Nontrivial spin structure of graphene on Pt(111) at the Fermi level due to spin-dependent hybridization

The electronic and spin structure of a graphene monolayer synthesized on Pt(111) has been investigated experimentally by angle- and spin-resolved photoemission with different polarizations of incident synchrotron radiation and using density functional theory calculations. It is shown that despite the observed total quasifreestanding character of the dispersion of the graphene $\ensuremath{\pi}$ state remarkable local distortions and breaks in the dispersions take place due to hybridization between the graphene $\ensuremath{\pi}$ and Pt $d$ states. Corresponding spin-dependent avoided-crossing effects lead to significant modification of the spin structure and cause an enhanced induced spin-orbit splitting of the graphene $\ensuremath{\pi}$ states near the Fermi level in the region of the $\overline{\mathrm{K}}$ point of the graphene Brillouin zone (BZ) with a magnitude of 80--200 meV depending on the direction in the BZ. Using $p$, $s$, and elliptical polarizations of the synchrotron radiation, the contributions of the graphene $\ensuremath{\pi}$ and Pt $d$ states were separated and their intersection at the Fermi level, which is important for effective spin injection between these states, was shown. Moreover, analysis of the data allows us to conclude that in the region of the Dirac point the spin structure of the system cannot be described by a Rashba splitting, and even a spin-orbit gap between lower and upper Dirac cones is observed.