AP Seminar - Twintronics of van der Waals Ferroelectrics

Over the recent years, several studies have established ferroelectric properties of rhombohedral transition-metal dichalcogenides (TMD), both grown as bulk crystals and assembled into twisted bilayers and multilayers [1-5]. For bilayers assembled from monolayer TMD crystals with parallel orientation of unit cells, lattice reconstruction (characteristic for small-angle twisted bilayers [6,7]) results in the out-of-plane polarised ferroelectric domains and networks of domain walls, switchable by mutual sliding of the monolayers, prompted by an out-of-plane electric field [3], manifested by the hysteretic field-effect transistor [4] and tunneling FET [8] operations, and readable optically by the linear Stark shift of the interlayer excitons [9].

In bulk 3R-TMD crystals, groups of layers with the same stacking order appear as three-dimensional twins separated by planes of twin boundaries. Here, we propose [10] the formation of two-dimensional (2D) electron/hole gases at twin boundaries, analyse their stable density in photo-doped structures, which appears to be in the range of n^*~8x10¹²cm^-2 for electrons at both intrinsic mirror twin boundaries in bulk crystals and twisted twin boundaries in structures assembled from two thin mono-domain films. We also predict ‘magic’ values of twist angles between the assembled twins, for which the commensurability between the accumulated carrier density, n^*, and moiré pattern would promote the formation of a strongly correlated state of electrons, such as a Wigner crystal.

[1]   F. Ferreira, et al, Scientific Reports 11, 13422 (2021)

[2]   F. Ferreira, et al, Phys Rev B 106, 125408 (2022)

[3]   V. Enaldiev, et al, Nano Letters 22, 1534 (2022)

[4]   A. Weston, et al, Nature Nanotechnology 17, 390 (2022)

[5]   L. Molino et al, Advanced Materials 35, 2370273 (2023)

[6]   V. Enaldiev, et al, Phys Rev Lett 124, 206101 (2020)

[7]   A. Weston, et al, Nature Nanotechnology 15, 592 (2020)

[8]   Y. Gao, et al, Nature Communications 15, 4449 (2024)

[9]   J. Sung, et al, Nature Nanotechnology 15, 750 (2020)

[10] J. McHugh, et al, Nature Communications 15, 6838 (2024)