Plenarvorträge - DPG-Tagungen

Oberflächenphysik Dienstag
structural distortion. It can be affected by thermal fluctuations and other
metallic states in the system.
Recent progress in photoelectron spectroscopy and synchrotron radi-
ation instrumentation allow to study these effects directly and k-space
resolved. Various examples on surfaces, in quasi-one dimensional crystals,
and in magnetic materials will be shown.
O 17 Elektronische Struktur (Experiment und Theorie) II
Zeit: Dienstag 11:15–13:15 Raum: H36
O 17.1 Di 11:15 H36
Stark Effect for Shockley Surface States of Au(111) and Cu(111)
— •J. Kröger 1 , L. Limot 1 , R. Berndt 1 , and P. Johansson 2
— 1 Christian-Albrechts-Universität zu Kiel, D-24098 Kiel, Germany —
2 University of Örebro, S-70182 Örebro, Sweden
We present scanning tunneling spectroscopy data of Au(111) and
Cu(111) acquired at 9 K. Increasing the electric field between tip and
sample surface leads to a gradual downward shift of the surface state
binding energies for both surfaces. The observed shift is attributed to
the Stark effect as was previously observed for Ag(111) by Limot et al.
[1]. Experimental data are compared with calculations performed within
a one-dimensional model. In the limit of infinitely large tip-surface
distances the photoemission binding energy values are obtained.
[1] L. Limot, T. Maroutian, R. Berndt, and P. Johansson, Phys. Rev.
Lett. 91, 196801 (2003).
O 17.2 Di 11:30 H36
Temperaturabhängige Untersuchung des Cr(001)-Oberflächenzustands
mittels SP-STM/STS — •Torben Hänke, Stefan
Krause, Robert Ravlić, Matthias Bode und Roland Wiesendanger
— Institut für Angewandte Physik, Universität Hamburg,
Jungiusstraße 11, 20355 Hamburg
Die magnetische Struktur der Cr(001)-Oberfläche läßt sich durch
den topologischen Antiferromagnetismus beschreiben, d.h. Terrassen,
die durch monoatomare Stufen getrennt sind, weisen eine alternierende
Magnetisierung auf. Typisch für eine solche bcc(001)-Oberfläche
ist ein spinpolarisierter d z 2-artiger Oberflächenzustand nahe der
Fermi-Kante [1]. Bei Raumtemperatur zeigt spinpolarisierte Rastertunnelspektroskopie
auf benachbarten Terrassen gemäß dieser Interpretation
eine alternierende Peak-Höhe dieses Zustands [2]. Demgegenüber steht
eine STS-Untersuchung bei 4,2 K [3], in der eine scharfe Resonanz
26meV oberhalb der Fermi-Kante beobachtet wird. Diese Resonanz des
Spektrums wird als orbitale Kondo-Resonanz (TK = 180K), hervorgerufen
durch die Bildung eines orbitalen Singlet-Zustandes, erklärt.
Dieser Zustand entsteht aus dem virtuellen Übergang zwischen dxy und
dyz Oberflächenzuständen über Hybridisierung mit dem sp-Band. In
diesem Beitrag wurde der Oberflächenzustand mittels SP-STM/STS bei
Temperaturen von 20-300 K untersucht, und die Ergebnisse werden im
Hinblick auf die verschiedenen Interpretationen diskutiert.
[1] J. A. Stroscio et al., Phys. Rev. Lett. 75 2960 (1995).
[2] M. Kleiber et al., Phys. Rev. Lett. 85 4606 (2000).
[3] O. Yu. Kolesnychenko et al., Nature 415 507 (2002).
O 17.3 Di 11:45 H36
STM-Thermovoltage measurements of surface electronic states
on Ag/Au(111) — •Winfried Langenkamp, Bastian Weyers,
and Rolf Möller — Universität Duisburg-Essen, Institut für Experimentelle
Physik, Universitätsstr. 3-5, 45141 Essen
Submonolayer films of silver on Gold(111) have been examined with
a thermovoltage-STM to determine the shift in the fermi wavelength of
surface state electrons.
If tip and sample of an STM are at different temperatures, a thermovoltage
arises which is correlated to the derivative of the sample’s density
of electronic states at the fermi level.
This voltage is measured by a feedback loop for a bias voltage tuning
the average current to zero. Simultaneously the tip-sample separation is
adjusted by the current resulting from a small alternating bias voltage.
