Ladder Ferroelectrics

Ferroelectricity in atomically thin bilayer structures has been recently predicted and measuredin two-dimensional (2D) materials with hexagonal non-centrosymmetric unit-cells. Interestingly, the crystal symmetry translates lateral shifts between parallel 2D layers to sign changes intheir out-of-plane electric polarization, a mechanism termed"Slide-Tronics"[1]. These observations have been restricted to switching between only two polarization states under low charge carrier densities, limiting the practical application of the revealed phenomena. To overcome these issues, one shouldexplore the nature of polarization in multi-layered van der Waals (vdW) stacks, how it is governed by intra-and inter-layer charge redistribution, and to whatextent it survives the additionof mobile charge carriers. To explore these questions, we conduct surface potential measurements of parallel WSe2and MoS2multi-layers with aligned and anti-aligned configurations of the polar interfaces[2]. We find evenly spaced, nearly decoupled potential steps, indicating highly confined interfacial electric fields, whichprovide means to design multi-state "Ladder-Ferroelectrics". Furthermore, we find that the internal polarization remains significant upon electrostatic doping of mobile charge carrier densitiesas high as 1013 cm-2, with substantial in-plane conductivity. Using density functional theory (DFT)calculations, we trace the extra charge redistribution in real and momentum spacesand identify an eventual doping-induced depolarization mechanism.