Structural Transition in Few-Layer Group-IV Monochalcogenides Induced by Mechanical Forces

Monolayers of group IV monochalcogenides are predicted to exhibit multiferroic behavior, combining both ferroelectric and ferroelastic properties at room temperature. However, the ferroelasticity of these materials has not yet been investigated experimentally. We theoretically report ferroelastic properties in few-layer SnSe, which demonstrate multiferroic behavior. Experimentally, polarization-resolved second-harmonic generation (SHG) measurements were conducted on few-layer SnSe flakes to reveal their in-plane polarization along the armchair direction. A force-induced structural transition was observed in several flakes of SnSe, as confirmed by SHG analysis. First-principles calculations elucidate the strain-dependent SHG response and suggest that the structural transition is associated with the process of ferroelastic or partial ferroelastic switching.