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1. Monolayer Superconductivity and Tunable Topological Electronic Structure at the Fe(Te,Se)/Bi 2 Te 3 Interface

   https://doi.org/10.1002/adma.202210940  

   Moore, Robert G. ; Lu, Qiangsheng ; Jeon, Hoyeon ; Yao, Xiong ; Smith, Tyler ; Pai, Yun‐Yi ; Chilcote, Michael ; Miao, Hu ; Okamoto, Satoshi ; Li, An‐Ping ; et al ( April 2023 , Advanced Materials)

   The interface between 2D topological Dirac states and ans‐wave superconductor is expected to support Majorana‐bound states (MBS) that can be used for quantum computing applications. Realizing these novel states of matter and their applications requires control over superconductivity and spin‐orbit coupling to achieve spin‐momentum‐locked topological interface states (TIS) which are simultaneously superconducting. While signatures of MBS have been observed in the magnetic vortex cores of bulk FeTe_0.55Se_0.45, inhomogeneity and disorder from doping make these signatures unclear and inconsistent between vortices. Here superconductivity is reported in monolayer (ML) FeTe_1–ySe_y(Fe(Te,Se)) grown on Bi₂Te₃by molecular beam epitaxy (MBE). Spin and angle‐resolved photoemission spectroscopy (SARPES) directly resolve the interfacial spin and electronic structure of Fe(Te,Se)/Bi₂Te₃heterostructures. Fory = 0.25, the Fe(Te,Se) electronic structure is found to overlap with the Bi₂Te₃TIS and the desired spin‐momentum locking is not observed. In contrast, fory = 0.1, reduced inhomogeneity measured by scanning tunneling microscopy (STM) and a smaller Fe(Te,Se) Fermi surface with clear spin‐momentum locking in the topological states are found. Hence, it is demonstrated that the Fe(Te,Se)/Bi₂Te₃system is a highly tunable platform for realizing MBS where reduced doping can improve characteristics important for Majorana interrogation and potential applications.

    

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2. Spintronic Quantum Phase Transition in a Graphene/Pb 0.24 Sn 0.76 Te Heterostructure with Giant Rashba Spin‐Orbit Coupling

   https://doi.org/10.1002/adfm.202311875  

   DeMell, Jennifer E. ; Naumov, Ivan ; Stephen, Gregory M. ; Blumenschein, Nicholas A. ; Sun, Y. ‐J. Leo ; Fedorko, Adrian ; Robinson, Jeremy T. ; Campbell, Paul M. ; Taylor, Patrick J. ; Heiman, Don ; et al ( December 2023 , Advanced Functional Materials)

   Mechanical stacking of two dissimilar materials often has surprising consequences for heterostructure behavior. In particular, a 2D electron gas (2DEG) is formed in the heterostructure of the topological crystalline insulator Pb_0.24Sn_0.76Te and graphene due to contact of a polar with a nonpolar surface and the resulting changes in electronic structure needed to avoid polar catastrophe. The spintronic properties of this heterostructure with non‐local spin valve devices are studied. This study observes spin‐momentum locking at lower temperatures that transitions to regular spin channel transport only at ≈40 K. Hanle spin precession measurements show a spin relaxation time as high as 2.18 ns. Density functional theory calculations confirm that the spin‐momentum locking is due to a giant Rashba effect in the material and that the phase transition is a Lifshitz transition. The theoretically predicted Lifshitz transition is further evident in the phase transition‐like behavior in the Landé g‐factor and spin relaxation time.

    

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3. Spin memory of the topological material under strong disorder

   https://doi.org/10.1038/s41535-020-0241-5  

   Korzhovska, Inna ; Deng, Haiming ; Zhao, Lukas ; Deshko, Yury ; Chen, Zhiyi ; Konczykowski, Marcin ; Zhao, Shihua ; Raoux, Simone ; Krusin-Elbaum, Lia ( June 2020 , npj Quantum Materials)

   Robustness to disorder is the defining property of any topological state. The ultimate disorder limits to topological protection are still unknown, although a number of theories predict that even in the amorphous state a quantized conductance might yet reemerge. Here we report that in strongly disordered thin films of the topological material Sb₂Te₃disorder-induced spin correlationsdominate transport of charge—they engender a spin memory phenomenon, generated by the nonequilibrium charge currents controlled by localized spins. We directly detect a glassy yet robust disorder-induced magnetic signal in filmsfree of extrinsic magnetic dopants, which becomes null in a lower-disorder crystalline state. This is where large isotropic negative magnetoresistance (MR)—a hallmark of spin memory—crosses over to positive MR, first with only one e²/hquantum conduction channel, in a weakly antilocalized diffusive transport regime with a 2D scaling characteristic of the topological state. A fresh perspective revealed by our findings is that spin memory effect sets a disorder threshold to the protected topological state. It also points to new possibilities of tuning spin-dependent charge transport by disorder engineering of topological materials.

    

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4. 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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5. Anomalous Landau quantization in intrinsic magnetic topological insulators

   https://doi.org/10.1038/s41467-023-40383-x  

   Chong, Su Kong ; Lei, Chao ; Lee, Seng Huat ; Jaroszynski, Jan ; Mao, Zhiqiang ; MacDonald, Allan H. ; Wang, Kang L. ( August 2023 , Nature Communications)

   The intrinsic magnetic topological insulator, Mn(Bi_1−xSb_x)₂Te₄, has been identified as a Weyl semimetal with a single pair of Weyl nodes in its spin-aligned strong-field configuration. A direct consequence of the Weyl state is the layer dependent Chern number,$$C$$$C$. Previous reports in MnBi₂Te₄thin films have shown higher$$C$$$C$states either by increasing the film thickness or controlling the chemical potential. A clear picture of the higher Chern states is still lacking as data interpretation is further complicated by the emergence of surface-band Landau levels under magnetic fields. Here, we report a tunable layer-dependent$$C$$$C$ = 1 state with Sb substitution by performing a detailed analysis of the quantization states in Mn(Bi_1−xSb_x)₂Te₄dual-gated devices—consistent with calculations of the bulk Weyl point separation in the doped thin films. The observed Hall quantization plateaus for our thicker Mn(Bi_1−xSb_x)₂Te₄films under strong magnetic fields can be interpreted by a theory of surface and bulk spin-polarised Landau level spectra in thin film magnetic topological insulators.

    

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