Scanning tunneling spectroscopy and Dirac point resonances due to a single Co adatom on gated graphene

Based on the standard tight-binding model of the graphene $\pi$-band electronic structure, the extended H\"uckel model for the adsorbate and graphene carbon atoms, and spin splittings estimated from density functional theory (DFT), the Dirac point resonances due to a single cobalt atom on graphene are studied. The relaxed geometry of the magnetic adsorbate and the graphene is calculated using DFT. The system shows strong spin polarization in the vicinity of the graphene Dirac point energy for all values of the gate voltage, due to the spin-splitting of Co 3d orbitals. We also model the differential conductance spectra for this system that have been measured in the scanning tunneling microscopy (STM) experiments of Brar {\em et al.} [Nat. Phys. {\bf 7}, 43 (2011)]. We interpret the experimentally observed behavior of the S-peak in the STM differential conductance spectrum as evidence of tunneling between the STM tip and a cobalt-induced Dirac point resonant state of the graphene, via a Co 3d orbital. The cobalt ionization state which is determined by the energy position of the resonance can be tuned by gate voltage, similar to that seen in the experiment.