Sliding and superlubric moir\'e twisting ferroelectric transition in HfO2

Abstract

Despite progress in HfO2 thin-film ferroelectrics, issues like high coercive fields persist, and the dynamics of twisted ferroelectricity remain largely unexplored. Here, we explore how sliding and twisting in bilayer HfO2 enables low barrier switching. Among 144 sliding configurations, two exhibit strong in-plane polarization (2360 pC/m) with a low switching barrier of 9.57 meV/f.u. Twisting generates polar textures associated with moir\'e patterns, which drive ferroelectricity via a soft zone-center mode, as revealed by machine-learning-assisted first-principles calculations. The in-plane (out-of-plane) polarization values for HfO2 at twist angles of 21.79{\deg}, 27.80{\deg}, and 46.83{\deg} are 430 (5.82), 367 (2.20), and 1057 (0.03) pC/m, respectively. For 21.79{\deg} and 27.80{\deg} twisting, switching barriers drop to 1.74 and 0.18 meV/f.u., indicating superlubric-like transitions. Notably, the 46.83{\deg} twisted bilayer shows an almost barrier-free polar evolution (0.03 meV /f.u.), attributed to sharply enhanced zone-center phonon linewidths. Our findings establish a moir\'e-engineered switching route for 2D ferroelectrics.