二维Ga $XY$（$X$ = Se，Te; $Y$ = Cl，Br，I）化合物中自旋螺旋的可逆巨面外Rashba效应
Reversible giant out-of-plane Rashba effect in two-dimensional Ga$XY$ ($X$= Se, Te; $Y$= Cl, Br, I) compounds for persistent spin helix
摘要：
铁电性和自旋轨道耦合（SOC）在非中心对称系统中的共存可以允许通过经由公知的铁电Rashba效应（FRE）切换铁电极化来非易失性地控制自旋自由度。尽管铁电体系统中的铁电损耗已被广泛观察到，但在二维铁电系统中的铁电损耗却很少被发现。基于第一性原理计算，辅以$\vec{k}\cdot\vec{p}$分析，我们报道了Ga $XY$（$X$ = Se，Te; $Y$ = Cl，Br，I）单层化合物中FRE的出现，这是一类具有面内铁电性的新型二维材料.由于大的面内铁电极化，在最高的价带中观察到巨大的面外Rashba效应，在动量空间中产生单向的面外自旋织构。重要的是，这样的面外自旋纹理，它可以托管一个长寿命的螺旋自旋模式，称为持久的自旋螺旋，可以完全逆转，通过切换方向的面内铁电极化。因此，我们的研究结果可以为二维材料中单向面外Rashba效应和面内铁电性之间的相互作用开辟途径，这对于高效和非易失性自旋电子器件是有用的。
英文摘要：
The coexistence of ferroelectricity and spin-orbit coupling (SOC) in noncentrosymmetric systems may allow for a nonvolatile control of spin degrees of freedom by switching the ferroelectric polarization through the well-known ferroelectric Rashba effect (FRE). Although the FER has been widely observed for bulk ferroelectric systems, its existence in two-dimensional (2D) ferroelectric systems is still very rarely discovered. Based on first-principles calculations, supplemented with $\vec{k}\cdot\vec{p}$ analysis, we report the emergence of the FRE in the Ga$XY$ ($X$= Se, Te; $Y$= Cl, Br, I) monolayer compounds, a new class of 2D materials having in-plane ferroelectricity. Due to the large in-plane ferroelectric polarization, a giant out-of-plane Rashba effect is observed in the topmost valence band, producing unidirectional out-of-plane spin textures in the momentum space. Importantly, such out-of-plane spin textures, which can host a long-lived helical spin mode known as a persistent spin helix, can be fully reversed by switching the direction of the in-plane ferroelectric polarization. Thus, our findings can open avenues for interplay between the unidirectional out-of-plane Rashba effect and the in-plane ferroelectricity in 2D materials, which is useful for efficient and non-volatile spintronic devices.
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