Rippled Moire Superlattices for Decoupled Ferroelectric Bits

Abstract

Symmetry considerations suggest that moire superlattices formed by twisted two-dimensional materials should preserve overall inversion symmetry. However, experiments consistently report robust ferroelectricity in systems such as twisted bilayer h-BN, posing a fundamental discrepancy between theory and experiment regarding its microscopic origin. Here, using large-scale finite-field molecular dynamics simulations, we challenge the prevailing defect-pinning hypothesis and instead identify an out-of-plane bending field, induced by in-plane compressive strain, as the key symmetry-breaking mechanism. This strain-induced rippling drives spatially heterogeneous interlayer sliding and distorts the moire domain wall network, resulting in a four-state ferroelectric system. Remarkably, we show this mechanism can be harnessed at the nanoscale, where localized nanobubbles designate the moire lattice's fundamental hexagonal domain clusters as the smallest individually addressable ferroelectric bits, thereby imposing local control on an otherwise globally defined structure. Our findings establish a geometry-driven framework for understanding and engineering moire ferroelectrics, offering not only a route toward ultra-high-density, rewritable memory, but also a strategy for locally tuning the moire potential itself, a critical step for manipulating emergent correlated and topological quantum phases.

Figures (4)

• Figure 1: Effect of point defects on polarization and domain walls in twisted bilayer $h$-BN. (a) Schematics of AB and BA stacking domains in twisted bilayer $h$-BN. (b) Comparison of energies computed with DFT and DP model for pristine and C$_\mathrm{B}$-doped configurations in the training database. (c) Polarization--electric field ($P$--$E$) hysteresis loops for pristine twisted bilayer (with twist angle $\theta=0.365$°) and systems with 0.1% and 1% C$_\mathrm{B}$ defects. All cases exhibit a non-hysteretic response with zero remanent polarization, indicating the absence of defect-induced ferroelectricity. Insets show domain patterns for a moiré superlattice with 0.1% C$_\mathrm{B}$ defects (black gray dots) at zero field. (d) Equilibrium structure of an extended $\Sigma_0$ 180 ° domain wall ke25p046201 in Bernal-stacked bilayer $h$-BN modeled using a 771840-atom supercell at 300 K. The supercell spans approximately $2500~\text{\AA} \times 420~\text{\AA}$ in the in-plane directions, with a defect-rich region on the left and a pristine region on the right. The domain wall propagates continuously across both regions without noticeable distortion or bending. Color indicates the local structural order parameter ($\lambda$) ke25p046201.
• Figure 2: Strain-induced moiré ferroelectricity in twisted bilayer $h$-BN. (a) Net out-of-plane polarization ($P$) and buckling amplitude ($\Delta z_{\rm max}$) as functions of in-plane biaxial strain ($\eta$). (b) Switching field ($E_s$) as a function of $\eta$. (c) $P$--$E$ hysteresis loops at $-0.1\%$ strain for moiré superlattices with upward ($\Delta z_{\rm AA} > 0$, black) and downward ($\Delta z_{\rm AA} < 0$, yellow) AA buckling. (d) Local polarization ($p$) and buckling ($\Delta z$) profiles for the four polar states. The orange lines in the $\Delta z$-profiles indicate the locations of domain walls. Electrical switching enables reversible transitions of $+P_1\leftrightarrow-P_2$ and $+P_2\leftrightarrow-P_1$ within a fixed buckling state. Mechanical switching reverses the sign of $\Delta z_{\rm AA}$, enabling transitions between $+P_1\leftrightarrow -P_1$ and $+P_2\leftrightarrow-P_2$.
• Figure 3: Nanobubble-induced global symmetry breaking and ferroelectric switching in moiré bilayer $h$-BN. (a) Schematic of a moiré bilayer $h$-BN with a nanobubble pinned at each AA region. (b) Variation of the relaxed in-plane lattice constant with interlayer spacing in AA-stacked bilayer $h$-BN. The inset shows vertical expansion at the nanobubble center leads to local in-plane lattice expansion (gray arrows), which imposes compressive strain on adjacent AB/BA regions. (c) Time evolution of the total polarization from MD simulations. Starting from an equilibrium structure with nanobubbles pinned at AA sites, an external out-of-plane electric field ($E=\pm0.5$ V/nm) is applied and then removed. The system exhibits reversible switching between $+P_s$ and $-P_s$ states, with a stable remanent polarization after the field is turned off. (d) Local polarization ($\mu$) and buckling heights ($\Delta z$) for the two degenerated polar states that can be reversibly switched.
• Figure 4: Individually addressable ferroelectric bits in moiré bilayer $h$-BN (a) Domain patterns in supercells containing two nanobubbles centered at circular regions $O_1$ and $O_2$. The top panel illustrates the switching of $O_2$ from $-P$ to $+P$ under a global upward electric field. The bottom panel shows the switching of $O_1$ from $+P$ to $-P$ under a localized downward electric field $E_1$. (b) Time evolution of local polarization in regions $O_1$ and $O_2$ corresponding to the processes shown in (a). (c) Conceptual device architecture resembling a crossbar array, where nanobubble-induced hexagonal moiré domains can be individually controlled to encode binary states "0" and "1".