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1. Electrically and magnetically switchable nonlinear photocurrent in РТ-symmetric magnetic topological quantum materials

   https://doi.org/10.1038/s41524-020-00462-9  

   Wang, Hua; Qian, Xiaofeng (December 2020, npj Computational Materials)

   null (Ed.)

   Abstract Nonlinear photocurrent in time-reversal invariant noncentrosymmetric systems such as ferroelectric semimetals sparked tremendous interest of utilizing nonlinear optics to characterize condensed matter with exotic phases. Here we provide a microscopic theory of two types of second-order nonlinear direct photocurrents, magnetic shift photocurrent (MSC) and magnetic injection photocurrent (MIC), as the counterparts of normal shift current (NSC) and normal injection current (NIC) in time-reversal symmetry and inversion symmetry broken systems. We show that MSC is mainly governed by shift vector and interband Berry curvature, and MIC is dominated by absorption strength and asymmetry of the group velocity difference at time-reversed ± k points. Taking $${\cal{P}}{\cal{T}}$$ P T -symmetric magnetic topological quantum material bilayer antiferromagnetic (AFM) MnBi 2 Te 4 as an example, we predict the presence of large MIC in the terahertz (THz) frequency regime which can be switched between two AFM states with time-reversed spin orderings upon magnetic transition. In addition, external electric field breaks $${\cal{P}}{\cal{T}}$$ P T symmetry and enables large NSC response in bilayer AFM MnBi 2 Te 4 , which can be switched by external electric field. Remarkably, both MIC and NSC are highly tunable under varying electric field due to the field-induced large Rashba and Zeeman splitting, resulting in large nonlinear photocurrent response down to a few THz regime, suggesting bilayer AFM- z MnBi 2 Te 4 as a tunable platform with rich THz and magneto-optoelectronic applications. Our results reveal that nonlinear photocurrent responses governed by NSC, NIC, MSC, and MIC provide a powerful tool for deciphering magnetic structures and interactions which could be particularly fruitful for probing and understanding magnetic topological quantum materials. 

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2. Proximity-magnetized quantum spin Hall insulator: monolayer 1 T’ WTe2/Cr2Ge2Te6

   https://doi.org/10.1038/s41467-022-32808-w  

   Li, Junxue; Rashetnia, Mina; Lohmann, Mark; Koo, Jahyun; Xu, Youming; Zhang, Xiao; Watanabe, Kenji; Taniguchi, Takashi; Jia, Shuang; Chen, Xi; et al (December 2022, Nature Communications)

   Abstract Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T’ WTe 2 , a two-dimensional quantum spin Hall insulator, and few-layer Cr 2 Ge 2 Te 6 , an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe 2 manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator. 

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3. Families of asymmetrically functionalized germanene films as promising quantum spin Hall insulators

   https://doi.org/10.1039/D0CP06231F  

   Shi, Lawrence; Li, Qiliang (February 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   Topological insulators (TIs), exhibiting the quantum spin Hall (QSH) effect, are promising for developing dissipationless transport devices that can be realized under a wide range of temperatures. The search for new two-dimensional (2D) TIs is essential for TIs to be utilized at room-temperature, with applications in optoelectronics, spintronics, and magnetic sensors. In this work, we used first-principles calculations to investigate the geometric, electronic, and topological properties of GeX and GeMX (M = C, N, P, As; X = H, F, Cl, Br, I, O, S, Se, Te). In 26 of these materials, the QSH effect is demonstrated by a spin–orbit coupling (SOC) induced large band gap and a band inversion at the Γ point, similar to the case of an HgTe quantum well. In addition, engineering the intra-layer strain of certain GeMX species can transform them from a regular insulator into a 2D TI. This work demonstrates that asymmetrical chemical functionalization is a promising method to induce the QSH effect in 2D hexagonal materials, paving the way for practical application of TIs in electronics. 

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4. Quantum metric nonlinear Hall effect in a topological antiferromagnetic heterostructure

   https://doi.org/10.1126/science.adf1506  

   Gao, Anyuan; Liu, Yu-Fei; Qiu, Jian-Xiang; Ghosh, Barun; V. Trevisan, Thaís; Onishi, Yugo; Hu, Chaowei; Qian, Tiema; Tien, Hung-Ju; Chen, Shao-Wen; et al (July 2023, Science)

   Quantum geometry in condensed-matter physics has two components: the real part quantum metric and the imaginary part Berry curvature. Whereas the effects of Berry curvature have been observed through phenomena such as the quantum Hall effect in two-dimensional electron gases and the anomalous Hall effect (AHE) in ferromagnets, the quantum metric has rarely been explored. Here, we report a nonlinear Hall effect induced by the quantum metric dipole by interfacing even-layered MnBi₂Te₄with black phosphorus. The quantum metric nonlinear Hall effect switches direction upon reversing the antiferromagnetic (AFM) spins and exhibits distinct scaling that is independent of the scattering time. Our results open the door to discovering quantum metric responses predicted theoretically and pave the way for applications that bridge nonlinear electronics with AFM spintronics. 

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5. Layer- and gate-tunable spin-orbit coupling in a high-mobility few-layer semiconductor

   https://doi.org/10.1126/sciadv.abe2892  

   Shcherbakov, Dmitry; Stepanov, Petr; Memaran, Shahriar; Wang, Yaxian; Xin, Yan; Yang, Jiawei; Wei, Kaya; Baumbach, Ryan; Zheng, Wenkai; Watanabe, Kenji; et al (January 2021, Science Advances)

   null (Ed.)

   Spin-orbit coupling (SOC) is a relativistic effect, where an electron moving in an electric field experiences an effective magnetic field in its rest frame. In crystals without inversion symmetry, it lifts the spin degeneracy and leads to many magnetic, spintronic, and topological phenomena and applications. In bulk materials, SOC strength is a constant. Here, we demonstrate SOC and intrinsic spin splitting in atomically thin InSe, which can be modified over a broad range. From quantum oscillations, we establish that the SOC parameter α is thickness dependent; it can be continuously modulated by an out-of-plane electric field, achieving intrinsic spin splitting tunable between 0 and 20 meV. Unexpectedly, α could be enhanced by an order of magnitude in some devices, suggesting that SOC can be further manipulated. Our work highlights the extraordinary tunability of SOC in 2D materials, which can be harnessed for in operando spintronic and topological devices and applications. 

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