Direct measurement of layer polarizability in a van der Waals ferroelectric semimetal

A ferroelectric semimetal has coexistence of spontaneous electric polarization and metallicity. A recent study [1] demonstrated bilayer and trilayer 1T^'-WTe₂ exhibit an out-of-plane switchable polarization using graphene as an electric-field sensor. However, a direct measurement of its layer polarizability is still lacking. Here, we detect and quantify the ferroelectricity by layer-resolved capacitance measurements [2] in dual-gated bilayer WTe₂, allowing us to control applied electric field and charge density independently. Moreover, we examine the density-dependent polarizability and extract the critical densities above which the polarization becomes weaker. All of our observations, including hysteresis, "near-layer capacitance enhancement", density dependence of the polarization, are consistent with the theoretical predictions based upon an electrostatic model of capacitance derived from a "tilted-Dirac-cone" model of bilayer WTe₂.
[1] Fei, Z. et al. Nature (2018).
[2] Hunt, B. M. et al. Nature Communications (2017).