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August 28 2003

Prof. E. W. Plummer
Department of Phyics and Astronomy, The University of Tennessee Knoxville and Solid State Division, Oak Ridge National Laboratory

Title: The Dynamic Role of Defects in Surface Phenomena

The variable temperature scanning tunneling microscope is presenting an unparalleled view of the importance of defects in all forms of surface phenomena. What is must surprising is that in many cases it is the dynamic behavior of the defects that is crucial. In this talk I will discuss two examples illustrating that we need a new paradigm to describe real surfaces at finite temperatures. The first example is associated with a complex symmetry lowering (square root of 3 x square root of 3) to (3x3) phase transition in Sn/Ge(111). The study of macroscopic properties of phase transitions in low-dimensional systems provides an understanding of the fundamental aspects of systems of interacting particles. Phase transitions are strongly affected by defects, especially in systems with lower dimensionality. In quasi-1D or –2D systems that exhibit a charge density wave (CDW) transition, a small proportion of microscopic disorder can control the global properties because of the collective nature of the phenomena. It has been speculated that the interaction of mobile defects with CDW leads to alignment of defects with the CDW, or the formation of Defect Density Waves1. In this dynamic picture, the distribution of defects is neither random nor static, instead defects align their positions to optimize the energy of the pinned CDW. In this system and similar systems the transition can be decomposed into two intertwined phase transitions: a second order CDW-like transition and a first order disorder-order transition in the defect distribution. The origin of the Sn/Ge(111) phase transition has been very controversial, primarily because the community tries to map the concepts developed for bulk phase transitions onto the surface. I will describe a new theory that starts with the unique properties of a surface and explains the essence of this transition, with and without defects4. In this theory a CDW can be present without Fermi surface nesting. The second example is from the beautiful work of the Salmeron group on the role of subsurface impurities in Pd(111)5.These mobile impurities, which have been identified as O and C, interact with adsorbed atoms and molecules, limiting surface diffusion, nucleating island growth, serving as active sites for surface reactions, and decreasing the dissociation energy of adsorbed molecules. Indeed, it is time to rethink our basic concepts about surface dynamics, defects and impurities are always present!

  1. H. Mutka, in Advances in the crystallographic and microstructural analysis of charge density wave modulated crystals, edited by F. W. Boswell and J. C. Bennet, Kluwer Academic Publishers, Dordrecht, 1999.
  2. A complex structural phase transition in a defect populated 2D system, A. V. Melechko, M. Simkin, N. F. Samatova, J. Braun, and E. W. Plummer, Phys. Rev. B, 64, 235424 (2001).
  3. Two-Dimensional Phase Transition Mediated by Extrinsic Defects, A. V. Melechko, J. Braun, H. H. Weitering, and E. W. Plummer, Phys. Rev. Letters, 83, 999 (1999). Defect-Mediated Condensation of a Charge Density Wave, (H. H. Weitering, A. Melechko, J. M. Carpinelli, and E. W. Plummer),H. H. Weitering, A. Melesko, J. M. Carpinelli, and E. W. Plummer, Science, 285, 2107-2110 (1999).
  4. Surface Phase Transitions Induced by Electron Mediated Adatom-Adatom Interations, , Junren Shi, Biao Wu, X. C. Xie, E. W. Pummer, and Zhenyu Zhang, submitted.
  5. Subsurface Impurities in Pd(111) Studied by Scanning Tunneling Microscopy, M. K. Rose, A. Borg, T. Mitsui, D. F. Ogletree, and M. Salmeron, J. of Chemical Physics, 115, 10927 (2001).

Additional information on Prof. Plummer can be found here http://www.phys.utk.edu/WPWebSite/ewp_scihistory.htm.