Condensed Matter Seminars

Scanning tunneling microscopy/spectroscopy on so-called Kondo systems consisting of magnetic atoms adsorbed on non-magnetic surfaces has shown that suitable two-dimensional nanostructures can influence the surface electron transport that is a consequential part of the observable STM process. Almost everyone has seen Eigler's stunning STM pictures in which individual atoms were assembled to form a chosen two-dimensional configuration, call it a nanostructure, on the metal substrate.1 Some of these shapes, when closed, are referred to as quantum corrals.2 A particularly intriguing example is an elliptical corral (major axis <15 nm) composed of up to 70 individually-placed atoms or molecules on a surface-state-supporting Cu(111) surface.3 It has been observed with the STM that both the pictorial image and the Fano-related spectroscopic signature of a single Kondo atom4 such as Co placed at one of the foci showed a mirage when STM measurements were made at the opposing unoccupied focus. The generic physics of the resonance electron transfer and transport occuring in a wide variety of surface dynamics processes including those responsible for the quantum mirages will be outlined. The consequent Fano-like spectra depend upon both the position of the STM tip and also on the size and shape, hence 2-D quantum states of the nanostructure confinement. For the 10's of nm corrals of experimental interest, the level spacings are comparable with the Kondo resonance width. This results in non-trivial spectra showing size-dependent oscillatory structure in both the energy-dependent amplitude and in the lineshape asymmetry. Calculated mirage spectra illustrate the useful inter-dependence upon the contrasting nm-scale confinement size and shape and the atomic-scale resonance-defining properties which depend upon the species.5 This is a nice example of a timely problem in high-visibility contemporary science and technology which has usefully and synergistically been addressed by complimentary experimental observation, analytic theory, and computationally-more-intensive modeling.