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High momentum resolution angle resolved photoemission spectra from the Au (111) sp-derived surface state exhibit a doublet. The separation between the peaks increases linearly with k_, and reaches a maximum of 110 meV at 0. 153^-1, when one of the bands crosses the Fermi level. These results are interpreted as spin-split surface state bands, with the spins aligned in the plane of the surface perpendicular to the electronic momentum. The origin of the splitting is spin-orbit coupling, which can break spin degeneracy in systems which lack inversion symmetry.
LaShell et al. (Mon,) studied this question.