Freezing out of a low-energy bulk spin exciton in SmB6

The Kondo insulator SmB 6 is purported to develop into a robust topological insulator at low temperatures. Yet there are several puzzling and unexplained physical properties of the insulating bulk. It has been proposed that bulk spin excitons may be the source of these anomalies and may also adversely affect the topologically protected metallic surface states. Here, we report muon spin rotation measurements of SmB 6 that show thermally activated behavior for the temperature dependence of the transverse-field relaxation rate below 20 K and a decreasing contact hyperfine field contribution to the positive muon Knight shift below 5–6 K. Our data are consistent with the freezing out of a bulk low-energy (~1 meV) spin exciton concurrent with the appearance of metallic surface conductivity. Furthermore, our results support the idea that spin excitons play some role in the anomalous low-temperature bulk properties of SmB 6 . Kondo insulators: the importance of spin excitons The role of spin excitons in the bulk properties of the Kondo insulator SmB6 is a matter of debate; now, Jeff E. Sonier at the Simon Fraser University in Canada and colleagues use muon spin spectroscopy to shed new light on the question. SmB6 is a good candidate 3D topological insulator with a strong insulating gap, but the physics of this material is still not fully elucidated. In particular, SmB-6 exhibits low-temperature thermodynamic and transport anomalies that have been attributed to spin excitons, which might negatively impact the topological surface states. The authors present evidence for a low-energy bulk spin exciton that freezes below the temperature at which the surface conductivity arises and seems to be related to the anomalous low-temperature bulk properties of SmB6, contributing to the understanding of this material.