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Switchable moiré potentials in ferroelectric WTe2/WSe2 superlattices

Nature Nanotechnology volume 18, pages 861–866 (2023)Cite this article

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Abstract

Moiré materials with superlattice periodicity many times the atomic length scale have shown strong electronic correlations and band topology with unprecedented tunability. Non-volatile control of the moiré potentials could allow on-demand switching of superlattice effects but has remained challenging to achieve. Here we demonstrate the switching of the correlated and moiré band insulating states, and the associated nonlinear anomalous Hall effect, by the ferroelectric effect. This is achieved in a ferroelectric WTe2 bilayer of the Td structure with a centred-rectangular moiré superlattice induced by interfacing with a WSe2 monolayer of the H structure. The results can be understood in terms of polarization-dependent charge transfer between two WTe2 monolayers, in which the interfacial layer has a much stronger moiré potential depth; ferroelectric switching thus turns on and off the moiré insulating states. Our study demonstrates the potential for creating new functional moiré materials by incorporating intrinsic symmetry-breaking orders.

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Fig. 1: Bilayer Td-WTe2/monolayer H-WSe2 moiré heterostructure.
Fig. 2: Switching moiré potential by ferroelectricity.
Fig. 3: Temperature-dependent electrical transport.
Fig. 4: Switching the nonlinear anomalous Hall transport by ferroelectricity.

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Source data are provided with this paper. Additional data related to this paper are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

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Acknowledgements

We thank A. Rubio, Y. Zhang and J. Zhang for the fruitful discussions. This work was supported by the Air Force Office of Scientific Research MURI under award number FA9550-18-1-0480 (device fabrication), the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under award number DE-SC0019481 (transport measurements), the Cornell Center for Materials Research through the NSF MRSEC programme under award DMR-1719875 (scanning probe characterization) and the NSF Platform for the Accelerated Realization, Analysis and Discovery of Interface Materials (PARADIM) under cooperative agreement DMR-2039380 (analysis). This work was also funded in part by the Gordon and Betty Moore Foundation. The growth of the hBN crystals was supported by the Elemental Strategy Initiative of MEXT, Japan, and CREST (JPMJCR15F3), JST. This work made use of the Cornell NanoScale Facility, an NNCI member supported by NSF grant NNCI-2025233. W.Z. acknowledges support from the Kavli Postdoctoral Fellowship.

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Authors and Affiliations

  1. School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA

    Kaifei Kang, Jie Shan & Kin Fai Mak

  2. Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, NY, USA

    Wenjin Zhao, Jie Shan & Kin Fai Mak

  3. Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY, USA

    Yihang Zeng, Jie Shan & Kin Fai Mak

  4. National Institute for Materials Science, Tsukuba, Japan

    Kenji Watanabe & Takashi Taniguchi

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  1. Kaifei Kang
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Contributions

K.K. fabricated the devices. K.K. and W.Z. performed the electrical measurements. Y.Z. and K.K. performed the PFM measurements. K.K. analysed the data. K.W. and T.T. grew the hBN crystals. K.K., J.S. and K.F.M. designed the scientific objectives, oversaw the project and co-wrote the paper. All authors discussed the results and commented on the paper.

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Correspondence to Jie Shan or Kin Fai Mak.

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Nature Nanotechnology thanks Menghao Wu, Daniel Rhodes and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 PFM images and analysis.

a, Raw PFM image of an angle-aligned monolayer Td-WTe2/monolayer H-WSe2 heterostructure. b, Fourier transform of the raw image. Fourier peaks corresponding to a centred-rectangular lattice can be clearly observed. c, d, The Fourier-filtered PFM image (c) and the corresponding Fourier transform (d). A high-pass filter is applied with a threshold of 72 % of the maximum intensity.

Extended Data Fig. 2 Device structures for aligned Td-WTe2/monolayer H-WSe2 devices.

a, schematics of an angle-aligned monolayer WTe2/monolayer WSe2 device. Top: cross-section of the device. A thin hBN spacer is inserted between the WTe2 and the platinum (Pt) electrodes to avoid direct edge contacts. Gr stands for few-layer graphite gate electrode. Bottom: top view of the device. The blue and red lines denote the helical edge states of a quantum spin Hall insulator. The measurement configuration is also shown. b, optical image of a typical angle-aligned monolayer WTe2/ monolayer WSe2 device. Red, blue and black dashed lines mark the WSe2, the WTe2, and the hBN spacer flakes, separately. c, optical image of the a typical angle-aligned bilayer WTe2/ monolayer WSe2 device. Red and blue dashed lines mark the WSe2 and the bilayer WTe2 flakes, separately.

