Two-dimensional electrons at mirror and twistronic twin boundaries in van der Waals ferroelectrics

TL;DR

This paper predicts that twin boundaries in ferroelectric 3R-MX$_2$ transition metal dichalcogenides can host high-density two-dimensional electrons and holes, and suggests that specific twisted bilayer structures may induce strongly correlated electronic states.

Contribution

It introduces the concept of using twin boundaries in ferroelectric TMDs to host 2D electron/hole gases and proposes moiré engineering with twist angles to realize correlated electron phases.

Findings

Twin boundaries can host 2D electrons and holes with densities up to 10^{13} cm^{-2}.

N-doped twin boundaries have more promising binding energies than p-doped ones.

Specific twist angles can promote strongly correlated states like Wigner crystals.

Abstract

Semiconducting transition metal dichalcogenides (MX$_2$) occur in 2H and rhombohedral (3R) polytypes, respectively distinguished by anti-parallel and parallel orientation of consecutive monolayer lattices. In its bulk form, 3R-MX$_2$ is ferroelectric, hosting an out-of-plane electric polarisation, the direction of which is dictated by stacking. Here, we predict that twin boundaries, separating adjacent polarization domains with reversed built-in electric fields, are able to host two-dimensional electrons and holes with an areal density reaching $\sim 10^{13} {\rm cm}^{-2}$. Our modelling suggests that n-doped twin boundaries have a more promising binding energy than p-doped ones, whereas hole accumulation is stable at external surfaces of a twinned film. We also propose that assembling pairs of mono-twin films with a `magic' twist angle $\theta^*$ that provides commensurability between the moir\'e pattern at the interface and the accumulated carrier density, should promote a regime of strongly correlated states of electrons, such as Wigner crystals, and we specify the values of $\theta^*$ for homo- and heterostructures of various TMDs.