A. N. Rudenko, F. J. Keil, M. I. Katsnelson, A. I. Lichtenstein
In this work we investigate the adsorption of a single cobalt atom (Co) on graphene by means of the complete active space self-consistent field approach (CASSCF), additionally corrected by the second-order perturbation theory. Systematic treatment of the electron correlation and possibility to study excited states allow us to reproduce the potential energy curves for different electronic configurations of Co. We find that upon approaching the surface, the ground-state configuration of Co undergoes several transitions, giving rise to two stable states. The first corresponds to the physisorption of the adatom in the high-spin $3d^74s^2$ ($S=3/2$) configuration, while the second results from the chemical bonding formed by strong orbital hybridization, leading to the low-spin $3d^9$ ($S=1/2$) state. Due to instability of the $3d^9$ configuration, the adsorption energy of Co is small in both cases and does not exceed 0.35 eV. We analyze the obtained results in terms of a simple model Hamiltonian that involves Coulomb repulsion ($U$) and exchange coupling ($J$) parameters for the 3$d$ shell of Co, which we estimate from first-principles. We show, while the exchange interaction remains constant upon adsorption ($\simeq$1.1 eV), the Coulomb repulsion significantly reduces for decreasing distances (from 5.3 to 2.6$\pm$0.2 eV). The screening of $U$ favors higher occupations of the 3$d$ shell and thus is largely responsible for the interconfigurational transitions of Co. Finally, we discuss limitations of the DFT-based approaches in respect to transition metal atoms on graphene and conclude that the proper account of the electron correlations is crucial for the description of adsorption in such systems.
View original:
http://arxiv.org/abs/1206.1222
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