  Strongly correlated electron systems show many exotic properties such as
unconventional superconductity, quantum criticality, and Kondo insulating
behavior. In addition, the Kondo insulator SmB6 has been predicted
theoretically to be a 3D topological insulator with a metallic surface state.
We report here transport measurements on doped SmB6, which show that ~3%
magnetic and non-magnetic dopants in SmB6 exhibit clearly contrasting behavior,
evidence that the metallic surface state is only destroyed when time reversal
symmetry is broken. We find as well a quantum percolation limit of impurity
concentration which transform the topological insulator into a conventional
band insulator by forming impurity band. Our careful thickness dependence
results show that SmB6 is the first demonstatrated perfect 3D topological
insulator with virtually zero residual bulk conductivity.
