The state-of-the-art knowledge of ferroelectric and ferroelastic group-IV monochalcogenide monolayers is surveyed. These semiconductors feature remarkable structural and mechanical properties, such as a switchable in-plane spontaneous polarization, soft elastic constants, structural degeneracies, and thermally driven two-dimensional structural transformations. Additionally, these 2D materials display selective valley excitations, valley Hall effects, and persistent spin helix behavior. After a description of their Raman spectra, a discussion of optical properties arising from their lack of centrosymmetry (such as an unusually strong second-harmonic intensity, large bulk photovoltaic effects, photostriction, and tunable exciton binding energies) is provided as well. The physical properties observed in these materials originate from (correlate with) their intrinsic and switchable electric polarization, and the physical behavior hereby reviewed could be of use in nonvolatile memory, valleytronic, spintronic, and optoelectronic devices: these 2D multiferroics enrich and diversify the 2D material toolbox.

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专题讨论会：IV组单硫属元素化物单层的物理性质

研究了铁电和铁弹性IV型单硫族化物单层的最新知识。这些半导体具有出色的结构和机械特性，例如可切换的面内自发极化，软弹性常数，结构简并性以及热驱动的二维结构转换。此外，这些2D材料显示出选择性的谷底激发，谷底霍尔效应和持续的自旋螺旋行为。在描述了它们的拉曼光谱之后，还讨论了由于缺乏中心对称性而引起的光学特性（例如异常强的二次谐波强度，大体积光伏效应，光致固结和可调谐激子结合能）。