CPM Seminar

High-resolution scanning tunneling microscopy:
gaining insights by comparing experiment and theory

Werner A. Hofer

University of Liverpool

The aim of the talk is to give an overview over the current state of theoretical research in scanning tunnelling microscopy. I shall introduce the basic theoretical concept, which unifies existing models of scattering (Landauer-Buttiker approach) and perturbation (Bardeen approach) in a common framework based on non-equilibrium Green's functions and discuss its limitations with respect to achievable currents and minimum distances [1,2]. The precision of the method is illustrated by recent simulations of oxygen covered ruthenium surfaces and detailed comparisons with experiment [3]. Interactions between STM tip and surface will be highlighted for close packed metal surfaces like gold, copper, or aluminium [4]; the method of inferring interaction energies and forces from the simulation of electron transitions, and the high elasticity of aluminum is shown to fully account for the observed giant corrugations. The second half of the talk will be devoted to recent simulations of tunnelling topographies and spectroscopies [5]. We show that the bias dependency derived within the scattering approach accounts for the shape of the spectrum on Fe(001), and solve the long standing problem of the position of the surface state on Cr(001), where we find that the spin-density wave of a Cr-interface changes the magnetic moment of the Cr-layers substantially. Recent work on HBC, adsorbed on Au(111) and Ag/Pt(111) dislocation networks is presented in detail. The final part of the talk deals with silicon surfaces [6] modified by organic molecules, where the find that electric fields modify the transport through single molecules [7], and also, that the electronic properties of the surface may be substantially altered by adsorbed polar molecules [8].

[1] W.A. Hofer, A.S. Foster, and A.L. Shluger, Reviews of Modern Physics 75, 1287 (2003)
[2] K. Palotas and W.A. Hofer, Journal of Physics:: Condensed Matter 17, 2705 (2005)
[3] F. Calleja, A. Arnau, J.J. Hinarejos, A.L. Vazquez de Parga, W.A. Hofer, P.M. Echenique; and R.Miranda, Physical Review Letters 92, 206101 (2004)
[4] W.A. Hofer, A. Garcia-Lekue, and H. Brune, Chemical Physics Letters 397, 354 (2004)
[5] W.A. Hofer and A. Garcia-Lekue, Physical Review B 71, 193407 (2005)
[6] L.A. Zotti, W.A. Hofer, and F. Giessibl, Chemical Physics Letters, in press
[7] P.G. Piva, G.A. DiLabio, J.L. Pitters, J. Zikovsky, M. Rezeq, S. Dogel, W.A. Hofer, and R.A. Wolkow, Nature 435, 658 (2005)
[8] S. Dobrin, R.V. Jones, K.R. Harikumar, N. Li, I. R. McNab, J.C. Polanyi, P.A. Sloan, Z. Waqar, J. Yang, S. Ayassi, and W.A. Hofer, Surface Science Letters, in press