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Electrically tunable low-density superconductivity in a monolayer topological insulator

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Science  23 Nov 2018:
Vol. 362, Issue 6417, pp. 926-929
DOI: 10.1126/science.aar4642
  • Fig. 1 Device schematic and superconductivity characteristics.

    (A) Cartoon illustration of the device structure and the crystal structure of monolayer WTe2. (B) Optical microscopy image of device 1, with the monolayer WTe2 (red) and graphite top gate (orange) highlighted. Circuit elements show the measurement configuration. V4p, four-probe voltage; ISD, source-to-drain current. (C) Temperature dependence of the resistance for Vbg = 4 V and Vtg = 5 V. The inset shows the resistance as a function of both gate voltages, at a base temperature of 60 mK. (D) V-I characteristics from base temperature (black) up to 940 mK (red). (E) Nonlinear V-I behavior, captured by differential resistance curves, at base temperature for different perpendicular magnetic fields.

  • Fig. 2 Switching superconductivity on and off with an electrostatic gate.

    Shown are data from device 1. (A) R(T) characteristic for different gate voltages, showing the transition from a superconducting state to an insulating state, in evenly spaced increments between the labeled curves. The dashed line is a guide to the gate voltage, VcMIT, that separates the two regimes. The inset shows a color plot of the same data, normalized to the normal state resistance Rn, with Tc marked in black. (B) The upper panel shows the gate-dependent critical temperature Tc, summarized from (A). The zero-resistance region is shaded dark blue. The lower panel shows the gate-dependent critical current, Ic and Ic′, summarized from (D). For Vbg < 1 V, Ic is found to bifurcate into two peaks, the values of which are determined by extrema of the second derivative of dV/dI (Idc) data. The electron density (n2D), corresponding to the bottom-gate voltage shown on the bottom axis, is estimated from the capacitance model and is shown on the top axis. (C) Differential resistance dV/dI versus current bias Idc for Vtg = Vbg = 5V. The inset shows a zoom-in to the zero-resistance region. (D) Differential resistance (Rdiff) dV/dI versus current bias Idc and gate voltage Vbg. Close to Vbg ~ 1 V, the observed Ic trace bifurcates, as more clearly indicated in (B). The dashed line indicates the gate voltage at which the peaks in differential resistance merge at zero bias, indicating destruction of superconductivity. (E) Hall resistivity ρxy(B) for three selected bottom-gate voltages taken at 2 K (Vtg = 5 V). See details in (23). (F) Extracted Hall density (nH) as a function of the gate voltage (red circles) and the estimated density from the capacitance model (gray dashed line).

  • Fig. 3 Electronic phase diagram of monolayer WTe2.

    (A) Schematic temperature-density phase diagram for the electronic ground state of monolayer WTe2, reminiscent of the inset of Fig. 2A. (B) Resistance versus temperature for device 2, which shows a clear superconducting transition. The residual resistance at low temperature is caused by a known imperfection in this device, as described in (23). The inset shows the gate-dependent conductance of device 2, where the observed plateau corresponds to the QSHI phase. The arrow indicates the gate voltage at which the temperature dependence was recorded.

  • Fig. 4 Effect of magnetic field on the monolayer superconductivity.

    Shown are data from device 1. (A) Perpendicular magnetic field dependence of the resistance, R(B), normalized to the normal state resistance Rn, at base temperature for different gate voltages: Vtg = 5 V and Vbg = 5, 4, 3, 2, or 1 V. The inset shows a schematic of the perpendicular and parallel field orientations. See Fig. 1B for the orientation of the parallel magnetic field in the plane of the device. (B) Comparison between effects of perpendicular and parallel fields. Orange represents the parallel magnetic field dependence of resistance, R(B), normalized to the normal state resistance Rn, for the same series of gate voltages as in (A). Blue represents the perpendicular magnetic field data of (A) plotted on the same scale for comparison. (C) Bc2-Tc phase diagram for both the parallel (orange) and perpendicular (blue) orientations. Vtg = Vbg = 5 V. Black dashed lines are fits to theoretical models (23). The inset shows the temperature dependence of resistance under different parallel magnetic fields.

Supplementary Materials

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

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

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S7
    • References

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