  To date, spin generation in three-dimensional topological insulators is
primarily modeled as a single-surface phenomenon, attributed to the
momentum-spin locking on each individual surface. In this article we propose a
mechanism of spin generation where the role of the insulating yet topologically
non-trivial bulk becomes explicit: an external electric field creates a
transverse pure spin current through the bulk of a three-dimensional
topological insulator, which transports spins between the top and bottom
surfaces. Under sufficiently high surface disorder, the spin relaxation time
can be extended via the Dyakonov-Perel mechanism. Consequently both the spin
generation efficiency and surface conductivity are largely enhanced. Numerical
simulation confirms that this spin generation mechanism originates from the
unique topological connection of the top and bottom surfaces and is absent in
other two dimensional systems such as graphene, even though they possess a
similar Dirac cone-type dispersion.
