On the mechanism for the 2D phase transition in freestanding group-IV monochalcogenide monolayers*

Group-IV monochalcogenides monolayers MX (M=Ge,Sn ; X=S,Se) are low-dimensional semiconductors which exhibit significant piezoelectric and ferroelectric responses. Monochalcogenide monolayers undergo a structural phase transition from a rectangular unit cell to a square unit cell at a critical temperature. There exist two intriguing descriptions of this behavior: (1) a Landau-Ginzburg effective model and (2) a description of the order-disorder transition based on ab initio molecular dynamics. We explore the phase transition by examining the softening of the phonon modes of SnSe as a function of temperature, in order to shed light on the possible physical mechanism underpinning the transition.