Nonequilibrium theory of scanning tunneling spectroscopy via adsorbate resonances: Nonmagnetic and Kondo impurities
Abstract
We report on a fully nonequilibrium theory of scanning tunneling microscopy (STM) through resonances induced by impurity atoms adsorbed on metal surfaces. The theory takes into account the effect of tunneling current and finite bias on the system, and is valid for arbitrary intraadsorbate electron correlation strength. It is thus applicable to recent STM experiments on Kondo impurities. We discuss the finitetemperature effects and the consequences of atomic scale resolution of the STM for the spectral property of such systems. We find that the tip position affects the resonance line shapes in two ways. As a function of the distance from the surface, the line shapes vary due to the different extents of the adsorbate and metal wave functions into the vacuum. However, we do not expect large variations in line shapes unless tunneling into the tightly bound adsorbate states is considerable, or nonequilibrium effects are significant. As a function of the lateral tip position, line shapes should not change significantly on length scales of R_{∥}<=10 Å under typical experimental conditions when the electrons tunnel into the perturbed bulk conduction states hybridized with the outer shell sp adsorbate orbitals. Tunneling into surfaces states on (111) surfaces of noble metals should be important for an observation of resonance at larger distances (>10 Å), and oscillatory variations in the line shape should develop. This longrange behavior was not resolved in recent experiments with Kondo impurities. The temperature dependence of the Kondo resonance cannot be deduced directly from the differential conductance, as the thermal broadening of the tip Fermi surface produces qualitatively similar effects of comparable and larger magnitudes. A careful deconvolution is necessary to extract the temperature dependence of the Kondo resonance. The finitebias currentinduced nonequilibrium effects in tunneling through Kondo impurities should produce a characteristic broadening of the resonance in the case of strong hybridization of the discrete state with the STM tip.
 Publication:

Physical Review B
 Pub Date:
 February 2001
 DOI:
 10.1103/PhysRevB.63.085404
 arXiv:
 arXiv:condmat/0007183
 Bibcode:
 2001PhRvB..63h5404P
 Keywords:

 68.37.Ef;
 72.10.Fk;
 Scanning tunneling microscopy;
 Scattering by point defects dislocations surfaces and other imperfections;
 Condensed Matter  Strongly Correlated Electrons
 EPrint:
 28 pages with 14 figures