Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films in which dimensionality, strain, and interface effects come into play. However, little is known of the tilt and rotational distortions in films a few unit cells thick including the question of if coherent tilt patterns survive at all in this ultrathin limit. Here, a series of films of perovskite LaNiO3 is studied and it is shown that the phonon mode related to oxygen octahedral tilts can be followed by Raman spectroscopy down to a film thickness of three pseudocubic perovskite unit cells (∼1.2 nm). To push the limits of resolution to the ultrathin regime, a statistical analysis method is introduced to separate the Raman signals of the film and substrate. Most interestingly, these analyses reveal a pronounced hardening of the tilt vibrational mode in the thinnest films. A comparison between the experimental results, first principles simulations of the atomic structure, and the standing wave model, which accounts for size effects on the phononic properties, reveals that in the ultrathin regime, the Raman spectra are a hybrid entity of both the bulk and surface phononic behavior. These results showcase Raman spectroscopy as a powerful tool to probe the behavior of perovskite films down to the ultrathin limit.

中文翻译：

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LaNiO3 薄膜在超薄状态下的振动特性

氧多面体的集体旋转和倾斜在复杂氧化物的物理性质（如磁性和导电性）中起着至关重要的作用。这种旋转可以通过制备维度、应变和界面效应发挥作用的薄膜来调整。然而，对于几个单位单元厚的薄膜中的倾斜和旋转失真知之甚少，包括相干倾斜模式是否能在这个超薄极限下存活的问题。在这里，研究了一系列钙钛矿 LaNiO3 薄膜，结果表明，与氧八面体倾斜相关的声子模式可以通过拉曼光谱追踪到三个伪立方钙钛矿晶胞（~1.2 nm）的薄膜厚度。将分辨率的极限推向超薄区域，引入了统计分析方法来分离薄膜和基板的拉曼信号。最有趣的是，这些分析揭示了最薄薄膜中倾斜振动模式的明显硬化。实验结果、原子结构的第一性原理模拟和驻波模型之间的比较表明，在超薄状态下，拉曼光谱是体积和体积的混合实体。表面声子行为。这些结果展示了拉曼光谱是一种强大的工具，可以探测钙钛矿薄膜的行为直至超薄极限。原子结构的第一性原理模拟和解释尺寸对声子特性影响的驻波模型表明，在超薄区域中，拉曼光谱是体声子行为和表面声子行为的混合实体。这些结果展示了拉曼光谱是一种强大的工具，可以探测钙钛矿薄膜的行为直至超薄极限。原子结构的第一性原理模拟和解释尺寸对声子特性影响的驻波模型表明，在超薄区域中，拉曼光谱是体声子行为和表面声子行为的混合实体。这些结果展示了拉曼光谱是一种强大的工具，可以探测钙钛矿薄膜的行为直至超薄极限。