Swarup Deb

In this presentation, I will talk about the emergence of ferroelectric ordering in various bilayer two-dimensional materials with hexagonal non-centrosymmetric unit cells. Interestingly, the crystal symmetry translates lateral shifts between parallel 2D layers to sign changes in their out-of-plane electric polarization, a mechanism termed "Slide-Tronics"[1, 2]. 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, we explored 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 what extent it survives the addition of mobile charge carriers. To explore these questions, we conduct surface potential measurements of parallel WSe$_2$ and MoS$_2$ multi-layers with aligned and anti-aligned configurations of the polar interfaces [1]. We find evenly spaced, nearly decoupled potential steps, indicating highly confined interfacial electric fields, which provide means to design multi-state "Ladder-Ferroelectrics". Furthermore, we find that the internal polarization remains significant upon electrostatic doping of mobile charge carrier densities as high as 10$^{13}$ cm$^{-2}$, with substantial in-plane conductivity. Using density functional theory (DFT) calculations, we trace the extra charge redistribution in real and momentum spaces and identify an eventual doping-induced depolarization mechanism.

1. Deb, S. et al. Cumulative polarization in conductive interfacial ferroelectrics. Nature 612, 465–469 (2022).
2. Vizner Stern, M. et al. Interfacial ferroelectricity by van der Waals sliding. Science 372, 1462–1466 (2021).