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"Adsorbed atoms on quantum corrals"
M. Weissmann and H. Bonadeo
Proc. of the "XXIV International Workshop on Condensed Matter Theories" (CMT24), Buenos Aires, Argentina, September 12-17, 2000. Ed. S. Hernández and J.W. Clark
Condensed Matter Theories, Volume 16, Nova Science Publishers, Inc. (2001) 109-118
ISBN: 978-1-59033-034-0
In an interesting new experiment the electronic structure of a magnetic atom adsorbed on the surface of Cu(111) could be projected into a remote location on the same surface. The STM tunneling spectrum of a cobalt atom located on one focus of an elliptical corral was observed as a mirage on the other focus. This raises many theoretical questions, the first one being the relation between this quantum system and classical image projection. Other questions regarding this experiment are: How does the appearance of a mirage depend on the surface type, on the adsorbed atom type, on the shape of the corral, etc. The purpose of the present paper is to address these questions using a very simple tight-binding model Hamiltonian. The ellipses considered have the size of the experimental ones, containing about 2500 atoms. The charge distribution for the different wavefunctions is analyzed, in particular, for those with energy close to the Fermi energy of copper, EF. Some of these relevant wavefunctions show two symmetric maxima, located on the principal axis of the ellipse but not necessarily at the foci. If a Co atom is adsorbed at the site where the wavefunction with energy EF has a maximum and the interaction is small, the main effect of the adsorbed atom is to split this particular wavefunction in two. Therefore, although the total charge density will remain the same, the local density of states at any site where the charge density is large enough will present a dip at EF. We relate the presence of this dip to the observation of the quantum mirages. This suggests that other sites, apart from the foci of the ellipses, could be used for projecting atomic images and also indicates the conditions for other adsorbates to produce mirages.
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