R. Matzdorf

Temperature dependence of Shockley-type surface energy bands on Cu(111), Ag(111) and Au(111)

Abstract Using angle-resolved photoemission at very high resolution we have measured thermally induced energy shifts of the Shockley surface states observed around the center \ gG of the surface Brillouin zones on the noble metal (111) surfaces. Based on calculations using the one-dimensional multiple reflection model we demonstrate that the observed shifts can be quantitatively traced back to the temperature-dependent shift of the relevant bulk band gaps which support these surface states. We have addressed care to a precise investigation of the \ gG state on Ag(111), since conflicting results had been reported earlier. Its initial state energy at \ gG is given by E 0 ( T ) = −(75 ± 5) meV + (0.17 meV/K) T in the temperature range up to about 600 K.

Investigation of line shapes and line intensities by high-resolution UV-photoemission spectroscopy — Some case studies on noble-metal surfaces

Line shapes in angle-resolved photoemission spectra from solid surfaces contain a wealth of information about many-body effects in the electron system. However, an analysis of experimental data depends on a very detailed understanding of the contributions to line width due to various other effects. This report reviews photoemission line shape studies of bulk- and surface-emission peaks measured on noble-metal surfaces and their interpretation in terms of electron and photohole lifetimes, electron-phonon interaction and defect scattering. Special attention is focussed on the possibility to measure the photohole lifetime in the vicinity of the Fermi level. Different channels of photohole decay like Auger processes, phonon creation and elastic scattering at surface defects and impurities are discussed along with high-resolution spectra from surface states. We discuss to which extent the various effects can be distinguished experimentally. Information can also be extracted from experimental data concerning properties of the many-electron system at elevated temperature.

Surfaces: a playground for physics with broken symmetry in reduced dimensionality

With our crystal ball in front of us, we attempt to articulate the opportunities and challenges for a surface physicist in the beginning of the new millennium. The challenge is quite clear: to use the unique environment of a surface or interface to do fascinating physics, while taking full advantage of the skills the community has developed over the last 30 years. The opportunities appear to be endless! In this age of Nanotechnology where the promise is to shape the world atom by atom, leading to the next industrial revolution [Nanotechnology: shaping the world atom by atom, National Science and Technology Council, Committee on Technology, 1999], surface science should be at the very forefront of both technological and scientific advances. The smaller objects become, the more important their surfaces become. In this article we focus on the role of a surface physicist in the emergence of nanoscale collective phenomena in complex materials.

Temperature-dependent photoemission spectra from Cu(100) and Cu(111) surfaces

Abstract We have studied high-resolution angle-resolved photoelectron energy distribution curves from Cu(100) and Cu(111) in the temperature interval 38 ⩽ T ⩽ 70.,,0 K . For several well defined peaks, originating from bulk direct transitions as well as from surface states, we determined separately the temperature dependence of emission amplitude, peak area and peak width, respectively. Our results clearly demonstrate that the description of thermal effects in angle-resolved photoemission spectra by the definition of a Debye-Waller factor with an adjustable Debye temperature is not adequate.

UV-photoelectron spectroscopy at highest resolution —direct access to lifetime effects in solids?

Lineshapes and linewidth in angle-resolved photoemission spectra from solid surfaces contain a wealth of contributions from e.g. the photohole lifetime, the lifetime of the final state electron, and from their respective interactions with phonons and lattice imperfections. In addition, finite energy and angular resolution contribute to the experimentally observed linewidths. Using photoelectron spectra from bulk and surface state transitions on copper as an example, we discuss to which extent the various contributions may be distinguished experimentally. The results indicate that relevant spectroscopic information can be directly derived from such studies at very high resolution. This will lead beyond the kinematical analysis of photoelectron data in terms of band structures and may enable us to extract quantities which refer to the dynamical properties of the many-electron system.

High-resolution photoemission study of the surface states near \̄gG on Cu(111) and Ag(111)

We present high-resolution angle-resolved photoemission results concerning the already well-known surface states at the center of the surface Brillouin zone on Cu(111) and Ag(111). Attention is focused on the influence of energy- and angle-resolution, sample temperature, and the proximity of the Fermi edge on the photoelectron line shape. We also discuss, to which extent photohole-lifetimes for Cu and Ag may be inferred from photoemission line-widths.

Quantization of electron states in ultrathin xenon layers

Abstract Physisorbed Xe was grown at temperatures T ⩽ 55 K in a layer-by-layer mode on Ag(111) and Cu(100). High-resolution normal-emission photoelectron spectra from the Xe 5p valence levels show clear evidence for a quantization of the electron states, depending on the layer thickness. Their energies and their numbers can be explained within a very simple potential well model. We have also determined values for the electron mean free path in solid xenon from the attenuation of core level photoemission intensities.

Self-organized atomic nanowires of noble metals on Ge(001): atomic structure and electronic properties

Atomic structures of quasi-one-dimensional (1D) character can be grown on semiconductor substrates by metal adsorption. Significant progress concerning study of their 1D character has been achieved recently by condensing noble metal atoms on the Ge(001) surface. In particular, Pt and Au yield high quality reconstructions with low defect densities. We report on the self-organized growth and the long-range order achieved, and present data from scanning tunneling microscopy (STM) on the structural components. For Pt/Ge(001), we find hot substrate growth is the preferred method for self-organization. Despite various dimerized bonds, these atomic wires exhibit metallic conduction at room temperature, as documented by low-bias STM. For the recently discovered Au/Ge(001) nanowires, we have developed a deposition technique that allows complete substrate coverage. The Au nanowires are extremely well separated spatially, exhibit a continuous 1D charge density, and are of solid metallic conductance. In this review, we present structural details for both types of nanowires, and discuss similarities and differences. A perspective is given for their potential to host a 1D electron system. The ability to condense different noble metal nanowires demonstrates how atomic control of the structure affects the electronic properties.

Localization of d-like Surface Resonances on Reconstructed Au(111)

We have investigated d-like surface resonances observed already earlier on the reconstructed Au(111)(22 × √3) surface at initial-state energies Ei = -5.65 and -4.25 eV relative to EF. At sample temperatures below T = 100 K a non-reconstructed Au(111) surface can be prepared by gentle bombardment with Ne+ at 200 eV. It exhibits sufficient order to show a clear (1 × 1) pattern. However, the above-mentioned surface resonances are missing in angle-resolved normal-emission photoelectron spectra from the (1 × 1) face. This observation proves that the surface resonance states are spatially localized within the outermost corrugated atomic layer of the reconstructed surface.

Influence of electron-phonon interactions on angle-resolved photoelectron spectra from metals

Abstract The influence of temperature on angle-resolved valence-band photoemission spectra from metals is not well understood. Despite considerable progress in recent years, both experimentally and theoretically, there is no profound understanding of how the electron-phonon coupling enters the experimental data. We present an improved description of temperature effects as revealed in peak intensities and line-widths. The basic constituent of this model is the incorporation of emission and absorption of phonons by the photoelectron. It has been applied to recent high-resolution data from bulk transitions in copper and a promising agreement could be obtained.