  Previous studies showed that, as ferroelectric films become thinner, their
Curie temperature (Tc) and polarization below Tc both typically decrease. In
contrast, a recent experiment [Chang et al., Science 353, 274 (2016)] observed
that atomic-thick SnTe films have a higher Tc than their bulk counterpart,
which was attributed to extrinsic effects. Here, we find, using
first-principles calculations, that the 0K energy barrier for the polarization
switching (which is a quantity directly related to Tc) is higher in most
investigated defect-free SnTe ultrathin films than that in bulk SnTe, and that
the 5-unit-cell (UC) SnTe thin film has the largest energy barrier as a result
of an interplay between hybridization interactions and Pauli repulsions.
Further simulations, employing a presently developed effective Hamiltonian,
confirm that free-standing defect-free SnTe thin films have a higher Tc than
bulk SnTe, except for the 1-UC case. Our work therefore demonstrates the
possibility to intrinsically enhance ferroelectricity of ultrathin films by
reducing their thickness.
