Multiferroicity of Non-Janus MXY (X = Se/S, Y = Te/Se) Monolayers with Giant In-Plane Ferroelectricity

Using first-principles calculation, we show that two non-Janus configurations, i.e., Se2Te1 and Se2Te2, of MSeTe (M = Mo or W) monolayers (MLs) are not only considerably more stable than Janus configuration but also dynamically and thermally stable at room temperature. Our Berry phase calculation shows that there is giant in-plane spontaneous electric polarization in the non-Janus MSeTe MLs as well as in the MSSe MLs, which is at least comparable to those predicted for the MLs of group-IV monochalcogenides and not present in the corresponding Janus MLs. Electronic band structure calculation indicates that both non-Janus configurations also exhibit giant spin splitting (160-480 meV) at the valence band maximum (VBM) due to the giant in-plane polarization, rendering them useful for miniaturized p-type spintronics based on two-dimensional (2D) materials. All of them exhibit direct gaps for the minority spin, which is in a strong contrast to the case of the corresponding Janus configuration. Calculation of the interconversion barrier shows that they are multiferroic with simultaneous ferroelectricity and ferroelasticity, which is enhanced under tensile strain. The multiferroic property can be used in the manipulation of carrier spin and band gap in spintronics and optospintronics.