One of the most important new concepts in modern condensed matter physics is that there are new states of matter where the bulk is an unusual insulator in that it has topologically-protected metallic states on the surface. Although many important breakthroughs in the study of these surface states have been achieved within the last a few years, a very important link still remains missing—the experimental discovery of a true 3D topological insulator. By definition, a topological insulator must have an insulating bulk and a topologically-protected metallic surface. However, among all the known 3D topological insulators (e.g. Bi1-xSbx, Bi2Se3 and Bi2Te3), their bulk has until now found to be a conductor in which the topological index is not well-defined. In other words, these materials are not really topological insulators. Meanwhile, in the seemingly unrelated system of heavy-fermion insulators known as Kondo insulators, some long-standing puzzles have remained unanswered for nearly 40 years. In particular, it has been found that the strange transport results for some of the Kondo insulators cannot be understood if one assumes that the transport is governed by the 3D bulk. Here we study the transport properties of the heavy-fermion Kondo insulator SmB6 with a novel configuration designed to distinguish bulk-dominated conduction from surface-dominated conduction. We find that this material is a true topological insulator with an insulating bulk and a metallic surface. This discovery also resolves some of the puzzles about the strange transport behavior of this material.
Note: The work is performed in collaboration with Steven Wolgast, Yun Suk Eo, Kai Sun, James W. Allen, and Zachary Fisk.