Extended Data Fig. 3 Repeatability of the data.

a, Two-terminal sample resistance as a function of filling factor and electric field in another angle-aligned 1lay-WTe2/1lay-WSe2 moiré device at T = 60 K. b,c, Four-terminal sample resistance as a function of filling factor and electric field in another angle-aligned 2lay-WTe2/1lay-WSe2 moiré device at T = 10 K. Both forward and backward field scans are shown. d, Filling factor dependence of the sample resistance at E = −0.25 V/nm for P<0. The moiré insulating states are weaker in this device likely caused by the larger unintentional twist angle introduced in the sample fabrication process.

Extended Data Fig. 4 Electric field dependent resistance at selected filling factors of ν=0,−1,−2.

Arrows denote the electric field scan directions. The measurement temperature is 10 K. A clear hysteresis corresponding to ferroelectric switching is observed. In addition to the ferroelectric switching, non-monotonic electric field dependence is also observed (see discussions in Methods).

Extended Data Fig. 5 Variable-range hopping transport at different filling factors.

a-d, Resistance (in log scale) versus T−1/2 for both P<0 and P>0 at ν=0 (a), −0.5 (b), −1.5 (c) and −2 (d). The linear dependence demonstrates the Efros–Shklovskii variable-range hopping.

Extended Data Fig. 6 Nonlinear anomalous Hall response.

a, Linear dependence of the second-harmonic Hall voltage on the bias current squared at varying filling factors. The current modulation frequency is 17 Hz. The measurement temperature is 25 K. b, Filling factor dependence of the nonlinear anomalous Hall response \(V_ \bot ^{2\omega }/I^2\) at varying excitation frequencies of 17, 37, 77, 107, and 137 Hz. The response is independent of the excitation frequency.

Extended Data Fig. 7 Temperature dependent nonlinear anomalous Hall effect at E=0.2 V/nm and P<0.

a, b, Filling factor dependence of \(\frac{{V_ \bot ^{2\omega }}}{{I^2}}\) (a) and the longitudinal resistance R (b) at varying temperatures from 30 K to 100 K. c, Extracted \(V_ \bot ^{2\omega }/V_\parallel ^2\) as a function of the sample conductance G at selected moiré filling factors from –2.5, to 0.15. Unlike the extrinsic NAHE in the coherent metallic transport regime, in which \(V_ \bot ^{2\omega }/V_\parallel ^2 \propto G^2\) is expected and has been observed26, complicated dependence of \(V_ \bot ^{2\omega }/V_\parallel ^2\) on G is observed for the variable-range hopping transport regime here. Future studies are required to better understand the NAHE in the hopping transport regime.

Extended Data Fig. 8 Transport studies along the WTe2 crystal b-axis.

a, b, Electric field and filling factor dependence for the longitudinal resistance of angle-aligned bilayer Td-WTe2/monolayer H-WSe2 heterostructure at 10 K. The current is biased along the crystal b-axis of WTe2. The black arrows label the forward (a) and backward (b) electric field scan directions. Hysteretic electric field dependence corresponding to ferroelectric switching is observed. c, Filling factor dependent longitudinal resistance extracted from a (blue) and b (red) at E = 0.2 V/nm (the black dashed lines in a, b). They denote the two spontaneous polarization states P>0 and P<0. Moiré insulating states are observed only for P<0.

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Source Data Fig. 2

Unprocessed data for Fig. 2a–d.

Source Data Fig. 3

Unprocessed data for Fig. 3a–d.

Source Data Fig. 4

Unprocessed data for Fig. 4a–c.

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Kang, K., Zhao, W., Zeng, Y. et al. Switchable moiré potentials in ferroelectric WTe2/WSe2 superlattices. Nat. Nanotechnol. 18, 861–866 (2023). https://doi.org/10.1038/s41565-023-01376-5

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