Robustness of the Insulating Bulk in the Topological Kondo Insulator SmB${}_6$

We used the inverted resistance method to extend the bulk resistivity of SmB${}_6$ to a regime where the surface conduction overwhelms the bulk. Remarkably, the bulk resistivity shows an intrinsic thermally activated behavior that changes ten orders of magnitude, suggesting that it is an ideal insulator that is immune to disorder. Non-stoichiometrically-grown SmB${}_6$ samples also show an almost identical thermally activated behavior. At low temperatures, however, these samples show a mysterious high bulk resistivity plateau, which may arise from extended defect conduction in a 3D TI.