The thermovoltage can be used to analyze atoms of different chemical
nature and standing waves of surface state electrons. [1]
In the experiments discussed the equivalent of less then a monolayer
of Ag was evaporated on Au(111) leading to the formation of silver islands
of monoatomic height. The thermovoltage provides information to
distinguish gold and silver areas below the tip as well as on the standing
waves of surface states on Au(111) and on 1 ML Ag/Au(111).
[1] D. Hoffmann et al., Thermovoltage in scanning tunnelling microscopy,
J. Electron Spectroscopy, 109 (2000) 117
O 17.4 Di 12:00 H36
Shockley surface states at close-packed surfaces — •Frederik
Schiller 1,2 , Vito Servedio 3 , and Clemens Laubschat 1 —
1 Institut für Festkörperphysik, TU Dresden, D-01062 Dresden, Germany
— 2 Donostia International Physics Center, E-20018 Donostia/San
Sebastian, Spain — 3 Sezione INFM and Dip. di Fisica, Università “La
Sapienza”, P. le A. Moro 2, I-00185 Roma, Italy
Valence-band photoemission spectra of close-packed surfaces of the divalent
alkali-earth metals Be and Mg reveal huge peaks due to Shockley
surface states with a free-electron like dispersion parallel to the surface.
Similar states have been observed for close-packed surfaces of the noble
metals Cu, Ag, Au, and the trivalent metal Al. Surprisingly, for the divalent
metals Ca, Sr, Eu, and Yb as well as for all trivalent rare-earth
metals such Shockley states are missing. Instead, rare-earth metals exhibit
a d-like Tamm state that reveals almost no dispersion. On the basis
of band-structure calculations, we show that this different behavior is
caused by the d bands that convert the Shockley-inverted energy gaps at
the close-packed surfaces of Be, Mg, Cu, Ag, Au, and Al into conventional
gaps in the latter group of metals.
Supported by DFG, project La 655/7-2 and SFB 463, TP B4.
O 17.5 Di 12:15 H36
Quantum-well states in thin films of Yb/W(110) studied by
scanning tunnelling spectroscopy — •Daniel Wegner, Andreas
Bauer, and Günter Kaindl — Freie Universität Berlin, Institut für
Experimentalphysik, Arnimallee 14, 14195 Berlin-Dahlem
We have studied the electronic structure of thin films of Yb/W(110)
by low-temperature scanning tunnelling spectroscopy at 10 Kelvin.
Quantum-well states around the Fermi energy, with small dispersion
parallel to the film plane, result in a series of sharp, well resolved peaks
in the tunnelling spectra. Due to the high lateral resolution of STS, the
quantum-well states and their film-thickness dependence can be nicely
observed on films with variations of the local thickness over a range
of 10 monolayers. Close to the Fermi level, the electronic structure is
complex.
This work was supported by the DFG, Sfb-290.
O 17.6 Di 12:30 H36
Confinement and quantization of the Ag(111) surface state in
hexagonal vacancy islands — •Henning Jensen, Jörg Kröger,
and Richard Berndt — Institut für Experimentelle und Angewandte
Physik, Universität Kiel, D-24098 Kiel, Germany
The Shockley surface state of the noble metal (111) faces provides an
experimentally accessible model system for a two-dimensional free electron
gas. We report on the scattering and confinement of the Ag(111) surface
state by hexagonal sputtering-induced surface defects of monatomic
depth. Low-temperature STS measurements of the local density of states
inside these defects reveal a series of resonance peaks, which are compared
with potential well calculations and related experimental results.
Examination of the line width vs. binding energy relation provides an
estimate of surface state electron life time.
O 17.7 Di 12:45 H36
Electronic properties of a 2D-Superlattice of Ce Adatoms on
Ag(111) — •M. Ternes 1 , C. Weber 2 , M. Pivetta 1 , F. Patthey 1 ,
J. Pelz 3 , T. Giamarchi 4 , F. Mila 2 , and W.-D. Schneider 1 —
1 Institut de Physique des Nanostructures, EPFL, CH-1015 Lausanne
— 2 Institut de Physique Théorique, EPFL, CH-1015 Lausanne —
3 Departement of Physics, The Ohio State University, Columbus, Ohio
43210, USA — 4 DPMC, Université de Genève, CH-1211 Genève
Recently we have shown that Ce adatoms on a clean Ag(111) surface
form a hexagonal superlattice at liquid He temperatures [1]. This
superlattice is created by long-range Ag(111) surface state mediated interactions
between the Ce adatoms. We present here local STS measure-