More Like this

1. 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.
2. 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.
3. Emergent helical edge states in a hybridized three-dimensional topological insulator

   https://doi.org/10.1038/s41467-022-33643-9  

   Chong, Su Kong ; Liu, Lizhe ; Watanabe, Kenji ; Taniguchi, Takashi ; Sparks, Taylor D. ; Liu, Feng ; Deshpande, Vikram V. ( October 2022 , Nature Communications)

   As the thickness of a three-dimensional (3D) topological insulator (TI) becomes comparable to the penetration depth of surface states, quantum tunneling between surfaces turns their gapless Dirac electronic structure into a gapped spectrum. Whether the surface hybridization gap can host topological edge states is still an open question. Herein, we provide transport evidence of 2D topological states in the quantum tunneling regime of a bulk insulating 3D TI BiSbTeSe₂. Different from its trivial insulating phase, this 2D topological state exhibits a finite longitudinal conductance at ~2e²/h when the Fermi level is aligned within the surface gap, indicating an emergent quantum spin Hall (QSH) state. The transition from the QSH to quantum Hall (QH) state in a transverse magnetic field further supports the existence of this distinguished 2D topological phase. In addition, we demonstrate a second route to realize the 2D topological state via surface gap-closing and topological phase transition mechanism mediated by a transverse electric field. The experimental realization of the 2D topological phase in a 3D TI enriches its phase diagram and marks an important step toward functionalized topological quantum devices.
4. Enhanced Superconductivity in Monolayer T _d -MoTe ₂

   https://doi.org/10.1021/acs.nanolett.0c04935  

   Rhodes, Daniel A. ; Jindal, Apoorv ; Yuan, Noah F. ; Jung, Younghun ; Antony, Abhinandan ; Wang, Hua ; Kim, Bumho ; Chiu, Yu-che ; Taniguchi, Takashi ; Watanabe, Kenji ; et al ( March 2021 , Nano Letters)

   Kim, Philip (Ed.)

   Crystalline two-dimensional (2D) superconductors (SCs) with low carrier density are an exciting new class of materials in which electrostatic gating can tune superconductivity, electronic interactions play a prominent role, and electrical transport properties may directly reflect the topology of the Fermi surface. Here, we report the dramatic enhancement of superconductivity with decreasing thickness in semimetallic Td-MoTe2, with critical temperature (Tc) increasing up to 7.6 K for monolayers, a 60-fold increase with respect to the bulk Tc. We show that monolayers possess a similar electronic structure and density of states (DOS) as the bulk, implying that electronic interactions play a strong role in the enhanced superconductivity. Reflecting the low carrier density, the critical temperature, magnetic field, and current density are all tunable by an applied gate voltage. The response to high in-plane magnetic fields is distinct from that of other 2D SCs and reflects the canted spin texture of the electron pockets.
5. Field-effect conductivity scaling for two-dimensional materials with tunable impurity density

   https://doi.org/10.1088/2053-1583/ac72b0  

   Wang, Chulin ; Peng, Lintao ; Wells, Spencer A ; Cain, Jeffrey D ; Huang, Yi-Kai ; Rhoads, Lawrence A ; Dravid, Vinayak P ; Hersam, Mark C ; Grayson, Matthew A ( June 2022 , 2D Materials)

   Abstract A scaling law is demonstrated in the conductivity of gated two-dimensional (2D) materials with tunable concentrations of ionized impurity scatterers. Experimental data is shown to collapse onto a single 2D conductivity scaling (2DCS) curve when the mobility is scaled by r , the relative impurity-induced scattering, and the gate voltage is shifted by V s , a consequence of impurity-induced doping. This 2DCS analysis is demonstrated first in an encapsulated 2D black phosphorus multilayer at T = 100 K with charge trap densities programmed by a gate bias upon cooldown, and next in a Bi 2 Se 3 2D monolayer at room temperature exposed to varying concentrations of gas adsorbates. The observed scaling can be explained using a conductivity model with screened ionized impurity scatterers. The slope of the r  vs.  V s plot defines a disorder-charge specific scattering rate Γ q = d r / d V s equivalent to a scattering strength per unit impurity charge density: Γ q > 0 indicates a preponderance of positively charged impurities with Γ q < 0 for negatively charged. This 2DCS analysis is expected to be applicable in arbitrary 2D materials systems with tunable impurity density, which will advance 2Dmore »materials characterization and improve performance of 2D sensors and transistors.« less