Nonlinear Optical Response in Layer-Stacked Gallenene with Ferroelectric Polarization.

Polar metals are very rare and challenging to realize due to the incompatibility of ferroelectricity and metallicity. Mobile electrons in polar metals effectively screen the static electric field and dipoles. Recent studies show that 2D van der Waals metals without an inversion center can have polar order due to specific layer stacking. However, room temperature reversible ferroelectricity and nonlinear second harmonic generation in non-centrosymmetric polar metals remain unrealized. Here, the experimental realization of AB-stacked gallenene (a100) nanocrystals with a room temperature ferroelectric polarization in a liquid gallium environment is reported. Using first-principles calculations, the origin of spontaneous polarization (Ps) due to a broken symmetry in multilayer gallenene structures, resulting in P1 (space group) and C1 (point group) symmetry is explained. The reversible polarization switching is characterized using piezoresponse force microscopy. This results demonstrate the reversible nonlinear optical response of the AB-stacked gallenene crystal through second harmonic generation (SHG) microscopy. The intensities of SHG signals are controlled via angular rotations and thermal heating, which indicate a phase transition at high temperatures. Furthermore, electrical perturbation enables the tunability of SHG intensity. Bipolar resistive switching is demonstrated in a two-terminal device. These findings open avenues for advancements in 2D ferroelectricity, piezoelectricity, and topological superconductivity.