We investigate the electronic and magnetic properties of hydrogenated carbon nanotubes
using ab initio spin-polarized calculations within both the local density approximation
(LDA) and the generalized gradient approximation (GGA). We find that the combination
of charge transfer and carbon network distortion makes the spin-polarized flat-band
appear in the tube's energy gap. Various spin-dependent ground state properties are
predicted with the changes of the radii, the chiralities of the tubes and the concentration
of hydrogen (H). It is found that strain or external electric field can effectively
modulate the flat-band spin-splitting, and even induce an insulator-metal transition.