The full three-dimensional spin texture of the Bi(110) surface states has been investigated by means of spin- and angle-resolved photoemission spectroscopy. The observed complex in-plane spin texture is in basic agreement with the prior reports and first-principles calculations, and the previously unreported out-of-plane spin polarization ($P_Z$) is observed to be substantial. More significantly, the $P_Z$ texture exhibits an unexpected breaking of spin degeneracy at the Brillouin zone boundary, in contradiction to standard symmetry-based expectations. Our first-principles calculations reveal that the anticipated vanishing of $P_Z$ arises from the quantum interference between two surface wave functions propagating with opposite out-of-plane spin polarizations. These findings strongly suggest that the photoemission process selectively probes one of them, exposing the hidden $P_Z$ polarization of the surface wave function; i.e., the apparent spin vanishing is not due to an absence of polarization, but rather results from interference at the wave function level.

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• Electronic structure
• Surface states
• Single crystal materials
• Angle-resolved photoemission spectroscopy
• Density functional theory
• First-principles calculations
• Photoemission spectroscopy
• Spin-resolved photoemission spectroscopy