Fig. 2: Structure and ferroelectricity of monolayer α-In₂Se₃.

a Top and side views of monolayer α-In₂Se₃, corresponding to the L₁, H, and L₂ phases, respectively. \(\frac{1}{3}{{{{\bf{a}}}}}+\frac{1}{3}{{{{\bf{b}}}}}\), the in-plane displacement of M (Se) in the ferroelectric phase transition is shown by the blue arrow. b NEB calculation of the energy barrier and evolution of the polarization intensity along the path similar to the one in Ref. ¹⁴. The amplitude represents the in-plane polarization magnitude along the [110] direction. The positive and negative signs indicate the polarization toward [110] and the [−1–10] direction, respectively. Here the polarizations of L₁ and L₂ are \({{{{{\bf{P}}}}}}_{1}=\frac{1}{3}{{{{\bf{Q}}}}}\) and \({{{{{\bf{P}}}}}}_{2}=-\frac{1}{3}{{{{\bf{Q}}}}}\), respectively. Q is the polarization quantum along the [110] direction. The polarization difference is \({{{{\boldsymbol{\triangle }}}}}{{{{\bf{P}}}}}=\frac{2}{3}{{{{\bf{Q}}}}}\).