Condensed Matter Physics Seminar, Michigan State University, Lansing, 04/21/08

Pattern Formation, Molecular Self-Assembly, and Novel Collective Excitations on Metallic Interfaces

Karsten Pohl

Department of Physics and Materials Science Program, University of New Hampshire, Durham, NH, U.S.A.

Well-ordered misfit dislocation networks of thin metal films are a viable candidate for the growth of two-dimensional ordered cluster arrays with specific symmetries, feature size and lattice spacing in the nanometer regime. Such bottom-up processes can be very complex involving collective effects from large numbers of atoms. Understanding of these self-assembly processes requires detailed experimental information at the atomic level of large ensembles of hundreds to thousands of atoms. I will demonstrate how the combination of variable temperature measurements from our home-built STM correlated with 2D Frenkel-Kontorova models based on first-principle interaction parameters can explain how uniform arrays can form with the strain in the thin film as the driving force responsible for the surface self-assembly process. This process is generally applicable to assemble many molecular species thus opening avenues towards complex self-assembled structures based on a lock-and-key type approach.

We recently discovered that acoustic plasmons exist on close-packed metal surfaces that due to their low excitation energy and linear dispersion are possible candidates to mediate the attractive interaction leading to the formation of Cooper pairs in high TC superconductors [1]. First-principle calculations show that the novel collective excitation is caused by the coexistence of a partially occupied quasi-2D surface state band with the underlying 3D continuum in the same region of space. While conventional surface plasmons are optical modes and have a finite excitation energy of a few eV, the novel acoustic mode can be excited with very low energies of a few meV. This allows, in principle, for coupling with visible light for signal processing and advanced microscopies as well as new catalysts on nano-structured metallic surfaces.

[1] B. Diaconescu, K. Pohl, L. Vattuone, L. Savio, Ph. Hofmann, V.M. Silkin, J.M. Pitarke, E.V. Chulkov, P.M. Echenique, D. Farías, and M. Rocca, Nature 448, 57 (2007)