Spin Splitting of an Au(111) Surface State Band Observed with Angle Resolved Photoelectron Spectroscopy

High momentum resolution angle resolved photoemission spectra from the Au(111) $\mathrm{sp}$-derived surface state exhibit a doublet. The separation between the peaks increases linearly with ${k}_{\ensuremath{\Vert}}$, and reaches a maximum of 110 meV at $0.153{\AA{}}^{\ensuremath{-}1}$, 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.