  Associated with spin-orbit coupling (SOC) and inversion symmetry breaking,
Rashba spin polarization opens a new avenue for spintronic applications that
was previously limited to ordinary magnets. However, spin polarization effects
in actual Rashba systems are far more complicated than what conventional
single-orbital models would suggest. By studying via first-principles DFT and a
multi-orbital k.p model a 3D bulk Rashba system (free of complications by
surface effects) we find that the physical origin of the leading spin
polarization effects is SOC-induced hybridization between spin and multiple
orbitals, especially those with nonzero orbital angular momenta. In this
framework we establish a general understanding of the orbital mapping, common
to the surface of topological insulators and Rashba system. Consequently, the
intrinsic mechanism of various spin polarization effects, which pertain to all
Rashba systems even those with global inversion symmetry, is understood as a
manifestation of the orbital textures. This finding suggests a route for
designing high spin-polarization materials by considering the atomic-orbital
content.
