【纯计算】二维滑移铁电：实现非同步体光伏效应

铁电极化和体光伏效应（BPVE）是凝聚态物质中由于对称性破缺而出现的两种基本物理现象。BPVE出现在没有中心反演对称的体系中，而铁电材料往往具备BPVE的条件。因此，电极化可切换的铁电体是研究最多的BPVE材料之一，其能量转换效率可以超过肖克利–奎瑟极限。

以往针对铁电材料BVPE的研究中，BPVE往往与铁电翻转同时出现。例如，在中心反演变换下，铁电极化和BPVE在所有方向上一起翻转，因此铁电极化可以用来调控BPVE的光电流方向。但是仅从对称性角度分析看，铁电极化只需满足极性矢量，而BPVE系数则是一个三阶张量。铁电与BPVE满足不同的对称性变换基本要求，原则上可以不同时翻转。因此，准确理解铁电序与BPVE之间的关系，对光电器件的基础研究和设计都具有重要意义。

Fig. 2 Calculated results of ferroelectric bilayer MoS2.

来自安徽大学李惠教授与中科大高阳教授团队，针对二维层间滑移铁电体进行研究，发现了滑移铁电有序可以满足非同步的BPVE。他们利用双层晶体模型的对称性分析，发现镜面对称切换与层间滑移铁电相关，但是该对称操作不会改变所有的BPVE张量元。以WTe₂和MoS₂为例，通过第一性原理计算，作者证明面内BPVE确实不改变铁电序符号。

Fig. 3 Calculated results of ferroelectric bilayer WTe2.

这一发现拓展了对铁电和BPVE关系的理解。一方面，面外BPVE能用于设计可控光电器件。另一方面，平面内的BPVE对铁电翻转具有鲁棒性，其不可切换的特点有利于构建更大尺寸的光电器件。相关论文发布于npj Computational Materials 8: 138 (2022)。

Editorial Summary

Ferroelectricity and bulk photovoltaic effect (BPVE) are two basic physical phenomena that emerge in condensed matter materials due to symmetry breaking. Generally speaking, BPVE appears in systems without inversion symmetry, and ferroelectric materials often have the conditions for BPVE. As a result, the ferroelectric materials with switchable electronic polarization are one of the most studied BPVE materials, and the corresponding energy conversion efficiency can exceed the Shockley-Queisser limit. Previous studies revealed that BPVE often appears in conjunction with ferroelectric order flipping. For example, under the inversion symmetry operation, ferroelectric polarization and BPVE switch together in all directions, and thus ferroelectric polarization can be used to manipulate the direction of BPVE photocurrent. However, only from the symmetry point of view, ferroelectric polarization only needs to satisfy a polar vector, and BPVE coefficients forms a rank-three tensor. Ferroelectric and BPVE meet different symmetry-transformation requirements, and in principle should not always synchronize. Thus, a precise understanding of the relationship between the ferroelectric order and the BPVE is of tremendous significance in both fundamental research and the design of photoelectric devices. 

Here, Prof. Hui Li from Anhui University and Prof. Yang Gao from University of Science and Technology of China, performed a systematic study on two-dimensional sliding ferroelectrics and found that sliding ferroelectric order can satisfy non-synchronous BPVE. Using symmetry analysis of the abstract bilayer crystal model, they found that opposite ferroelectric states are related to the mirror operator, but this mirror-symmetry cannot flip all the BPVE tensor elements. Taking WTe₂ and MoS₂ as examples, they confirmed through first-principles calculations that in-plane BPVE does not change the sign with ferroelectric order. This finding extends the understanding of the relationship between ferroelectricity and BPVE. On one hand, the switchable out-of-plane BPVE can be used to design switchable photoelectric devices. On the other hand, the in-plane BPVE is robust against the ferroelectric flipping, and the unswitchable character is beneficial to construct larger-scale photoelectric devices. This article was previously published in npj Computational Materials 8: 138 (2022).

原文Abstract及其翻译

Non-synchronous bulk photovoltaic effect in two-dimensional interlayer-sliding ferroelectrics (二维层间滑移铁电体中的非同步体光伏效应)

Rui-Chun Xiao, Yang Gao, Hua Jiang, Wei Gan, Changjin Zhang & Hui Li

Abstract Spontaneous polarization and bulk photovoltaic effect (BPVE) are two concomitant physical properties in ferroelectric materials. The flipping of ferroelectric order usually accompanies the switching of BPVE in all directions because both of them are reversed under the inversion symmetry. In this study, we report the non-synchronous BPVE in two-dimensional (2D) interlayer-sliding ferroelectric materials featuring unswitchable in-plane BPVE (light-induced photocurrent in the xy plane) and switchable out-of-plane BPVE (light-induced polarization along the z-direction). Symmetry analysis within the abstract bilayer crystal model and first-principles calculations validate these BPVE properties. It is because the positive and negative ferroelectric states caused by interlayer sliding are related by mirror symmetry which cannot flip all the BPVE tensor elements. This finding extends the understanding of the relationship between ferroelectricity and BPVE. On one hand, the switchable out-of-plane BPVE can be used to design switchable photoelectric devices. On the other hand, the in-plane BPVE is robust against the ferroelectric flipping, and the unswitchable character is beneficial to construct larger-scale photoelectric devices.

摘要自发极化和体光伏效应（BPVE）是铁电材料中伴随出现的两种物理现象。铁电序的翻转通常伴随着BPVE在各个方向上的切换，因为它们在中心反演对称操作下都需要反转。在本研究中，我们报道了二维（2D）层间滑移铁电材料中的非同步BPVE，它具有不可切换的面内BPVE（xy平面上的光诱导电流）和可切换的面外BPVE（沿z方向的光诱导极化）。基于抽象双层晶体模型的对称分析和第一性原理计算验证了这些BPVE性质，因为层间滑移引起的正、负铁电态与镜面对称有关，而镜面对称不能翻转所有的BPVE张量元。这一发现拓展了对铁电和BPVE关系的理解。一方面，面外BPVE能用于设计可控光电器件。另一方面，平面内的BPVE对铁电翻转具有鲁棒性，其不可切换的特点有利于构建更大尺寸的光电器件。