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Effects due to the tip in STM and STS: many-particle theory

NQD theory is used to solve problems of scanning tunnelling microscopy in imaging adsorbates on metal surfaces and problems of scanning tunnelling spectroscopy on clean and adsorbate covered metal surfaces. To list a few, these are the imaging of single CO molecules on Cu(111) as indentations using a clean metal tip and as protrusions using a tip with a carbon monoxide molecule on the apex, the nature of spectral features below the Fermi level and the missing of spectral features above the Fermi level on the clean Cu(111) surface. The NQD theory gives answers to the questions: Why is the contrast of the STM image of CO on Cu(111) reversed with tip change? Why do the spectra with STS differ when different tips are used?

Tunnelling is regarded as an excited state problem, i.e. the injection of charge in the electrodes is local and drives the interacting tip-sample system in transient electronically excited states with one electron more or one electron less compared to the ground state. These transient negative or positive ion resonance states are calculated in separate self-consistent calculations and serve as basis for the representation of the exact many-particle Green function, which is needed for the evaluation of the tunnelling current. Many-particle effects in the transient negative and positive ion resonances, i.e. dynamic relaxation effects in the local region and long-range screening, are explicitely taken into account via coupling to plasmons in the metal surfaces.

cu111cleanstm.gif (77k)

coinducedchange.gif (91k)

Left: Theoretical constant height image of Cu(111). A clean aluminium tip is used and the distance between the Al tip atom on the apex and the Cu(111) surface equals 20 bohr.

Right: Induced current change due to a single CO molecule adsorbed on top of a copper atom in Cu(111) using an aluminium tip (the distance between the Al tip atom and the Cu(111) surface equals 20 bohr)


With a platinum tip terminated with a CO molecule on the apex the image of the carbon monoxide molecule adsorbed on Cu(111) is inverted.

E. Koetter, D. Drakova and G. Doyen Phys. Rev. 53, 16595 (1996).

D. Drakova, M. Nedjalkova and G. Doyen, Int. J. Quant. Chem. 106, pp 1419-1431 (2006).