇异相和物理现象（1-5）。近年来，由于电子能带结构以及材料中自旋和自旋轨道耦合之间的相互作用，出现了复杂的自旋拓扑结构-（6，7）。在这里，我们通过利用钛酸铅和钛酸锶交替层的超晶格中电荷、轨道和晶格自由度之间的竞争，产生了复杂的电极化拓扑结构，即纳米级的涡旋-反涡旋（即顺时针-反涡旋）阵列，这让人想起了旋转自旋拓扑结构（6）。通过扫描透射电子显微镜对极性原子位移进行原子尺度映射，揭示了长程有序涡旋-反涡旋阵列的存在，这些阵列表现出几乎连续的极化旋转。相场模型证实，涡旋阵列是一个超晶格周期的范围内的低能量状态。在此范围内，来自涡旋结构的大梯度能量被整体静电能量（否则将由钛酸铅/钛酸锶界面处的极性不连续性引起）和与外延约束和畴形成相关的弹性能量的相应大减少抵消。这些观测结果对创造新的物质状态（如偶极skyrmions，刺猬态）和铁性材料中的相关现象（如电可控手性）具有影响。
The complex interplay of spin, charge, orbital and lattice degrees of freedom provides a plethora of exotic phases and physical phenomena(1-5). In recent years, complex spin topologies have emerged as a consequence of the electronic band structure and the interplay between spin and spin-orbit coupling in materials-(6,7). Here we produce complex topologies of electrical polarization-namely, nanometre-scale vortex-antivortex (that is, clockwise-anticlockwise) arrays that are reminiscent of rotational spin topologies(6)-by making use of the competition between charge, orbital and lattice degrees of freedom in superlattices of alternating lead titanate and strontium titanate layers. Atomic-scale mapping of the polar atomic displacements by scanning transmission electron microscopy reveals the presence of long-range ordered vortex-antivortex arrays that exhibit nearly continuous polarization rotation. Phase-field modelling confirms that the vortex array is the low-energy state for a range of superlattice periods. Within this range, the large gradient energy from the vortex structure is counterbalanced by the corresponding large reduction in overall electrostatic energy (which would otherwise arise from polar discontinuities at the lead titanate/strontium titanate interfaces) and the elastic energy associated with epitaxial constraints and domain formation. These observations have implications for the creation of new states of matter (such as dipolar skyrmions, hedgehog states) and associated phenomena in ferroic materials, such as electrically controllable chirality.