Scanning Tunneling Microscope Spectroscopy of Atomically Engineered Surfaces: Kondo vs. Non-magnetic Systems.

J. W. Gadzuk

Objective:To understand and theoretically quantify STM spectroscopic measurements of solid surfaces containing isolated adsorbed atoms and/or single-atom-by-atom-tailored surface nanostructures.

Problem:Resonance tunneling STM studies of transition metal atoms adsorbed on noble metal surfaces, some combinations forming Kondo systems (defined by the presence of a localised magnetic moment on the adsorbate and a spatially extended spin compensation cloud within the substrate), have recently been reported. The important additional influence of artificially-synthesized boundaries such as other atoms, quantum wires, and quantum corrals on the surface is also receiving much attention in the electronics and information technology industries. The spectroscopic results are given as spatially-dependent differential conductance versus voltage spectra, usually observed as asymmetric Fano lineshapes (first described in the most highly-ever- cited NBS/NIST journal article). This form is characteristic of the interfering "decay" of an initially-excited localized state into a continuum which has been perturbed by a discrete localized state. The asymmetry of the tip-position-dependent spectral profile contains detailed information on electron, hence information-transport-properties of the composite surface nanostructure that could benefit device engineers, if proper theory existed for quantitative data interpretation.
 

Approach: The basic ideas behind the Fano profile of isolated atoms have been adapted to apply to STM spectroscopy of atoms adsorbed on surfaces. The state of the tunneling electron within an STM tip that is biased with respect to the surface is regarded as the initially excited state. The atom on the surface plays the role of Fano’s discrete state embedded in the continuum. The normalized differential conductivity associated with the resonance, the STM equivalent of a spectral lineshape, is described by an expression that is related to, but more complicated than the Fano formula. It also contains more information, particularly that due to electron surface transport and scattering from the surface nanostructure.

Results and Future Plans: Temperature and tip-position-dependent tunneling characteristics have been obtained for Kondo systems such as Co or Ce adsorbed on the (111) face of Au and some characteristic results of differential conductance vs. energy are shown in Fig.1, for several transverse tip positions labeled in Å. The dramatic cycling in line shape is due to quantum interference in electron transport. Future work will address issues such as the relative importance of ballistic vs. quantum vs. chaotic transport within both quantum corrals and other novel nanostructures, as potentially revealed in STM spectroscopic measurements.

Web Contact micro@nist.gov