Research ArticleCONDENSED MATTER PHYSICS

Imaging quantum spin Hall edges in monolayer WTe2

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Science Advances  08 Feb 2019:
Vol. 5, no. 2, eaat8799
DOI: 10.1126/sciadv.aat8799
  • Fig. 1 Imaging edge conductivity in monolayer WTe2.

    (A) Schematics of the technique and device structure. (B) Optical image of a WTe2 monolayer exfoliated onto SiO2 and covered with a 10-nm-thick hBN. (C to E) MIM-Im images of the regions marked in (B), measured at T = 8 K and B = 0. Scale bars, 5 μm.

  • Fig. 2 Gate and magnetic field dependence of the edge conduction.

    (A) MIM-Im and MIM-Re images of part of a monolayer WTe2 flake between two thin graphite contacts, measured at B = 0. (B) MIM-Im and MIM-Re obtained along the dashed line in (A) and stacked as a function of gate voltage. The corresponding carrier density is plotted on a separate axis, which includes an offset by the charge neutrality point. The upper right panel plots the averaged linecuts over the bulk region indicated by the two dashed lines in the MIM-Im channel, from Vg = −20 to 40 V. (C and D) Real-space images and gate voltage dependence at B = 9 T. (E) MIM-Im and MIM-Re signals as a function of 2D resistivity. The colored bands match those in the linecuts in (B), and the dashed lines indicate the 2D resistivity at the edge for B = 0 and 9 T, estimated from the line traces in (F). a.u., arbitrary units. (F) Averaged MIM-Im (top) and MIM-Re (bottom) traces of linecuts in between the two contacts with the edge position aligned at both B = 0 and 9 T. Insets are the comparison between the 9 T line traces and the simulated MIM responses. The measurement temperature is 5 K. Scale bars, 1 μm.

  • Fig. 3 Conduction at oxidized edges and internal cracks in monolayer WTe2.

    (A) Optical image of a monolayer WTe2 flake partially covered by hBN. (B) MIM image of the same flake measured at T = 480 mK and B = 0. High conductivity is observed both at the physical edges and along lines in the interior. (C) Polarized Raman spectroscopy and angular dependence measured at 5 K, for spot #5 marked in (B). (D) Polar plot of the 163 cm−1 Raman peak intensity for the five spots marked in (B). All have the same angular dependence, showing that the crystal axes are the same and implying that the lines are cracks. (E and F) MIM-Im images of another monolayer WTe2 sample at 77 K (E) and 100 K (F) (Note that the scanned areas are slightly shifted and include a piece of graphite contact near the upper left corner in the 100 K image.) Scale bars, 5 μm.

  • Fig. 4 Conductivity features near contacts and around defects in monolayer WTe2.

    (A and B) MIM-Im and MIM-Re images of part of a monolayer (1L) WTe2 device between two Pt contacts, measured at T = 480 mK, Vg = 0 V, and B = 12 T. Scale bars, 500 nm. (C and D) MIM-Im and topography images of part of a second monolayer WTe2 device, measured at T = 10 K, Vg = 3.3 V, and B = 0 T. In the topography, the flake appears continuous, but MIM reveals that the regions around the contacts are highly insulating (dark). Scale bars, 1 μm. (E and F) MIM-Im and topography images of part of a third monolayer WTe2 device, measured at T = 10 K, Vg = 0 V, and B = 9 T. The small rings visible in the MIM-Im image correspond to the blisters in the topography image. Scale bars, 3 μm. (G) Cartoon illustrating various conductivity features observed in our experiments.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaat8799/DC1

    Section S1. Sample structures

    Section S2. MIM images of monolayer WTe2 flake

    Section S3. Gate dependence of transport and MIM signals

    Section S4. Simulation of the edge parameters

    Section S5. The monolayer/bilayer WTe2 flake in Fig. 3

    Section S6. Gate dependence of internal cracks

    Fig. S1. MIM-Im images taken for a large monolayer WTe2 flake, overlaid on the optical image at T = 8 K and B = 0.

    Fig. S2. Gate-dependent MIM images of the sample presented in Fig. 4C.

    Fig. S3. Gate dependence of transport and MIM.

    Fig. S4. Simulation of MIM near the edge.

    Fig. S5. Conceptual model of the effects of disorder on the edge conduction in a magnetic field.

    Fig. S6. AFM topography image of the monolayer WTe2 flake presented in Fig. 3 (A to D).

    Fig. S7. Gate dependence of internal cracks.

    Table S1. Information of the monolayer WTe2 samples presented in the main text.

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Sample structures
    • Section S2. MIM images of monolayer WTe2 flake
    • Section S3. Gate dependence of transport and MIM signals
    • Section S4. Simulation of the edge parameters
    • Section S5. The monolayer/bilayer WTe2 flake in Fig. 3
    • Section S6. Gate dependence of internal cracks
    • Fig. S1. MIM-Im images taken for a large monolayer WTe2 flake, overlaid on the optical image at T = 8 K and B = 0.
    • Fig. S2. Gate-dependent MIM images of the sample presented in Fig. 4C.
    • Fig. S3. Gate dependence of transport and MIM.
    • Fig. S4. Simulation of MIM near the edge.
    • Fig. S5. Conceptual model of the effects of disorder on the edge conduction in a magnetic field.
    • Fig. S6. AFM topography image of the monolayer WTe2 flake presented in Fig. 3 (A to D).
    • Fig. S7. Gate dependence of internal cracks.
    • Table S1. Information of the monolayer WTe2 samples presented in the main text.

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