  Ferroelectricity usually fades away when materials are thinned down below a
critical value. Employing the first-principles density functional theory and
modern theory of polarization, we show that the unique ionic-potential
anharmonicity can induce spontaneous in-plane electrical polarizations and
ferroelectricity in monolayer group-IV monochalcogenides MX (M=Ge, Sn; X=S,
Se). Using Monte Carlo simulations with an effective Hamiltonian extracted from
the parameterized energy space, we show these materials exhibit a
two-dimensional ferroelectric phase transition that is described by
fourth-order Landau theory. We also show the ferroelectricity in these
materials is robust and the corresponding Curie temperature is higher than room
temperature, making these materials promising for realizing ultra-thin
ferroelectric devices of broad interest.
