Two-dimensional ferroelectrics provide a unique platform for exploring symmetry breaking and electromechanical coupling at the atomic limit. Using first-principles calculations, we investigate the shear piezoelectric response of a black-phosphorus-like bismuth monolayer. Bismuthene exhibits a giant shear piezoelectric coefficient of $d_{26}=311.1$ pC/N, placing it among the highest values reported for two-dimensional materials. We show that this exceptional response originates from shear-strain-induced polarization rotation between symmetry-equivalent in-plane ferroelectric states, enabled by an intrinsically flat potential-energy landscape with a rotational barrier of only $\sim 10\mathrm{meV}$. This flat landscape also gives rise to an anomalous barrier inversion, where the in-plane switching barrier is lower than the conventional out-of-plane barrier. These findings establish that multiaxial ferroelectricity and an intrinsically flat potential-energy landscape are the two essential physical ingredients for giant shear piezoelectricity, providing a symmetry-guided computational strategy for discovering high-performance low-dimensional piezoelectric materials.

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• Ferroelectricity
• Piezoelectricity
• 2-dimensional systems
• Density functional theory