Coexistence of Antiferroelectric and Ferroelectric Phases in Ultrathin SnTe Films*

Two-dimensional (2D) SnTe films with a thickness of as little as 2 atomic layers (AL) have recently been shown to be ferroelectric with in-plane polarization. Remarkably, they exhibit transition temperatures (T_C) that reach above room temperature, much higher than that of bulk SnTe. The mystery of such a dramatic enhancement of T_C in 2D SnTe was not understood. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy and ab initio calculations, we have unveiled the underlying mechanism of the T_C enhancement, which relies on the formation of a van der Waals structural phase with antiferroelectric inter-layer coupling in the few-AL thick SnTe films. This newly discovered structure has an “AB” bilayer stacking sequence distinct from the “AA” stacking of the bulk phase. In the AB phase, the 4n − 2 AL (n = 1, 2, 3…) thick films are found to be ferroelectric, while the 4n AL thick films are antiferroelectric. In the AB phase films thicker than 4 AL, significant lattice distortions persist to above 400 K.