Credit: David Hsieh, Yuqi Xia, Andrew Wray/Princeton University
Band structure of bismuth selenide, a topological insulator. The red areas represent surface states and the vertical space between the yellow areas is the bulk band gap. At lower right, a 3-D schematic of the cone-shaped surface band structure. The spin states (yellow arrows) indicate that electrons on the surface won’t backscatter from disorder and impurities in the material.