Natural materials usually consist of isotopic mixtures, for which different isotopic ratios can lead to distinct material properties such as thermal conductivity and nucleation process. However, the knowledge of isotopic interface remains largely unexplored mainly due to the challenges in isotopic identification and property measurement at an atomic scale. Here, by using monochromated electron energy-loss spectroscopy in a scanning transmission electron microscope, we reveal momentum-transfer-dependent lattice vibration behavior at an artificial h-10BN/h-11BN heterostructure with sub-unit-cell resolution. We find the vibrational energy changes across the isotopic interface gradually, featuring a wide transition regime, which suggests strong delocalization of the out-of-plane optical phonons at the interface. In addition, we identify phonons near the Brillouin zone center have a transition regime ~3.34 nm (10 atomic layers), whereas phonons at the Brillouin zone boundary transition in ~1.66 nm (5 atomic layers). We propose that the isotope-induced charge effect at the interface accounts for the distinct delocalization behavior. Moreover, intra-atomic layer variation of vibration energy is also sensitive to the momentum transfer, thus at the interface it depends on both of momentum transfer and mass change. The revealed lattice vibration behavior at an isotopic interface provides new insights to understand the isotopic effects on properties in natural materials.

中文翻译：

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跨同位素界面的声子跃迁

天然材料通常由同位素混合物组成，不同的同位素比会导致不同的材料特性，例如导热性和成核过程。然而，同位素界面的知识在很大程度上仍未得到探索，这主要是由于在原子尺度上同位素识别和性质测量的挑战。在这里，通过在扫描透射电子显微镜中使用单色电子能量损失光谱，我们揭示了具有亚晶胞分辨率的人工 h-10BN/h-11BN 异质结构中动量传递相关的晶格振动行为。我们发现跨同位素界面的振动能量逐渐变化，具有广泛的过渡范围，这表明界面处的面外光学声子存在强烈的离域。此外，我们发现布里渊区中心附近的声子具有~3.34 nm（10 个原子层）的跃迁状态，而布里渊区边界跃迁处的声子在~1.66 nm（5 个原子层）。我们建议界面处的同位素诱导电荷效应解释了不同的离域行为。此外，振动能量的原子层内变化对动量传递也很敏感，因此在界面处它取决于动量传递和质量变化。在同位素界面上揭示的晶格振动行为为理解同位素对天然材料特性的影响提供了新的见解。我们建议界面处的同位素诱导电荷效应解释了不同的离域行为。此外，振动能量的原子层内变化对动量传递也很敏感，因此在界面处它取决于动量传递和质量变化。在同位素界面上揭示的晶格振动行为为理解同位素对天然材料特性的影响提供了新的见解。我们建议界面处的同位素诱导电荷效应解释了不同的离域行为。此外，振动能量的原子层内变化对动量传递也很敏感，因此在界面处它取决于动量传递和质量变化。在同位素界面上揭示的晶格振动行为为理解同位素对天然材料特性的影响提供了新的见解。