Kondo Effect of Molecular Complexes at Surfaces: Ligand Control of the Local Spin Coupling

The spin state of single magnetic atoms and molecules at surfaces is of fundamental interest and may play an important role in future atomic-scale technologies. We demonstrate the ability to tune the coupling between the spin of individual cobalt adatoms with their surroundings by controlled attachment of molecular ligands. The strength of the coupling is determined via the Kondo resonance by low-temperature scanning tunneling spectroscopy. Spatial Kondo resonance mapping is introduced as a novel imaging tool to localize spin centers in magnetic molecules with atomic precision.

Figure 1

Models, STM topographies, and STS spectra in the center of the Co adatoms and complexes under investigation: (a) Cobalt adatom, (b) $\mathrm{Co}(\mathrm{CO}{)}_2$ (constantly flipping; from the spectrum a linear background has been removed), (c) $\mathrm{Co}(\mathrm{CO}{)}_3$, (d) $\mathrm{Co}(\mathrm{CO}{)}_4$, (e) $\mathbf{(}\mathrm{Co}(\mathrm{CO}{)}_2{\mathbf{)}}_{\mathbf{2}}$, (f) $\mathbf{(}\mathrm{Co}(\mathrm{CO}{)}_3{\mathbf{)}}_{\mathbf{2}}$. Models and topographies in (a)–(f) drawn to the same scale. The solid lines in the spectra are fits of a Fano function.

Figure 2

Scaling behavior of a cobalt impurity with $n$ CO molecules attached to it. Data for dicobalt carbonyls are shown as crosses, the number of ligands refers to the number of CO molecules per cobalt atom. The solid curve shows a fit of Eq. (2) to the data.

Figure 3

(a) Topography and (b) simultaneously acquired Kondo map of a cobalt tetracarbonyl molecule. (c) and (d) show topography and Kondo map for $\mathbf{(}\mathrm{Co}(\mathrm{CO}{)}_2{\mathbf{)}}_{\mathbf{2}}$, (e) and (f) for $\mathbf{(}\mathrm{Co}(\mathrm{CO}{)}_3{\mathbf{)}}_{\mathbf{2}}$.