  We propose a nonlocal manipulation method to build topological devices
through emerging robust helical surface states in Z_2=0 topological systems.
Specifically, in a ribbon of Z_2=0 Bernevig- Hughes-Zhang (BHZ) model with
finite-size effect, if magnetic impurities are doped on the top (bottom) edge,
the edge states on the bottom (top) edge can be altered according to the
strengths and directions of these magnetic impurities. Consequently, the
backscattering between the emerging robust helical edge states and gapped
normal edge states due to finite-size confinement is also changed, which makes
the system alternate between a perfect one-channel conductor and a perfect
insulator. This effect allows us to fabricate topological devices with high
on-off ratio. Moreover, it can also be generalized to 3D model and more
realistic Cd3As2 type Dirac semimetals.
