  Samarium hexaboride (SmB6) has recently been considered to be a topological
Kondo insulator (TKI), the first strongly correlated electron system to exhibit
topological surface conduction states. In this contribution, results of
electrical resistivity measurements between 80 K and 0.08 K of various SmB6
single crystalline samples are presented, analyzed and discussed. The received
results imply that the residual conductivity of SmB6 below about 4 K is of
non-activated (metallic-like) nature. It is shown that this metallic-like
behavior can be attributed both to surface (2D) conduction states, as may be
expected in case of a topological insulator, as well as to the highly
correlated many-body (3D) bulk ground state which is formed within the gap of
this compound. From this it follows that in SmB6, where surface conductivity
states are clearly present, there is in parallel also a bulk contribution to
residual electrical conductivity originating from the strongly correlated
electron system with valence fluctuations. This raises the question whether
SmB6 does not form a new / special type of topological insulator in which in
the energy gap besides the surface conduction states, there is also a
conducting narrow in-gap band originating from the bulk strongly correlated
electron system.
