  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.
