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1. Electric control of a canted-antiferromagnetic Chern insulator

   https://doi.org/10.1038/s41467-022-29259-8  

   Cai, Jiaqi ; Ovchinnikov, Dmitry ; Fei, Zaiyao ; He, Minhao ; Song, Tiancheng ; Lin, Zhong ; Wang, Chong ; Cobden, David ; Chu, Jiun-Haw ; Cui, Yong-Tao ; et al ( March 2022 , Nature Communications)

   The interplay between band topology and magnetism can give rise to exotic states of matter. For example, magnetically doped topological insulators can realize a Chern insulator that exhibits quantized Hall resistance at zero magnetic field. While prior works have focused on ferromagnetic systems, little is known about band topology and its manipulation in antiferromagnets. Here, we report that MnBi₂Te₄is a rare platform for realizing a canted-antiferromagnetic (cAFM) Chern insulator with electrical control. We show that the Chern insulator state with Chern numberC = 1 appears as the AFM to canted-AFM phase transition happens. The Chern insulator state is further confirmed by observing the unusual transition of theC = 1 state in the cAFM phase to theC = 2 orbital quantum Hall states in the magnetic field induced ferromagnetic phase. Near the cAFM-AFM phase boundary, we show that the dissipationless chiral edge transport can be toggled on and off by applying an electric field alone. We attribute this switching effect to the electrical field tuning of the exchange gap alignment between the top and bottom surfaces. Our work paves the way for future studies on topological cAFM spintronics and facilitates the development of proof-of-concept Chern insulator devices.

    

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2. Topological current divider in a Chern insulator junction

   https://doi.org/10.1038/s41467-022-33645-7  

   Ovchinnikov, Dmitry ; Cai, Jiaqi ; Lin, Zhong ; Fei, Zaiyao ; Liu, Zhaoyu ; Cui, Yong-Tao ; Cobden, David H. ; Chu, Jiun-Haw ; Chang, Cui-Zu ; Xiao, Di ; et al ( December 2022 , Nature Communications)

   Abstract A Chern insulator is a two-dimensional material that hosts chiral edge states produced by the combination of topology with time reversal symmetry breaking. Such edge states are perfect one-dimensional conductors, which may exist not only on sample edges, but on any boundary between two materials with distinct topological invariants (or Chern numbers). Engineering of such interfaces is highly desirable due to emerging opportunities of using topological edge states for energy-efficient information transmission. Here, we report a chiral edge-current divider based on Chern insulator junctions formed within the layered topological magnet MnBi 2 Te 4 . We find that in a device containing a boundary between regions of different thickness, topological domains with different Chern numbers can coexist. At the domain boundary, a Chern insulator junction forms, where we identify a chiral edge mode along the junction interface. We use this to construct topological circuits in which the chiral edge current can be split, rerouted, or switched off by controlling the Chern numbers of the individual domains. Our results demonstrate MnBi 2 Te 4 as an emerging platform for topological circuits design. 

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3. Quantized resistance revealed at the criticality of the quantum anomalous Hall phase transitions

   https://doi.org/10.1038/s41467-023-40784-y  

   Deng, Peng ; Zhang, Peng ; Eckberg, Christopher ; Chong, Su Kong ; Yin, Gen ; Emmanouilidou, Eve ; Che, Xiaoyu ; Ni, Ni ; Wang, Kang L. ( September 2023 , Nature Communications)

   In multilayered magnetic topological insulator structures, magnetization reversal processes can drive topological phase transitions between quantum anomalous Hall, axion insulator, and normal insulator states. Here we report an examination of the critical behavior of two such transitions: the quantum anomalous Hall to normal insulator (QAH-NI), and quantum anomalous Hall to axion insulator (QAH-AXI) transitions. By introducing a new analysis protocol wherein temperature dependent variations in the magnetic coercivity are accounted for, the critical behavior of the QAH-NI and QAH-AXI transitions are evaluated over a wide range of temperature and magnetic field. Despite the uniqueness of these different transitions, quantized longitudinal resistance and Hall conductance are observed at criticality in both cases. Furthermore, critical exponents were extracted for QAH-AXI transitions occurring at magnetization reversals of two different magnetic layers. The observation of consistent critical exponents and resistances in each case, independent of the magnetic layer details, demonstrates critical behaviors in quantum anomalous Hall transitions to be of electronic rather than magnetic origin. Our finding offers a new avenue for studies of phase transition and criticality in QAH insulators.

    

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4. Evidence for one-dimensional chiral edge states in a magnetic Weyl semimetal Co3Sn2S2

   https://doi.org/10.1038/s41467-021-24561-3  

   Howard, Sean ; Jiao, Lin ; Wang, Zhenyu ; Morali, Noam ; Batabyal, Rajib ; Kumar-Nag, Pranab ; Avraham, Nurit ; Beidenkopf, Haim ; Vir, Praveen ; Liu, Enke ; et al ( July 2021 , Nature Communications)

   The physical realization of Chern insulators is of fundamental and practical interest, as they are predicted to host the quantum anomalous Hall (QAH) effect and topologically protected chiral edge states which can carry dissipationless current. Current realizations of the QAH state often require complex heterostructures and sub-Kelvin temperatures, making the discovery of intrinsic, high temperature QAH systems of significant interest. In this work we show that time-reversal symmetry breaking Weyl semimetals, being essentially stacks of Chern insulators with inter-layer coupling, may provide a new platform for the higher temperature realization of robust chiral edge states. We present combined scanning tunneling spectroscopy and theoretical investigations of the magnetic Weyl semimetal, Co₃Sn₂S₂. Using modeling and numerical simulations we find that depending on the strength of the interlayer coupling, chiral edge states can be localized on partially exposed kagome planes on the surfaces of a Weyl semimetal. Correspondingly, our dI/dVmaps on the kagome Co₃Sn terraces show topological states confined to the edges which display linear dispersion. This work provides a new paradigm for realizing chiral edge modes and provides a pathway for the realization of higher temperature QAH effect in magnetic Weyl systems in the two-dimensional limit.

    

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5. Exchange‐Biased Quantum Anomalous Hall Effect

   https://doi.org/10.1002/adma.202300391  

   Zhang, Peng ; Balakrishnan, Purnima P. ; Eckberg, Christopher ; Deng, Peng ; Nozaki, Tomohiro ; Chong, Su Kong ; Quarterman, Patrick ; Holtz, Megan E. ; Maranville, Brian B. ; Qiu, Gang ; et al ( June 2023 , Advanced Materials)

   The quantum anomalous Hall (QAH) effect is characterized by a dissipationless chiral edge state with a quantized Hall resistance at zero magnetic field. Manipulating the QAH state is of great importance in both the understanding of topological quantum physics and the implementation of dissipationless electronics. Here, the QAH effect is realized in the magnetic topological insulator Cr‐doped (Bi,Sb)₂Te₃(CBST) grown on an uncompensated antiferromagnetic insulator Al‐doped Cr₂O₃. Through polarized neutron reflectometry (PNR), a strong exchange coupling is found between CBST and Al‐Cr₂O₃surface spins fixing interfacial magnetic moments perpendicular to the film plane. The interfacial coupling results in an exchange‐biased QAH effect. This study further demonstrates that the magnitude and sign of the exchange bias can be effectively controlled using a field training process to set the magnetization of the Al‐Cr₂O₃layer. It demonstrates the use of the exchange bias effect to effectively manipulate the QAH state, opening new possibilities in QAH‐based spintronics.

    

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