  A ferroelectric is a material with a polar structure whose polarity can be
reversed by applying an electric field. In metals, the itinerant electrons tend
to screen electrostatic forces between ions, helping to explain why polar
metals are very rare. Screening also excludes external electric fields,
apparently ruling out the possibility of polarity reversal and thus
ferroelectric switching. In principle, however, a thin enough polar metal could
be penetrated by an electric field sufficiently to be switched. Here we show
that the layered topological semimetal WTe2 provides the first embodiment of
this principle. Although monolayer WTe2 is centrosymmetric and thus nonpolar,
the stacked bulk structure is polar. We find that two- or three-layer WTe2
exhibits a spontaneous out-of-plane electric polarization which can be switched
using gate electrodes. We directly detect and quantify the polarization using
graphene as an electric field sensor. Moreover, the polarization states can be
differentiated by conductivity, and the carrier density can be varied to modify
the properties. The critical temperature is above 350 K, and even when WTe2 is
sandwiched in graphene it retains its switching capability at room temperature,
demonstrating a robustness suitable for applications in combination with other
two-dimensional materials.
