A ferroelectric is a material with a polar structure whose polarity can be reversed (switched) by applying an electric field1,2. In metals, itinerant electrons screen electrostatic forces between ions, which explains in part why polar metals are very rare3,4,5,6,7. Screening also excludes external electric fields, apparently ruling out the possibility of ferroelectric switching. However, in principle, a thin enough polar metal could be sufficiently penetrated by an electric field to have its polarity switched. Here we show that the topological semimetal WTe2 provides an embodiment of this principle. Although monolayer WTe2 is centro-symmetric and thus non-polar, the stacked bulk structure is polar. We find that two- or three-layer WTe2 exhibits spontaneous out-of-plane electric polarization that can be switched using gate electrodes. We directly detect and quantify the polarization using graphene as an electric-field sensor8. Moreover, the polarization states can be differentiated by conductivity and the carrier density can be varied to modify the properties. The temperature at which polarization vanishes is above 350 kelvin, and even when WTe2 is sandwiched between graphene layers it retains its switching capability at room temperature, demonstrating a robustness suitable for applications in combination with other two-dimensional materials9,10,11,12.
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We thank J. Folk, E. Sajadi, A. Levanyuk, T. Birol and A. Andreev for substantial insights. D.H.C. and X.X. were supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under awards DE-SC0002197 and DE-SC0018171, respectively. Synthesis efforts at ORNL were also supported by the same division of the Department of Energy. Z.Z. was partially supported by the CEM, and NSF MRSEC, under grant DMR-1420451. T.A.P. was supported by AFOSR FA9550-14-1-0277. Z.F., W.Z. and B.S. were supported by the above awards and also by NSF EFRI 2DARE 1433496 and NSF MRSEC 1719797.
Nature thanks L. Bartels, T. Birol and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Science China Materials (2019)