MPISF - Göttingen
Abt. Molekulare Wechselwirkungen, Projekt 407

Structure, Phonons, and Diffusion on the Surfaces of Metals


Phonons on metal surfaces

Phonons are elementary excitations in a solid in which the motions of all the atoms in the lattice are correlated. These excitations are elastic waves exhibiting dispersion, i.e. the vibrational frequencies, w, are, due to the interaction of the atoms in the lattice, a function of the wave lenght l. Within the picture of phonons as quasi-particles they are attributed the energy E=(h/2p)w and the momentum q=2p/l. In a so called "phonon dispersion curve" one plots E(q) versus the wavevector q.

Such dispersion curves for phonons in the bulk of a solid are routinely determined from inelastic neutron scattering experiments. The dispersion of surface phonons, which in general differs from that of the bulk phonons, can be measured using a technique that has been developed about 20 years ago in this institute:

The atoms in a helium atom beam of sub-thermal energy exchange energy and momentum in a collision with the surface resulting in the excitation or the annihilation of surface phonons. On measuring the discrete energy loss or -gain of the projectile one determines the phonon energies. This information can be obtained using time-of-flight spectroscopy using a chopped atom beam and detecting the scattered projectiles according to their arrival times at the detector. The associated phonon momentum is determined by the angles between the incoming beam, the scattered beam, and the surface. In this way one can measure all the points of the surface phonon dispersion curves and thus obtain detailed information about the inter-atomic forces in the surface.

Phonon anomalies on adsorbate covered metal surfaces

A few years ago we had discovered that the surface phonon dispersion curve on W(110) undergoes significant changes if this surface is saturated with a monolayer of hydrogen atoms. This 'anomalous' behavior is shown in figure 1. As compared to the clean surface, which exhibits a normal shape of the dispersion curve, that of the hydrogen saturated surface displays a deep incision at a critical wavevector Qcrit.
Abb.1.
The top panel shows the phonon dispersion curves measured on a tungsten (110) surface. The upper branch refers to longitudinally polarized phonons and the lower branch to the so called 'Rayleigh phonon'. The bottom panel shows the effect of hydrogen adsorption on the shape of the dispersion curves: in addition to the dramatic softening in the neighborhood of Qcrit one notices a more shallow indentation centered around the same point.
Up to now, the reasons for the anomalous behavior are not perfectly clear. A possible explanation is based on an enhanced interaction between the metal electrons in the surface and the phonons at the wavevector Qcrit. Such an effect is well known from bulk phonons in certain layered compound crystals and are called 'Kohn anomalies'. One condition for the enhanced electron-phonon coupling is that the electrons in such a system behave like a one-dimensional gas. An alternative explanation would be that the energy- and momentum transfer to the surface, detected in the experiment, are due to a direct excitation of electron hole pairs induced by the collision of the projectile with the surface.

Meanwhile we have found such 'anomalies' in a number of other adsorbate covered surfaces e.g. for H/Mo(110) and the (110) surfaces of a Mo-Re compound with various rhenium concentrations. These new data have, however, not provided any better clues as to the real nature of the effect. Therefore we are continuing the search for other systems that exhibit a similar behavior. Special emphasis is put on surfaces with an electronic band structure resembling a one-dimensional electronic system. Kohn anomalies should also be observable in surfaces of a two-dimensional electronic character, like the Nb terminated sapphire surface, a genuinely two-dimensional metal which is frequently used as a substrate for ultra-thin magnetic films.

Original literature:
E. Hulpke und J. Lüdecke, Phys. Rev. Lett. 68 2846 (1992)

Earlier work on metal surfaces in the department

Present Staff

Alumni

  • Dr. Björn Flach, Infineon Technologies, D-81730 München
  • Dr. Werner Steinhoegl, Infineon Technologies, D-81730 München
  • Dr. Jens Lüdecke, Carl Zeiss, D-73447 Oberkochen
  • Dr. Detlef-Mathias Smilgies, Chess Wilson Lab, Cornell University, Ithaca, NY, USA
  • Dr. Hans-Joachim Ernst, CEA Saclay, Gif sur Yvette, France

International collaboration

  • Prof. Dr. E.W. Plummer; University of Tennessee, Knoxville TN, USA
  • Dr. Michio Okada; Chemistry Department, Graduate School of Science, Osaka University, Osaka, Japan
  • Dr. A. Menzel; University of Innsbruck, Dept. of Physical Chemistry, Innsbruck, Austria
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last revision: E. Hulpke, Dec. 12, 2000; e-mail to Webmaster: mailto:Webmaster