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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. Electrically tunable low-density superconductivity in a monolayer topological insulator

   https://doi.org/10.1126/science.aar4642  

   Fatemi, Valla ; Wu, Sanfeng ; Cao, Yuan ; Bretheau, Landry ; Gibson, Quinn D. ; Watanabe, Kenji ; Taniguchi, Takashi ; Cava, Robert J. ; Jarillo-Herrero, Pablo ( October 2018 , Science)

   Turning on superconductivity in a topologically nontrivial insulator may provide a route to search for non-Abelian topological states. However, existing demonstrations of superconductor-insulator switches have involved only topologically trivial systems. Here we report reversible, in situ electrostatic on-off switching of superconductivity in the recently established quantum spin Hall insulator monolayer tungsten ditelluride (WTe₂). Fabricated into a van der Waals field-effect transistor, the monolayer’s ground state can be continuously gate-tuned from the topological insulating to the superconducting state, with critical temperaturesT_cup to ~1 kelvin. Our results establish monolayer WTe₂as a material platform for engineering nanodevices that combine superconducting and topological phases of matter.

    

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3. Electric-double-layer-gated transistors based on two-dimensional crystals: recent approaches and advances

   https://doi.org/10.1088/2515-7639/ab8270  

   Xu, Ke ; Fullerton-Shirey, Susan K. ( May 2020 , Journal of Physics: Materials)

   Electric-double-layer (EDL) gated transistors use ions in an electrolyte to induce charge in the channel of the transistor by field-effect. Because a sub-nanometer gap capacitor is created at the electrolyte/channel interface, large capacitance densities (∼µF cm⁻²) corresponding to high sheet carrier densities (10¹⁴cm⁻²) can be induced, exceeding conventional gate dielectrics by about one order of magnitude. Because it is an interfacial technique, EDL gating is especially effective on two-dimensional (2D) crystals, which—at the monolayer limit—are basically interfaces themselves. Both solid polymer electrolytes and ionic liquids are routinely used as ion-conducting gate dielectrics, and they have provided access to regimes of transport in 2D materials that would be inaccessible otherwise. The technique, now widely used, has enabled the 2D crystal community to study superconductivity, spin- and valleytronics, investigate electrical and structural phase transitions, and create abruptp-njunctions to generate tunneling, among others. In addition to using EDL gating as a tool to investigate properties of the 2D crystals, more recent efforts have emerged to engineer the electrolyte to add new functionality and device features, such as synaptic plasticity, bistability and non-volatility. Example of potential applications include neuromorphic computing and non-volatile memory. This review focuses on using ions forelectrostaticcontrol of 2D crystal transistors both to uncover basic properties of 2D crystals, and also to add new device functionalities.

    

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4. Imaging quantum spin Hall edges in monolayer WTe 2

   https://doi.org/10.1126/sciadv.aat8799  

   Shi, Yanmeng ; Kahn, Joshua ; Niu, Ben ; Fei, Zaiyao ; Sun, Bosong ; Cai, Xinghan ; Francisco, Brian A. ; Wu, Di ; Shen, Zhi-Xun ; Xu, Xiaodong ; et al ( February 2019 , Science Advances)

   A two-dimensional (2D) topological insulator exhibits the quantum spin Hall (QSH) effect, in which topologically protected conducting channels exist at the sample edges. Experimental signatures of the QSH effect have recently been reported in an atomically thin material, monolayer WTe 2 . Here, we directly image the local conductivity of monolayer WTe 2 using microwave impedance microscopy, establishing beyond doubt that conduction is indeed strongly localized to the physical edges at temperatures up to 77 K and above. The edge conductivity shows no gap as a function of gate voltage, and is suppressed by magnetic field as expected. We observe additional conducting features which can be explained by edge states following boundaries between topologically trivial and nontrivial regions. These observations will be critical for interpreting and improving the properties of devices incorporating WTe 2 . Meanwhile, they reveal the robustness of the QSH channels and the potential to engineer them in the monolayer material platform. 

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5. Tunable magnetic confinement effect in a magnetic superlattice of graphene

   https://doi.org/10.1038/s41699-024-00468-7  

   Tosun, Onur ; Sarkar, Preetha ; Qian, Chang ; Gilbert, Matthew ; Chen, Qian ; Mason, Nadya ( April 2024 , npj 2D Materials and Applications)

   Two-dimensional van der Waals materials such as graphene present an opportunity for band structure engineering using custom superlattice potentials. In this study, we demonstrate how self-assemblies of magnetic iron-oxide (Fe₃O₄) nanospheres stacked on monolayer graphene generate a proximity-induced magnetic superlattice in graphene and modify its band structure. Interactions between the nanospheres and the graphene layer generate superlattice Dirac points in addition to a gapped energy spectrum near the K and K′ valleys, resulting in magnetic confinement of quasiparticles around the nanospheres. This is evidenced by gate-dependent resistance oscillations, observed in our low temperature transport measurements, and confirmed by self-consistent tight binding calculations. Furthermore, we show that an external magnetic field can tune the magnetic superlattice potential created by the nanospheres, and thus the transport characteristics of the system. This technique for magnetic-field-tuned band structure engineering using magnetic nanostructures can be extended to a broader class of 2D van der Waals and topological materials.

    

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