Research ArticleMATERIALS SCIENCE

A room-temperature ferroelectric semimetal

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Science Advances  05 Jul 2019:
Vol. 5, no. 7, eaax5080
DOI: 10.1126/sciadv.aax5080
  • Fig. 1 Crystal and electronic structure of WTe2.

    (A) Atomistic model of the Td phase of WTe2. (B) XRD θ-2θ scan on a WTe2 single crystal. Inset shows a zoom-in on the (002) diffraction peak. a.u., arbitrary units. (C) Temperature-dependent resistance of a WTe2 sample with a thickness of ~50 nm measured from room temperature to 10 K. Inset shows the optical micrograph of the sample. (D and E) SdH oscillations of a WTe2 sample with a thickness of ~30 nm measured at 30 mK (D) and corresponding fast Fourier transform (E).

  • Fig. 2 Ferroelectric domains resolved in WTe2 single crystals.

    (A) Schematic of the experimental setup for imaging domains in WTe2 single crystals. (B) High-magnification optical image of the WTe2 single crystal sample. Inset shows optical image of a bulk WTe2 single crystal. (C to E) Topography (C), corresponding PFM amplitude (D), and PFM phase images (E). (F to H) High-magnification imaging of small ferroelectric domains enclosed within the rectangle shown in (E), topography (F), corresponding PFM amplitude (G), and PFM phase images (H). (I to K) Cross-sectional profiles of topography (I), PFM amplitude (J), and phase (K) across the dashed white line shown in (H) for respective images in (F) to (H).

  • Fig. 3 Probing ferroelectricity in a metal-gated WTe2 thin film sample.

    (A) Schematic of the experimental setup and geometry. Inset shows zoomed-in side view of the device structure. (B) Topography image of the metal (Ti/Au)–encapsulated WTe2 flake on a Si/SiO2 substrate covered with a conductive buffer layer of Ti/Au. Inset shows the profile of surface morphology along dashed white line. (C and D) Spectroscopic bias-dependent piezoresponse phase (C) and amplitude (D) hysteretic curves acquired through the top metal electrode gating the WTe2 flake. (E and F) Topography image showing zoom-in on the metal-gated WTe2 (E) and the corresponding piezoresponse phase image (F). (G) PFM phase image after application of a bias pulse of −2.5 V.

  • Fig. 4 First-principles DFT calculations.

    (A) Band structure of Td-WTe2. (B) Planar averaged conduction charge densities contributed from the electron pocket (top) and the hole pocket (bottom). (C) Schematic of different distortion vectors in 1T-WTe2. The solid straight arrows denote three equivalent distortion vectors for the polarization up states (Pup1, Pup2, and Pup3), while the dashed straight arrows denote three equivalent distortions vectors for the polarization down states (Pdown1, Pdown2, and Pdown3). The purple curved arrows denote the possible transition paths from Pup1 to Pdown1, involving the switching of different distortion vectors. (D) The energy evolution between opposite polarization states of Td-WTe2. The two opposite polarization states (Pup1 and Pdown1) are connected by the inversion operation (top), while the polarization down state (Pdown2) has a different distortion vector, which can be obtained by application of a threefold rotation on Pdown1 (bottom). The energies of intermediate states are calculated using the atomic position r(α) = rup + α(rdownrup), where 0 < α < 1 and rup and rdown are the atomic positions of Pup and Pdown, respectively. Insets show the top views of the different states. The red dashed boxes in the insets denote the unit cell of Td-WTe2.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/7/eaax5080/DC1

    Section S1. Electrical transport behavior of WTe2 single-crystal samples in c-AFM

    Section S2. Ellipsometry measurements on a WTe2 single-crystal

    Section S3. Electrical transport behavior of WTe2 thin films in a capacitor geometry

    Section S4. Polarization switching in the metal-gated WTe2 thin film sample

    Section S5. Piezoelectric coefficient of WTe2

    Section S6. Calculation of Debye length of WTe2

    Section S7. Extrinsic mechanisms as the origin of PFM response in WTe2?

    Table S1. The calculated piezoelectric constant e3i (C/m2) and d3i (pm/V) for WTe2 with a polarization along +z direction.

    Fig. S1. Spectroscopic current-bias curves recorded in c-AFM mode on a pristine surface of a freshly cleaved WTe2 single-crystal sample in a controlled N2 environment.

    Fig. S2. Ellipsometry measurements determining the thickness of the surface oxide on WTe2.

    Fig. S3. Current-bias characteristics of metal-gated WTe2 thin film samples.

    Fig. S4. Polarization switching in a metal-gated WTe2 thin film sample.

    Fig. S5. The polarization change ∆P with respect to the strain ξ along z direction.

    Fig. S6. The calibrated piezoresponse measurements versus bias for a metal-gated WTe2 thin film sample.

    Fig. S7. Stable ferroelectric domains in a WTe2 single crystal.

    References (5862)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Electrical transport behavior of WTe2 single-crystal samples in c-AFM
    • Section S2. Ellipsometry measurements on a WTe2 single-crystal
    • Section S3. Electrical transport behavior of WTe2 thin films in a capacitor geometry
    • Section S4. Polarization switching in the metal-gated WTe2 thin film sample
    • Section S5. Piezoelectric coefficient of WTe2
    • Section S6. Calculation of Debye length of WTe2
    • Section S7. Extrinsic mechanisms as the origin of PFM response in WTe2?
    • Table S1. The calculated piezoelectric constant e3i (C/m2) and d3i (pm/V) for WTe2 with a polarization along +z direction.
    • Fig. S1. Spectroscopic current-bias curves recorded in c-AFM mode on a pristine surface of a freshly cleaved WTe2 single-crystal sample in a controlled N2 environment.
    • Fig. S2. Ellipsometry measurements determining the thickness of the surface oxide on WTe2.
    • Fig. S3. Current-bias characteristics of metal-gated WTe2 thin film samples.
    • Fig. S4. Polarization switching in a metal-gated WTe2 thin film sample.
    • Fig. S5. The polarization change ∆P with respect to the strain ξ along z direction.
    • Fig. S6. The calibrated piezoresponse measurements versus bias for a metal-gated WTe2 thin film sample.
    • Fig. S7. Stable ferroelectric domains in a WTe2 single crystal.
    • References (5862)

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