Photoelectron spin-polarization control in the topological insulator Bi$_2$Se$_3$

We study Bi$_2$Se$_3$ by angle-resolved photoemission spectroscopy (ARPES) and density functional theory. We find that the topological surface state (TSS) is characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through photoelectron interference effects in ARPES. This explains the discrepancy between the spin polarization obtained in spin-ARPES--ranging from 20\% to 85\%--and the 100\% value assumed in phenomenological models [1]. We demonstrate how to probe the intrinsic spin texture of TSS by spin-ARPES, and continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to +/-100\% by an appropriate choice of photon energy, polarization, and angle of incidence [2]. As illustrated by a minimal two-atomic-layer model, photoelectron spin-polarization control is generically achievable in systems with a layer-dependent entangled spin-orbital texture as a direct manifestation of dipole selection rules, photoelectron interference, and TSS complex structure [2].\\[4pt] [1] Z.-H. Zhu et al., Phys. Rev. Lett. 110, 216401 (2013)\\[0pt] [2] Z.-H. Zhu et al., submitted to Phys. Rev. Lett. (2013)