Intrinsic Origin of Enhancement of Ferroelectricity in SnTe Ultrathin Films

TL;DR

This study reveals that defect-free SnTe ultrathin films can intrinsically exhibit higher ferroelectric Curie temperatures than bulk SnTe due to increased energy barriers for polarization switching, challenging previous assumptions about size effects.

Contribution

The paper demonstrates, through first-principles calculations and effective Hamiltonian simulations, that ultrathin SnTe films can intrinsically enhance ferroelectricity as their thickness decreases.

Findings

0K energy barrier for polarization is higher in ultrathin films

5-unit-cell SnTe film has the largest energy barrier

Ultrathin SnTe films can intrinsically have higher Tc than bulk

Abstract

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.