鐵電極化和體光伏效應（BPVE）是凝聚態(tài)物質(zhì)中由于對稱性破缺而出現(xiàn)的兩種基本物理現(xiàn)象。BPVE出現(xiàn)在沒有中心反演對稱的體系中，而鐵電材料往往具備BPVE的條件。因此，電極化可切換的鐵電體是研究最多的BPVE材料之一，其能量轉(zhuǎn)換效率可以超過肖克利–奎瑟極限。

以往針對鐵電材料BVPE的研究中，BPVE往往與鐵電翻轉(zhuǎn)同時出現(xiàn)。例如，在中心反演變換下，鐵電極化和BPVE在所有方向上一起翻轉(zhuǎn)，因此鐵電極化可以用來調(diào)控BPVE的光電流方向。但是僅從對稱性角度分析看，鐵電極化只需滿足極性矢量，而BPVE系數(shù)則是一個三階張量。鐵電與BPVE滿足不同的對稱性變換基本要求，原則上可以不同時翻轉(zhuǎn)。因此，準確理解鐵電序與BPVE之間的關(guān)系，對光電器件的基礎(chǔ)研究和設計都具有重要意義。

Fig. 2 Calculated results of ferroelectric bilayer MoS2.

來自安徽大學李惠教授與中科大高陽教授團隊，針對二維層間滑移鐵電體進行研究，發(fā)現(xiàn)了滑移鐵電有序可以滿足非同步的BPVE。他們利用雙層晶體模型的對稱性分析，發(fā)現(xiàn)鏡面對稱切換與層間滑移鐵電相關(guān)，但是該對稱操作不會改變所有的BPVE張量元。以WTe₂和MoS₂為例，通過第一性原理計算，作者證明面內(nèi)BPVE確實不改變鐵電序符號。

Fig. 3 Calculated results of ferroelectric bilayer WTe2.

這一發(fā)現(xiàn)拓展了對鐵電和BPVE關(guān)系的理解。一方面，面外BPVE能用于設計可控光電器件。另一方面，平面內(nèi)的BPVE對鐵電翻轉(zhuǎn)具有魯棒性，其不可切換的特點有利于構(gòu)建更大尺寸的光電器件。相關(guān)論文發(fā)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及其翻譯

Non-synchronous bulk photovoltaic effect in two-dimensional interlayer-sliding ferroelectrics (二維層間滑移鐵電體中的非同步體光伏效應)

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.

摘要自發(fā)極化和體光伏效應（BPVE）是鐵電材料中伴隨出現(xiàn)的兩種物理現(xiàn)象。鐵電序的翻轉(zhuǎn)通常伴隨著BPVE在各個方向上的切換，因為它們在中心反演對稱操作下都需要反轉(zhuǎn)。在本研究中，我們報道了二維（2D）層間滑移鐵電材料中的非同步BPVE，它具有不可切換的面內(nèi)BPVE（xy平面上的光誘導電流）和可切換的面外BPVE（沿z方向的光誘導極化）。基于抽象雙層晶體模型的對稱分析和第一性原理計算驗證了這些BPVE性質(zhì)，因為層間滑移引起的正、負鐵電態(tài)與鏡面對稱有關(guān)，而鏡面對稱不能翻轉(zhuǎn)所有的BPVE張量元。這一發(fā)現(xiàn)拓展了對鐵電和BPVE關(guān)系的理解。一方面，面外BPVE能用于設計可控光電器件。另一方面，平面內(nèi)的BPVE對鐵電翻轉(zhuǎn)具有魯棒性，其不可切換的特點有利于構(gòu)建更大尺寸的光電器件。