  The Kondo insulator SmB6 is purported to develop into a robust topological
insulator at low temperature. 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 SmB6 that show thermally-activated behavior for the
temperature dependences of the transverse-field (TF) relaxation rate below 20 K
and 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 SmB6.
