RESUMEN
Topological insulators such as Bi2Se3 and Bi2Te3 have extremely promising transport properties, due to their unique electronic behavior: they are insulators in the bulk and conducting at the surface. Recently, the coexistence of two types of surface conducting channels has been observed for Bi2Se3, one being Dirac electrons from the topological state and the other electrons from a conventional two-dimensional gas. As an explanation for this effect, a possible structural modification of the surface of these materials has been hypothesized. Using scanning tunneling microscopy we have directly observed the coexistence of a conducting bilayer and the bare surface of bulk-terminated Bi2Te3. X-ray crystal truncation rod scattering was used to directly show the stabilization of this epitaxial bilayer which is primarily composed of bismuth. Using this information, we have performed density functional theory calculations to determine the electronic properties of the possible surface terminations. They can be used to understand recent angular resolved photoemission data which have revealed this dual surface electronic behavior.