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The scientific interest in 2D ferroic (ferroelectric/ferromagnetic) materials experienced an explosion over the past three years. These 2D ferroic materials are promising for the development of future fast, low-power and miniaturized non-volatile logic and memory devices. In this talk, I will introduce the discovery of robust ferroelectricity in 2D SnTe films grown on graphene substrates, and the strong electronic state confinement effect induced by its 90° domain walls. Combining van der Waals molecular beam epitaxy (vdW-MBE) and variant temperature scanning tunneling microscopy (VT-STM), we revealed a switchable in-plane spontaneous polarization in monolayer SnTe that has a transition temperature Tc = 270 K, much higher than the Tc of bulk SnTe (~100 K) [1]. Such a Tc enhancement is ascribed to a 3D-to-2D structural phase transition as the thickness of SnTe decreases to atomic limit. Featuring an antipolar inter-layer coupling, the spontaneous polarization in the 2D γ-phase can survive even above 400 K [2]. A memory device based on 2Dferroelectrics with in-plane polarizations was proposed [3]. Furthermore, we found strong electronic standing waves induced by the reflection from the electrically neutral 90° domain walls in monolayer SnTe. Despite of the flat potential at the domain walls, the reflectivity is found to be comparable to that at the atomic steps on metal surfaces, and no sign of transmission was seen. Our results show potential for polarization-tuned valleytronics in 2D ferroelectrics.
Dr. Kai Chang is a postdoctoral associate inthe NISE department of Max Planck Institute of Microstructure Physics, directedby Prof. Stuart Parkin. He received his B.S. degree in Shandong University in2009 and Ph.D. degree from Tsinghua University in 2015 (supervised by Prof.Qi-Kun Xue). His research interests focus on the bottom-up synthesis andin-situ scanning tunneling microscopy studies of novel functionalnanostructures and heterostructures, including 2D ferroic materials,topological materials, spintronic and valleytronic materials, etc. He has published13 papers in high-impact journals including Science, Nature Physics, Phy. Rev.Lett., Advanced Materials, etc. and received over 1200 citations.