Orbital mapping of energy bands and the truncated spin polarization in three-dimensional Rashba semiconductors

Associated with spin-orbit coupling (SOC) and inversion symmetry breaking, Rashba spin polarization opens an 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 density functional theory and a multiorbital $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ model a three-dimensional bulk Rashba system (free of complications by surface effects), BiTeI, 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 the 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.