Colloquium: Physical properties of group-IV monochalcogenide monolayers

We survey the state-of-the-art knowledge of ferroelectric and ferroelastic\ngroup-IV monochalcogenide monolayers. These semiconductors feature remarkable\nstructural and mechanical properties, such as a switchable in-plane spontaneous\npolarization, soft elastic constants, structural degeneracies, and\nthermally-driven two-dimensional structural transformations. Additionally,\nthese 2D materials also display selective valley excitations, valley Hall\neffects, and persistent spin helix behavior. After a description of their Raman\nspectra, a discussion of optical properties arising from their lack of\ncentrosymmetry---such as an unusually strong second-harmonic intensity, large\nbulk photovoltaic effects, photostriction, and tunable exciton binding\nenergies---is provided as well. The physical properties observed in these\nmaterials originate from (correlate with) their intrinsic and switchable\nelectric polarization, and the physical behavior hereby reviewed could be of\nuse in non-volatile memory, valleytronic, spintronic, and optoelectronic\ndevices: these 2D multiferroics enrich and diversify the 2D materials toolbox.\n