Intrinsic Origin of Enhancement of Ferroelectricity in SnTe Thin Films

Understanding the ferroelectricity in thin films is a fundamental issue for several decades. Previous studies show that ferroelectricity is usually suppressed in a thin film: as the film becomes thinner, ferroelectric transition temperature (T_c) and ferroelectric polarization will decrease. Interestingly and surprisingly, a recent experiment showed that atomic-thick SnTe films have a higher T_c than bulk SnTe. Here, we perform first-principles calculations to investigate the intrinsic origin of such an unusual behavior. We find that the energy barrier for the polarization switching in SnTe thin-films are higher than that in bulk SnTe, and 5-UC SnTe thin-film has the largest energy barrier. This unusual phenomenal is attributed to the interplay between hybridization interactions which are essential for stabilizing ferroelectricity and Pauli repulsions that can suppress ferroelectricity: The increase of barrier with thinner film thickness is due to the fact that the surface Sn atom has weaker Pauli repulsions and thus a smaller force constant than the inner Sn atoms; The decrease of barrier from 5-UC to 1-UC arise from the fact that the HIs decrease with the decrement of film thickness for very thin films.