Samarium Hexaboride - First True 3D Topological Insulator

Although many important breakthroughs in the study of topological states of matter have been achieved within the last few years, a very important link still remains missing--the experimental discovery of a true 3D topological insulator. Materials currently known to have topological surface states (e.g. Bi$_{1-x}$Sb$_{x}$, Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$) are also bulk conductors, and thus do not have a well-defined topological index. Recent calculations of the heavy-fermion Kondo insulator Samarium Hexaboride (SmB$_{6}$) have predicted the possibility of in-gap topological surface states in this material. Meanwhile, the conjectured existence of a topologically-protected surface state in SmB$_{6}$ could resolve many of the long-standing puzzles surrounding its low-temperature transport properties. Here we study the transport properties of SmB$_{6}$ with a novel configuration designed to distinguish bulk-dominated conduction from surface-dominated conduction. We find that SmB$_{6}$ is a true topological insulator with an insulating bulk and a metallic surface. This discovery resolves the standing puzzles about the strange transport behavior of this material, and it provides the first material in which transport properties of a 3D topological state can be studied.