Topological insulators have recently emerged as a new and exciting form of quantum matter, in which topologies of the system (rather than symmetries) dictate the physical properties much like the situation found for the quantum Hall effect. To date, the majority of research has focused on non-interacting electron materials such as the bismuth-dichalcogenides, where the nearly free-electron model is sufficient to explain the non-trivial electronic structures that give rise to the observed topological states. An extension of this approach to strongly-correlated electron systems is challenging owing to the difficulty in determining the correct band structure by calculation, and remains as the next grand challenge in condensed matter physics.

A pioneering advance in this direction was made in 2009 by the team of Victor Galitski, who showed that a particularly simple type of heavy-electron material called a Kondo insulator, in which a filled band of heavy quasiparticles gives rise to a narrow band insulator, can host a three-dimensional topological insulating phase analogous to the non-interacting systems such as Bi2Se3. They developed a topological classification scheme for the emergent band structures of these systems, and predicted that at least one compound, samarium hexaboride, may be an ideal candidate.

Recently, this work and experiments performed in the Center for Nanophysics and Advanced Materials by the teams of Johnpierre Paglione and Richard Greene were highlighted in Nature [vol. 492, 165 (2012)] and Scientific American [December 12, 2012]. The experiments, using a technique called point-contact spectroscopy, explored the nature of the Kondo insulator SmB6 and found that the material indeed hosts an unusual metallic surface state as predicted by Dzero and co-workers, even though the bulk of the material becomes insulating upon cooling. Confirmed by other groups studying the properties of the surface state transport, this observation paves the way for the discovery of a new class of strongly-correlated topological insulator materials.

December 14, 2012