Photoelectron spin polarization in the Bi sub(2) Te sub(3)(0001) topological insulator: Initial- and final-state effects in the photoemission process

The photoelectron spin polarization in angle-resolved photoemission from the topological surface state in Bi sub(2) Te sub(3)(0001) has been investigated in a combined experimental and theoretical study. Our measurements show significant photon-energy-dependent deviations in the three-dimensional spin polarization of the photoelectron when compared to the expected intrinsic spin polarization of the surface state. The experimental observations are in line with relativistic one-step photoemission calculations. Our theoretical analysis confirms that spin-orbit coupling in the initial-state wave functions in combination with the dipole selection rules strongly influences the photoelectron spin polarization. Furthermore, spin-dependent final-state effects are found to influence the spin polarization significantly. A quantitative access to the three-dimensional spin-polarization vector in topological insulators is thus challenged by a complex interplay of initial- and final-state effects in the photoemission process.