Theory of Scanning Tunneling Spectroscopy of Magnetic Adatoms in Graphene

Bruno Uchoa, Ling Yang, S.-W. Tsai, N. M. R. Peres, and A. H. Castro Neto

We examine theoretically the signatures of magnetic adatoms in graphene probed by scanning tunneling spectroscopy (STS). When the adatom hybridizes equally with the two graphene sublattices, the broadening of the local adatom level is anomalous and can scale with the cube of the energy. In contrast to ordinary metal surfaces, the adatom local moment can be suppressed by the proximity of the probing scanning tip. We propose that the dependence of the tunneling conductance on the distance between the tip and the adatom can provide a clear signature for the presence of local magnetic moments. We also show that tunneling conductance can distinguish whether the adatom is located on top of a carbon atom or in the center of a honeycomb hexagon.

Analysis

1. This paper is published in Physics Review Letters in November, 2009 and presents theoretical study of Scanning Tunneling Spectroscopy of magnetic adatoms in graphene.

2. Authors propose that the dependence of tunneling conductance on the distance between the tip and the adatom can provide clear signature for the local magnetic moments.

3. Authors show that tunneling conductance can distinguished the position of adatom i.e. whether the magnetic adatom is on the top of carbon atom or in the middle of hexagon.
4. Authors propose that an adatom localized level can hybridize strongly with STM tip unlike ordinary metals that have low density of states and by analyzing the dependence of differential conductance (DC) with the distance between nonmagnetic tip and adatom one can get experimental signature of local magnetic moment.

5. Electrons have different sub-lattice quantum number in graphene so the destructive interference between different tunneling paths causes substantial change to the form of Fano Factor and shape of differential conductance (DC) when the magnetic adatom sits at top of carbon atom as opposed to when it sits in the middle of a hexagon.
6. Observation in (5) gives STM a capability to identify adatoms and defects in graphene.

7. It is know that adatom can occupy different sites in graphene. In this study authors have considered two cases (i) when adatom is sitting at the top of a carbon atom where adatom breaks local sublattice symmetry, (ii) when adatom is sitting in the center of hexagon without breaking any symmetry.

8. This study indicates that when the distance between the tip and the adatom become progressively becomes very small differential conductance DC peaks can shift strongly. If this is the case the peak at the right of the Fermi energy will have a red shift and it will eventually cross the experimentally accessible bias window providing an experimental signature for the presence of local magnetic moment using a nonmagnetic tip even in the presence of Kondo peak.

Conclusion

9. To conclude, authors have derived the fingerprint of Fano resonance of a magnetic adatom in graphene.

10. It also concludes that presence of adatom adsorbed on graphene can be identified using shift in differential conductance (DC) with the changing tip-adatom distance.

11. This study also gives information about the possibility of local magnetic moment away from Kondo regime in graphene.