SmB$_6$: Topological insulator or semiconductor with valence-fluctuation induced hopping transport?

We advert to the fact that presence of valence fluctuations (VFs) in semiconductors with in-gap impurity bands unconditionally leads to dynamical changes (fluctuations) of energies of localized impurity states. We provide arguments that in the impurity subnetwork consisting of centers having energy levels fluctuating around the Fermi energy there exist favorable conditions for hops from occupied states to empty states of less energy. Consequently, we propose original valence-fluctuation induced hopping mechanism as a new possibility to explain unusual metallic-like conduction of SmB$_6$ and other Kondo insulators experimentally observed at lowest temperatures. Interestingly, the proposed mechanism infers enhanced metallic-like surface conductivity of SmB$_6$, what resembles a characteristic property of topological insulator, and is in agreement with experimental observations attempting to prove the existence of topologically protected surface state in SmB$_6$.