Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO₃ thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

在铁电材料中已根据经验描述了倾斜的能带结构，以解释最近开发的铁电隧穿结（FTJ）的功能。非易失性铁电随机存取存储器（FeRAM）和基于FTJ系统的人工神经网络设备正在迅速发展。但是，由于尚未阐明实际的铁电带结构，因此必须通过适当的试探法来推进器件的精确设计。在这里，我们对铁电BaTiO ₃进行角分辨硬X射线光发射光谱薄膜用于直接观察铁电带的偏斜结构，作为原子轨道的深度剖面。深度分解的电子带结构由三个深度区域组成：沿着纤芯中的电极化的电势斜率，表面和界面表现出细微的变化。我们还证明了能量转移的方向是由极化反转控制的。在铁电斜带结构中，我们发现原子轨道能量位移的差异与反映声波有效电荷的软声子模式的原子构型有关。这些发现使人们对电极化的起源有了更好的了解。