  We have demonstrated that the evolution of the two-dimensional electron gas
(2DEG) system at an interface of metal and the model topological insulator (TI)
Bi$_2$Se$_3$ can be controlled by choosing an appropriate kind of metal
elements and by applying a low temperature evaporation procedure. In
particular, we have found that only topological surface states (TSSs) can exist
at a Mn/Bi$_2$Se$_3$ interface, which would be useful for implementing an
electric contact with surface current channels only. The existence of the TSSs
alone at the interface was confirmed by angle-resolved photoemission
spectroscopy (ARPES). Based on the ARPES and core-level x-ray photoemission
spectroscopy measurements, we propose a cation intercalation model to explain
our findings.
