Liang Jing, Tu Teng, Chen Guanchu, Sun Yuanwei, Qiao Ruixi, Ma He, Yu Wentao, Zhou Xu, Ma Chaojie, Gao Peng, Peng Hailin, Liu Kaihui, Yu Dapeng
State Key Laboratory for Mesoscopic Physics, Frontiers Science Center for Nano-optoelectronics, School of Physics, Peking University, Beijing, 100871, China.
Academy for Advanced Interdisciplinary Studies, Collaborative Innovation Center of Quantum Matter, Peking University, Beijing, 100871, China.
Adv Mater. 2020 Aug;32(31):e2002831. doi: 10.1002/adma.202002831. Epub 2020 Jun 25.
Bismuth oxyselenide (Bi O Se), a new type of 2D material, has recently attracted increased attention due to its robust bandgap, stability under ambient conditions, and ultrahigh electron mobility. In such complex oxides, fine structural distortion tends to play a decisive role in determining the unique physical properties, such as the ferrorotational order, ferroelectricity, and magnetoelasticity. Therefore, an in-depth investigation of the fine structural symmetry of Bi O Se is necessary to exploit its potential applications. However, conventional techniques are either time consuming or requiring tedious sample treatment. Herein, a noninvasive and high-throughput approach is reported for characterizing the fine structural distortion in 2D centrosymmetric Bi O Se by polarization-dependent third-harmonic generation (THG). Unprecedentedly, the divergence between the experimental results and the theoretical prediction of the perpendicular component of polarization-dependent THG indicates a fine structural distortion, namely, a <1.4° rotation of the oxygen square in the tetragonal (Bi O ) layers. This rotation breaks the intrinsic mirror symmetry of 2D Bi O Se, eventually reducing the symmetry from the D to the C point group. The results demonstrate that THG is highly sensitive to even fine symmetry variations, thereby showing its potential to uncover hidden phase transitions and interacting polarized sublattices in novel 2D material systems.
氧化铋硒（Bi₂O₂Se）作为一种新型二维材料，因其具有稳健的带隙、在环境条件下的稳定性以及超高的电子迁移率，最近受到了越来越多的关注。在这类复杂氧化物中，精细结构畸变往往在决定独特物理性质（如铁旋转序、铁电性和磁弹性）方面起决定性作用。因此，深入研究Bi₂O₂Se的精细结构对称性对于开发其潜在应用至关重要。然而，传统技术要么耗时，要么需要繁琐的样品处理。在此，报道了一种非侵入性且高通量的方法，用于通过偏振相关的三次谐波产生（THG）来表征二维中心对称Bi₂O₂Se中的精细结构畸变。前所未有的是，偏振相关THG垂直分量的实验结果与理论预测之间的差异表明存在精细结构畸变，即四方（Bi₂O₂）层中氧正方形发生了<1.4°的旋转。这种旋转打破了二维Bi₂O₂Se的固有镜面对称性，最终使对称性从D点群降低到C点群。结果表明，THG对即使是细微的对称性变化也高度敏感，从而显示出其在揭示新型二维材料系统中隐藏的相变和相互作用的极化子晶格方面的潜力。
