Department of Nano and Chemical Engineering, University of California San Diego, La Jolla, CA 92093, USA.
Nanoscale. 2023 Mar 16;15(11):5193-5200. doi: 10.1039/d2nr05732h.
Understanding the role of ferroelectric polarization in modulating the electronic and structural properties of crystals is critical for advancing these materials for overcoming various technological and scientific challenges. However, due to difficulties in performing experimental methods with the required resolution, or in interpreting the results of methods therein, the nanoscale morphology and response of these surfaces to external electric fields has not been properly elaborated. In this work we investigate the effect of ferroelectric polarization and local distortions in a BaTiO perovskite, using two widely used computational approaches which treat the many-body nature of X-ray excitations using different philosophies, namely the many-body, delta-self-consistent-field determinant (mb-ΔSCF) and the Bethe-Salpeter equation (BSE) approaches. We show that in agreement with our experiments, both approaches consistently predict higher excitations of the main peak in the O-K edge for the surface with upward polarization. However, the mb-ΔSCF approach mostly fails to capture the L separations at the Ti-L edge, due to the absence of spin-orbit coupling in Kohn-Sham density functional theory (KS-DFT) at the generalized gradient approximation level. On the other hand, and most promising, we show that application of the GW/BSE approach successfully reproduces the experimental XAS, both the relative peak intensities as well as the L separations at the Ti-L edges upon ferroelectric switching. Thus simulated XAS is shown to be a powerful method for capturing the nanoscale structure of complex materials, and we underscore the need for many-body perturbation approaches, with explicit consideration of core-hole and multiplet effects, for capturing the essential physics in these systems.
理解铁电极化在调节晶体电子和结构性质方面的作用对于推进这些材料克服各种技术和科学挑战至关重要。然而，由于在执行所需分辨率的实验方法或解释其中方法的结果方面存在困难，这些表面的纳米级形态及其对外部电场的响应尚未得到充分阐述。在这项工作中，我们使用两种广泛使用的计算方法研究了钙钛矿型 BaTiO 中铁电极化和局部变形的影响，这两种方法采用不同的哲学处理 X 射线激发的多体性质，即多体、Delta 自洽场行列式（mb-ΔSCF）和 Bethe-Salpeter 方程（BSE）方法。我们表明，与我们的实验一致，这两种方法都一致地预测了表面向上极化时 O-K 边主峰的更高激发。然而，由于在广义梯度近似水平下，Kohn-Sham 密度泛函理论（KS-DFT）中缺乏自旋轨道耦合，mb-ΔSCF 方法大多无法捕捉 Ti-L 边的 L 分离。另一方面，最有希望的是，我们表明 GW/BSE 方法的应用成功地再现了实验性 XAS，包括铁电开关时 Ti-L 边的相对峰强度和 L 分离。因此，模拟 XAS 被证明是一种捕获复杂材料纳米级结构的强大方法，我们强调需要采用多体微扰方法，并明确考虑核心空穴和多重态效应，以捕获这些系统中的基本物理。
