Title: Comment on "Spin-Orbit Coupling Induced Gap in Graphene on Pt(111) with Intercalated Pb Monolayer",
Abstract: Recently a paper of Klimovskikh et al. was published presenting experimental
and theoretical analysis of the graphene/Pb/Pt(111) system. The authors
investigate the crystallographic and electronic structure of this
graphene-based system by means of LEED, ARPES, and spin-resolved PES of the
graphene $\pi$ states in the vicinity of the Dirac point of graphene. The
authors of this paper demonstrate that an energy gap of approx. 200 meV is
opened in the spectral function of graphene directly at the Dirac point of
graphene and spin-splitting of 100 meV is detected for the upper part of the
Dirac cone. On the basis of the spin-resolved photoelectron spectroscopy
measurements of the region around the gap the authors claim that these
splittings are of a spin-orbit nature and that the observed spin structure
confirms the observation of the quantum spin Hall state in graphene, proposed
in earlier theoretical works. Here we will show that careful systematic
analysis of the experimental data presented in this manuscript is needed and
their interpretation require more critical consideration for making such
conclusions. Our analysis demonstrates that the proposed effects and
interpretations are questionable and require further more careful experiments.