Defect induced modification of the surface gap and optical properties of C(111)2 X 1 surface

To resolve the long standing controversy between experimentally measured reflectance anisotropy (RA) of C(111)2 x 1 surface and theoretically calculated optical response, we demonstrated that defects at the C(111) surface in the form of minor hydrogen contamination, missing carbon atoms and/or terraces can be responsible for the appearance of the gap between the surface states, which is absent for the Pandey reconstructed ideal C(111)2 x 1. Ab initio DFT LDA calculations for such non-ideal systems indeed demonstrate that even 6% of residual hydrogen (which is below the standard experimental detection limits) or 40 angstrom wide terraces are enough to sufficiently open the surface gap and account for the observed optical anisotropy.