| With their spin-helical metallic surface state, topological insulators (TIs) define a new class of materials with strong application potential in spintronics. Technological exploitation depends on the degree of spin polarization of the topological surface state (TSS), assumed to be 100% in phenomenological models. Yet in Bi2Se3, an archetypical TI material, spin- and angle-resolved photoemission spectroscopy (S-ARPES) detects a spin polarization ranging from 20 to 85%, a striking discrepancy which undermines the applicability of real TIs. Here we show - studying Bi2Se3 by polarization-dependent ARPES and density-functional theory slab calculations - that the TSS Dirac fermions are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. This explicitly solves the puzzle of the TSS spin polarization in S-ARPES, and suggests how 100% spin polarization of photoelectrons and photocurrents can be achieved and manipulated in TI devices by using linearly polarized light. |
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