Department of Physics, Cornell University, Ithaca, NY, 14853, USA.
Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, NJ, 08854, USA.
Nat Commun. 2017 Dec 1;8(1):1883. doi: 10.1038/s41467-017-02156-1.
In charge-ordered phases, broken translational symmetry emerges from couplings between charge, spin, lattice, or orbital degrees of freedom, giving rise to remarkable phenomena such as colossal magnetoresistance and metal-insulator transitions. The role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior. Here we directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature charge-ordered manganite BiSrCaMnO using aberration-corrected scanning transmission electron microscopy. We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped domains as small as ~5 nm, despite apparent bidirectionality over larger length scales. Further, we observe a direct link between disorder in one lattice modulation, in the form of dislocations and shear deformations, and nascent order in the perpendicular modulation. By examining the defects and symmetries of periodic lattice displacements near the charge ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.
在有序电荷相中，电荷、自旋、晶格或轨道自由度之间的耦合会产生打破平移对称性的现象，从而导致巨磁电阻和金属-绝缘体转变等显著现象。晶格在有序电荷相中所起的作用仍然特别神秘，这促使人们对微观晶格行为进行了表征。在这里，我们使用经过像差校正的扫描透射电子显微镜，直接绘制了室温有序钙钛矿锰氧化物中单个原子列的皮米级周期性晶格位移图。我们测量了阳离子的横向、离散晶格调制，这与现有的锰氧化物有序电荷模型不同。我们揭示了局部单向条纹畴，其尺寸小至约 5nm，尽管在较大的长度尺度上表现出明显的双向性。此外，我们还观察到一种直接的联系，即一种晶格调制中的无序（以位错和剪切变形的形式存在）与垂直调制中的初生有序之间的联系。通过检查电荷有序相变附近周期性晶格位移的缺陷和对称性，我们直接可视化了复杂氧化物中空间异质性的局部竞争。
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