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Direct Evidence of Surface State Contribution to the Kondo Resonance

Y. Hasegawa, S.-J. Kahng, and Q. K. Xue

The Kondo effect is a many-body phenomenon which results from a correlation of an electron spin in a magnetic impurity with conduction electrons of a host metal. Below the Kondo temperature, a singlet ground state is formed, giving rise to a resonance, called the Kondo resonance, near the Fermi energy at the site of the impurity. Low-temperature scanning tunneling microscopy/spectroscopy (STM/S), which has atomic-scale spatial resolution and meV energy resolution, is an ideal tool to observe the Fano-shaped resonance on a magnetic impurity adsorbed on metal surfaces. So far, most of the Kondo resonance observations by STM/S were performed on noble metal (111) substrates, which have both metallic surface states and the bulk states, and therefore, there have been arguments on the contribution of the surface state to the observed Kondo resonance. In order to provide direct evidence of the surface state contribution to the Kondo resonance, we performed its observation on a surface which has metallic free-electron-like surface states formed on a semiconducting substrate.

Fig.1. STM image of a TBrPP-Co molecule adsorbed on a Si(111)-√3×√3Ag surface and a dI/dV spectrum taken above the molecular center showing a resonance around the Fermi energy.

Fig.2. a series of dI/dV spectra taken with various lateral tip distances (r) from the molecular center.

An STM image in Figure 1 shows a 5, 10, 15, 20-Tetrakis-(4-bromophenyl)-porphyrin-Co (TBrPP-Co) molecule adsorbed on a Si(111)-root 3x root 3Ag surface together with Ag adsorbates. 4 lobes of the molecule are clearly resolved in the highly resolved image. The Ag-induced reconstructed surface is known to have metallic surface states as revealed by angle-resolved photoemission spectroscopy and standing wave observation with STM/S. Existence of a spin at the central Co atom of the molecule has been confirmed by STM observation of the Kondo resonance on Cu(111) surface.

For the Kondo resonance measurements, we picked up an isolated molecule whose closest Ag adsorbate is more than 5 nm away to minimize the influence of the Ag adsorbates. The right panel in Figure 1 shows a dI/dV spectrum taken above the molecular center, showing a clear peak near the zero bias voltage. The peak can be fitted well with the expected Fano resonance as demonstrated with a dotted fitting line, and from the fitting the Kondo temperature and the Fano interference parameter were given as ~180 K and ~ 8, respectively.

Figure 2 shows a series of dI/dV spectra taken with various lateral tip distances (r) from the molecular center. The resonance peak amplitude decreases with the lateral tip displacement, and up to 1.4 nm the peak is visible. At the distances larger than 2.5 nm, however, the obtained spectra do not have a significant feature near the Fermi energy, like the case on the clean root 3~root 3 Ag region. We found that the Kondo temperature and the Fano interference parameter are nearly constant in this distance range. Previous theoretical studies suggested that the Kondo resonance due to bulk states decays rapidly in the lateral direction whereas the surface state contribution stays longer (> 1 nm) from the magnetic impurity. The spectra we observed obviously demonstrate the characteristic slow-decaying features of the surface-state contributing Kondo resonance. The distinct Fano-shaped peak near the Fermi energy and its long decay length observed around the TBrPP-Co molecule adsorbed on the bulk-state-absent (near the Fermi energy) substrate are an direct evidence that the surface state does play a significant role in the Kondo resonance.


References
  • Q. Li, et al. Jpn. J. Appl. Phys. 48, 08JB01 (2009); Phys. Rev. B 80, 115431 (2009).
Authors
  • bTsinghua University, China
  • aKorea University, Korea
  • Q. Li, S. Yamazaki, T. Eguchi, H. Kima, S.-J. Kahnga, J. F. Jiab, Q. K. Xueb, and Y. Hasegawa