

Topological insulators are a new state of matter, topologically distinct from ordinary insulators. They comprise surface states which cross the bulk semiconducting gap  a consequence of band inversion due to spinorbit coupling. The crossing is protected by a topological invariant, and is robust to perturbations that do not break time reversal symmetry. The resulting metallic surface is predicted to have interesting properties, arising partly from the massless Dirac spectrum, and partly from the strong spinorbit coupling. The general electronic structure of bulk and surface states in several prototypical topological insulators has been beautifully revealed by ARPES (as illustrated above for the case of Bi2Se3, from Yulin Chen at Stanford). Our work in this field explores the properties of the surface states, and their interaction with the bulk, with a particular emphasis on revealing the signature of the surface state in transport measurements.
