ON THE COVER
Topological insulators. This issue of MRS Bulletin focuses on topological insulators, which represents a new state of matter based on the topology of the electronic structure. Topological insulators are materials with a distinguished electronic structure, equivalent to a Möbius strip. Most semiconductors and insulators, including the vacuum of the universe, are topologically “trivial,” with an electronic structure like a cylinder or donut shape. The Möbius strip electronic structure on the other hand cannot be continuously transformed into a donut, and, consequently, a metallic surface state (Dirac-type cone) develops in topological insulators while the bulk material remains an insulator, as represented on the cover. See the technical theme that begins on page 843.