The scientific interests in 2D ferroic materials, which are potential building blocks for the next-generation fast, low-power cost and miniaturized non-volatile logic and memory devices, have experienced an explosion over the past few years. In this talk, I will introduce the discovery of robust ferroelectricity in 2D monochalcogenide films with staggered black phosphorus lattices, as well as the interesting electronic phenomena associated with their domain walls and spin-valley coupled band structures. Combining van der Waals molecular beam epitaxy (vdW-MBE) and variable temperature scanning tunneling microscopy (VT-STM), we revealed switchable in-plane spontaneous polarizations in monolayer SnTe and SnSe with transition temperatures Tc = 270 K  and 380 K, respectively. Surprisingly, the Tc of monolayer SnTe is much higher than that in its bulk counterpart (~100 K), which is ascribed to a thickness-dependent phase transition . A non-volatile memory device based on 2D ferroelectrics with in-plane polarizations was proposed . We have also discovered bending induced conductive domain walls and nanoplate edges in ultrathin SnTe films, which are promising for the fabrication of 1D logic devices. Between the parallel, electrically neutral 90° domain walls in monolayer SnTe, we have observed clear energetically dispersive electronic standing waves, which are found to be induced by a valley mismatch in k space, rather than high potential barriers at the 1D defects like the atomic steps on noble metal surfaces. Furthermore, applying quasiparticle interference technique, we have demonstrated a valley-dependent Zeeman-like spin splitting as large as 220 meV at the valence band maximum of monolayer Pb0.5Sn0.5Te. Intrinsic valley and spin Hall effects are expected in this noncentrosymmetric monolayer. In the end, I will briefly discuss the exciting future of 2D ferroic material heterostructures.
Dr. Kai Chang is currently a postdoctoral associate in Department NISE, Max Planck Institute of Microstructure Physics, directed by Prof. Stuart Parkin. He received his B.S. degree from Shandong University in 2009 and Ph.D. degree from Tsinghua University in 2015. His research interests focus on the bottom-up synthesis and in-situ scanning tunneling microscopy studies of novel functional nanostructures and heterostructures, including 2D materials, topological materials, unconventional superconductors, etc. He is especially interested in the pursuit of 2D multiferroic materials and intrinsic topological ferromagnets in the heterostructures based on the newly discovered 2D ferroelectric and ferromagnetic materials.
 K. Chang et al., Science 353, 274 (2016).
 K. Chang et al., Adv. Mater. 31, 1804428 (2018).
 H. Shen et al., Phys. Rev. Applied 11, 024048 (2019).