Spatially Extended Kondo State in Magnetic Molecules Induced by Interfacial Charge Transfer

An extensive redistribution of spin density in TBrPP-Co molecules adsorbed on a Cu(111) surface is investigated by monitoring Kondo resonances at different locations on single molecules. Remarkably, the width of the Kondo resonance is found to be much larger on the organic ligands than on the central cobalt atom—reflecting enhanced spin-electron interactions on molecular orbitals. This unusual effect is explained by means of first-principles and numerical renormalization-group calculations highlighting the possibility to engineer spin polarization by exploiting interfacial charge transfer.

Figure 1

Molecular self-assembly. (a) Chemical structure of TBrPP-Co, (b) an STM image of a self-assembled TBrPP-Co on Cu(111) [1 V, 1.3 nA, $6.3\times 5.6{\mathrm{nm}}^2$], and (c) the adsorption model of the molecules.

Figure 2

Kondo map. (a) STM image showing a TBrPP-Co molecule inside a SAM. (b) The Kondo map ($dI/dV$ spectroscopic image) of the molecule acquired with 5 mV bias [$1.3\times 1.6{\mathrm{nm}}^2$]. The black and blue region indicates the Kondo effect, which extends over the entire porphyrin molecule. A molecular drawing is added on top of the image for eye guidance.

Figure 3

Kondo temperature across the molecule. (a) The arrows 1 and 2 indicate the paths along which the Kondo signatures are recorded [$3.8\times 3.2{\mathrm{nm}}^2$]. (b),(c) Sequences of Kondo resonances recorded along paths 1 and 2, respectively. The spectra from top to bottom are measured from the center towards the edge and are vertically and horizontally displaced for clarity. The solid line represents the Fano line-shape fit to the experimental data. (d) Kondo temperature versus distance from the center of the molecule for both paths. Error bars are produced from the statistical distribution of measured data on different molecules with different tips (separate experimental runs). Each data point here is an average between 8 to 20 curves.

Figure 4

Calculated spin density and Kondo temperature. (a) A gas phase TBrPP-Co has its spin concentrated at a central Co atom. (b) TBrPP-Co on a Cu(111) surface reveals its spin density spreads over the entire porphyrin ring. (c) A perspective view of TBrPP-Co on Cu(111) showing the extent of charge transfer from surface Cu atoms to the molecule (blue: highest, grey: medium, and red; the smallest charge transfer). (d) Calculated spin density averaged over paths 1 and 2 (shown in inset). (e) Illustration of $3d$ states where the total occupied carbon-derived states of the molecule (grey) and the occupied bulk Cu states (brown) are indicated by shaded regions. (f) Kondo temperature extracted from the DFT total spin density in Fig. 4d and the NRG calculation shows excellent agreement with experimental data for both paths.