  The production of spin-polarized currents in pristine carbon nanotubes with
Rashba spin-orbit interaction has been shown to be very sensitive to the
symmetry of the tubes and the geometry of the setup. Here we analyze the role
of defects on the spin quantum conductances of metallic carbon nanotubes due to
an external electric field. We show that localized defects, such as adsorbed
hydrogen atoms or pentagon-heptagon pairs, increase the Rashba spin-polarized
current. Moreover, this enhancement takes place for energies closer to the
Fermi energy as compared to the response of pristine tubes. Such increment can
be even larger when several equally spaced defects are introduced into the
system. We explore different arrangements of defects, showing that for certain
geometries there are flips of the spin-polarized current and even transport
suppression. Our results indicate that spin valve devices at the nanoscale may
be achieved via defect engineering in carbon nanotubes.
