  Silicene is a one-atom-thick 2D crystal of silicon with a hexagonal lattice
structure that is related to that of graphene but with atomic bonds that are
buckled rather than flat. This buckling confers advantages on silicene over
graphene, because it should, in principle, generate both a band gap and
polarized spin-states that can be controlled with a perpendicular electric
field. Here we use first-principles calculations to show that field-gated
silicene possesses two gapped Dirac cones exhibiting nearly 100%
spin-polarization, situated at the corners of the Brillouin zone. Using this
fact, we propose a design for a silicene-based spin-filter that should enable
the spin-polarization of an output current to be switched electrically, without
switching external magnetic fields. Our quantum transport calculations indicate
that the proposed designs will be highly efficient (nearly 100% spin
polarization) and robust against weak disorder and edge imperfections. We also
propose a Y-shaped spin/valley separator that produces spin-polarized current
at two output terminals with opposite spins.
