Scanning-tunneling-microscope imaging of clean and alkali-metal-covered Cu(110) and Au(110) surfaces.

Abstract

A combined experimental and theoretical study of the scanning-tunneling-microscope (STM) imaging properties of clean and alkali-metal covered Cu(110) and Au(110) surfaces is presented. The clean surfaces are imaged in the STM experiments as parallel strings of Cu or Au atoms, respectively, which represent the close-packed rows in the topmost layer. For the (1\ifmmode\times\else\texttimes\fi{}2) missing-row reconstructed Au(110) surface the corrugation amplitude of the reconstruction shows a maximum as a function of tip-sample distance. On the Cu(110) surface, which does not reconstruct spontaneously in its clean state, adsorbed alkali-metal atoms (K, Cs) induce a missing-row reconstruction with the missing substrate metal rows running along the densely packed [11\ifmmode\bar\else\textasciimacron\fi{}0] direction. On both surfaces, adatoms are located in the missing-row furrows. The alkali atoms are usually not visible in the STM picture but, rather, images are obtained typical for the reconstructed metal substrate. For certain tunneling conditions, image inversion is observed. K-covered Au(110)-(1\ifmmode\times\else\texttimes\fi{}2) and Cu(110)-(1\ifmmode\times\else\texttimes\fi{}2) surfaces exhibit distinct corrugation maxima similar to the clean Au(110)-(1\ifmmode\times\else\texttimes\fi{}2) surface, if the tip-sample distance is varied. A theory of scanning-tunneling microscopy is applied that accounts for a realistic treatment of the electronic structure of the sample surface. The tunnel current is evaluated using a Green-function technique. In the theory, adsorbed potassium atoms appear transparent on Cu(110) because they substitute for sample metal atoms and are embedded in the first layer of Cu atoms. Therefore, the 4s resonance is centered energetically well below the Fermi level and has only a small spectral weight at the Fermi level. The corrugation maximum and the image inversion are found to be a consequence of the tip-sample interaction.