Realization of Iodinene with Tunable Topological Edge States and Flat Bands.

Halogens, known for their diatomic molecular structures, typically do not form extended covalent materials. The development of 2D elemental materials from halogens is therefore significant for both fundamental research and practical applications. Here, we report the realization of a monolayer iodine sheet, namely iodinene, with multiple exotic properties. Using angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and first-principles calculations, we show that iodinene hosts 2D topological crystalline insulator states, a long-sought topological state previously observed only in 3D materials. Moreover, iodinene is exceptionally stable under ambient conditions. By applying a tensile strain of 48%, we realize two nearly flat bands with robust topological edge states in between, paving the way for the design and fabrication of tunable topological and spintronic devices.