Sublattice asymmetry and spin-orbit interaction induced out-of-plane spin polarization of photoelectrons

We study theoretically the effect of spin-orbit coupling and sublattice asymmetry in graphene on the spin polarization of photoelectrons. We show that sublattice asymmetry in graphene not only opens a gap in the band structure but in case of finite spin-orbit interaction it also gives rise to an out-of-plane spin polarization of electrons close to the Dirac point of the Brillouin zone. This can be detected by measuring the spin polarization of photoelectrons and therefore spin resolved photoemission spectroscopy can reveal the presence of a band gap even if it is too small to be observed directly by angle resolved photoemission spectroscopy because of the finite resolution of measurements or because the sample is $p$-doped. We present analytical and numerical calculations on the energy and linewidth dependence of photoelectron intensity distribution and spin polarization.