本文通过第一性原理计算研究二维Janus材料W2COX（X=S, Se, Te）中非线性Rashba效应，发现需引入三阶动量项才能描述其自旋劈裂，并揭示其拓扑非平庸性质带来的可观自旋霍尔电导率和自旋霍尔角，展示了其在自旋电子器件中的应用潜力。
摘要：
Rashba自旋轨道耦合（RSOC）无需依赖外部磁场即可实现自旋操控，为先进自旋电子器件开辟了令人兴奋的可能性。在本工作中，我们研究了晶体动量（$k$）非线性和各向异性对传统Rashba效应的影响，特别关注它们对新预测的二维Janus材料家族W$_2$COX（X = S, Se, Te）中自旋霍尔电导率（SHC）的影响。利用第一性原理密度泛函理论计算，我们确认了所研究二维材料的动力学稳定性和力学稳定性。引人注目的是，该材料家族在费米能级附近的布里渊区$Γ$点表现出显著的非线性Rashba自旋劈裂，这无法由线性-$k$ Rashba模型充分描述。因此，必须将三阶动量贡献（{$k^3$}）纳入Rashba哈密顿量。我们的分析表明，在所研究的体系中，W$_2$COS表现出最高的{$k^3$}贡献，为$-45.9$ eV Å$^3$，尽管其线性Rashba常数最低。对电子结构的详细分析揭示了这些二维材料中的拓扑非平庸行为，产生了可观的SHC，且该SHC主要由非线性Rashba效应主导。值得注意的是，这些材料还表现出大的自旋霍尔角（0.018--2.5），可与体相拓扑绝缘体Bi$_2$Se$_3$和Bi$_2$Te$_3$相媲美，并超过窄带隙体相半导体GeTe和SnTe以及Pt等重金属。可观的SHC、大的自旋霍尔角，以及在保持拓扑性质不变的情况下通过电场调控SHC的能力（其根源在于晶体场劈裂），凸显了这些材料在自旋电子应用中的潜力。
英文摘要：
Rashba spin-orbit coupling (RSOC) facilitates spin manipulation without relying on an external magnetic field, opening up exciting possibilities for advanced spintronic devices. In this work, we examine the effects of crystal momentum ($k$) nonlinearity and anisotropy on the conventional Rashba effect, with a particular focus on their impact on the spin Hall conductivity (SHC) in a newly predicted family of 2D Janus materials, W$_2$COX (X = S, Se, Te). Using first-principles density functional theory calculations, we confirm the dynamical and mechanical stability of the studied 2D materials. Strikingly, this materials family exhibits pronounced nonlinear Rashba spin splitting at the $Γ$ point of Brillouin zone near the Fermi level, which cannot be adequately described by the linear-$k$ Rashba model. Therefore, third-order momentum contributions ({$k^3$}) must be incorporated into the Rashba Hamiltonian. Our analysis reveals that among the studied systems, W$_2$COS exhibits the highest {$k^3$} contribution of $-45.9$\,eV\,{Å$^3$}, despite having the lowest linear Rashba constant. A detailed analysis of electronic structure reveals topological nontrivial behaviour in these 2D materials, yielding sizable SHC that is primarily governed by the nonlinear Rashba effect. Notably, these materials also exhibit large spin Hall angle (0.018 -- 2.5), which is comparable to that of in bulk topological insulators like Bi$_2$Se$_3$ and Bi$_2$Te$_3$, and surpassing those in narrow bandgap bulk semiconductors GeTe and SnTe, as well as heavy metals such as Pt. Sizable SHC, large spin Hall angles, and the ability to tune SHC via electric fields without altering the topological properties, rooted in the crystal field splitting, underscore the potential of these materials for spintronic applications.
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Institute of Sustainable and Autonomous Maritime Systems, University of Duisburg-Essen（杜伊斯堡-埃森大学可持续与自主海事系统研究所）；Department of Physics and Energy Engineering, Amirkabir University of Technology（阿米尔卡比尔理工大学物理与能源工程系）；Autonomous and Energy-efficient Maritime Technologies, Technische Universität Berlin（柏林工业大学自主与节能海事技术部门）；Institute of Fundamental Technological Research, Polish Academy of Sciences（波兰科学院基础技术研究所以）
Emergence of Rashba splitting and spin-valley properties in Janus MoGeSiP2As2 and WGeSiP2As2 monolayers
International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所）；Department of Physics and Energy Harvest Storage Center, University of Ulsan（蔚山大学物理与能量收集中心）；College of Physics and Optoelectronic Engineering, Shenzhen University（深圳大学物理与光电工程学院）
When Energy and Information Revolutions Meet 2D Janus
Huazhong University of Science and Technology（华中科技大学）；Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology（华中科技大学武汉国家高磁场科学中心）；Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology（华中科技大学武汉光电国家研究中心）；School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology（华中科技大学化学与化工学院）；Charles University（查理大学）
