Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure. Issue 3 (2nd November 2018) - Chang, Kai
Abstract: 2D SnTe films with a thickness of as little as 2 atomic layers (ALs) have recently been shown to be ferroelectric with in‐plane polarization. Remarkably, they exhibit transition temperatures ( Tc ) much higher than that of bulk SnTe. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy, and ab initio calculations, the underlying mechanism of the Tc enhancement is unveiled, which relies on the formation of γ‐SnTe, a van der Waals orthorhombic phase with antipolar inter‐layer coupling in few‐AL thick SnTe films. In this phase, 4 n − 2 AL ( n = 1, 2, 3…) thick films are found to possess finite in‐plane polarization (space group Pmn 21 ), while 4 n AL thick films have zero total polarization (space group Pnma ). Above 8 AL, the γ‐SnTe phase becomes metastable, and can convert irreversibly to the bulk rock salt phase as the temperature is increased. This finding unambiguously bridges experiments on ultrathin SnTe films with predictions of robust ferroelectricity in GeS‐type monochalcogenide monolayers. The observed high transition temperature, together with the strong spin‐orbit coupling and van der Waals structure, underlines the potential of atomically thin γ‐SnTe films for the development of novel spontaneous polarization‐based devices. Abstract : An antipolar orthorhombic van der Waals structure is found to be responsible for the enhanced ferroelectric transition temperature in ultrathin SnTe films. Isostructural to GeS, the several‐layer SnTe films can maintain their spontaneous polarization above 400 K. This discovery directly bridges the experiments of SnTe and predictions on the robust ferroelectricity in GeS‐type monochalcogenide monolayers.