Profiling spin and orbital texture of a topological insulator in full momentum space

We investigate the coupled spin and orbital textures of the topological surface state in ${\mathrm{Bi}}_{2}$(Te,${\mathrm{Se})}_{3}$(0001) across full momentum space using spin- and angle-resolved photoelectron spectroscopy and relativistic one-step photoemission theory. For an approximately isotropic Fermi surface in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$, the measured intensity and spin momentum distributions, obtained with linearly polarized light, qualitatively reflect the orbital composition and the orbital-projected in-plane spin polarization, respectively. In ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, the in-plane lattice potential induces a hexagonal anisotropy of the Fermi surface, which manifests in an out-of-plane photoelectron spin polarization with a strong dependence on light polarization, excitation energy, and crystallographic direction.