Plenarvorträge - DPG-Tagungen

Oberflächenphysik Mittwoch
For Cs deposition at around 160 K we observe discontinuities of the
surface conductivity at 0.5, 1, and 1.5 monolayer coverage, respectively.
Those at 0.5 and 1 ML can be associated with transitions of the surface
reconstruction from 3 × 4 to 3 × 2 and 3 × 2 to 2 × 1, respectively.
Based on EELS data, electrical phase transitions (metal - semiconductor
- metal) in the submonolayer coverage range have been reported by others
[1]. While according to EELS the wetting layer should be metallic,
conductivity measurements show that the electrical transport in the layer
is activated. Refined models of the surface structure will be presented.
[1] Ki-Dong Lee and Jinwook Chung, Phys. Rev. B 55 (1997) 12906.
O 28.9 Mi 16:00 Bereich C
STM-Simulation der Selbstassemblierung von Adenin auf
Graphit in Abhängigkeit der Scanrichtung — •Thomas
Markert 1 , Thomas Markert 1 , Ferdinand Jamitzky 2 , Frank
Trixler 3 , Axel Groß 1 , Wolfgang Heckl 3 , Ferdinand Jam
itzky 2 , Frank Trixler 3 und Axel Groß 1 — 1 Physik-Department,
TU München, D-85747 Garching — 2 MPI für extraterrestrische Physik,
D-85740 Garching — 3 Department für Geo- und Umweltwissenschaften,
LMU München, D-80333 München
STM-Bilder von Molekülnetzwerken, die über Wasserstoff-Brückenbindungen
stabilisiert sind, zeigen i.a. eine deutliche Abhängigkeit von
der Scanrichtung, da kleine Verschiebungen der Moleküle, die während
des Scanvorgangs durch die Spitze verursacht werden, abhängig von der
Orientierung der Wasserstoff-Brückenbindungen sind. Bei gleicher Auslenkung
sind die Rückstellkräfte parallel zur Wasserstoff-Brückenbindung
größer als senkrecht zu ihr. Mit Hilfe von LDOS-Bildern, die aus DFT-
Rechnungen erstellt wurden, wird am Beispiel der Selbstassemblierung
von Adenin auf Graphit versucht die experimentell gewonnenen Bilder
einer Drehserie zu simulieren.
O 28.10 Mi 16:00 Bereich C
Elctronic structure of MgSO4 crystals — •V.V. Maslyuk 1,2 ,
C. Tegenkamp 1 , T. Bredow 3 , and H. Pfnür 1 — 1 Institut für
Festkörperphysik, Appelstr.2 D-30167 Hannover, Germany — 2 Uzhhorod
National University, Pidgirna St.2 88000 Uzhhorod, Ukraine — 3 Institut
für Theoretische Chemie, Am Kleinen Felde 30 D-30167 Hannover, Germany
Typically, the crystallographic structures of minerals are quite well investigated,
but there is not much knowledge about their electronic structures.
As one candidate of an insulator consisting chemically of more than
two different elements, we investigated the crystallographic and electronic
structure of MgSO4 and kieserite (MgSO4 ·H2O), respectively. Starting
with kieserite with 36 atoms per unit cell, we determined the electronic
structure by ab–initio calculations. Using CRYSTAL03 the lattice parameters
were calculated within an accuracy better than 0.01 ˚A by choosing
basis sets for magnesium and oxygen taken from MgO and for sulfur
taken from CaSO4 including also d–orbitals. In order to calculate the
band gap of kieserite, the method of hybrid mixing of Hartree–Fock and
DFT hamiltonians was used. The band gap at the Γ–point was detemined
to be 7.4 eV. To support the theoretical calculations, first photoelectron–
and electron energy loss spectroscopy experiments have been performed.
Thereby, bulk kieserite crystals have been used. Obviously, the effect of
surface relaxation effects and defects are small, because the measured
band gap agrees quite well with the theoretical findings. Nevertheless,
our calculations will be further refined.
O 28.11 Mi 16:00 Bereich C
Valence band photoemission spectroscopy of chemically
disordered ultrathin NixPd1−x films on Cu3Au(100) — •Frank
Matthes 1 , Liu-Niu Tong 1 , Alexej Rzhevskii 2 , and C.M. Schneider
2 — 1 Leibniz Institut für Festkörper- und Werkstoffforschung
Dresden, PF 27 01 16, D-01171 Dresden — 2 Institut für Elektronische
Eigenschaften, Forschungszentrum Jülich, 52425 Jülich
We have studied the valence band structure of ultrathin NixPd1−x
alloys on a single crystalline Cu3Au(100) substrate. Ni and Pd are isoelectronic,
but while Ni is ferromagnetic with an average exchange splitting
of 0.3 eV along the ∆ direction, Pd exhibits a higher spin orbit
coupling (band splitting 0.3 eV). Via the alloy concentration one can
conveniently vary the relative weight of these two spin-dependent interactions
with profound effects for the electronic structure. We will present
first results of our high-resolution photoemission measurements for 15
monolayer thick NixPd1−x films with different compositions. The measurements
were performed along the ∆ direction for excitation energies
from 16 eV to 40 eV. As a main spectral feature, we observed contri-
butions from bands originating from 3d Ni and 4d Pd electrons near
the Fermi energy. This feature only disperses weakly with the excitation
energy. Starting with pure Ni films, we observed a broadening of
the spectra while increasing the Pd concentration up to 30%. This may
be caused by the chemically induced disorder. To distinguish the spin
dependent contributions and thus arrive a more detailed interpretation
of the photoemission data, we performed for three different Ni rich alloy
compositions also spin-resolved photoemission experiments.
O 28.12 Mi 16:00 Bereich C
Site-specific Valence-electronic Structure of SrTiO3 by X-ray
Standing Waves and XPS — •Sebastian Thiess 1 , Tien-Lin Lee 1 ,
Bruce C. C. Cowie 1 , Joe C. Woicik 2 , and Jörg Zegenhagen 1
— 1 ESRF, Grenoble, France — 2 NIST, Gaithersburg, USA
We used the x-ray standing waves (XSW) technique and x-ray photoelectron
spectroscopy (XPS) to determine the contribution of the constituent
elements Sr, Ti and O of a SrTiO3 (STO) crystal to the STO
valence band, thus deriving their partial density of states.
Lattice site-specific electronic information - which is not available from
standard XPS - is obtained by utilising the spatial intensity modulation
of an x-ray standing wave (XSW) interference field, generated by the coherent
superposition of an incident and a Bragg-reflected x-ray beam. By
proper positioning the antinodes of the XSW within the STO unit cell,
photoemission from specific lattice sites can be preferentially excited and
their valence electronic contribution identified.
XSW fields were generated by the STO(111) and STO(112) reflections
at photon energies of 2.7 and 3.9 keV. XSW modulated, high-resolution
core and valence-electron emission spectra were recorded from an in-situ
UHV annealed STO single crystal at beamline ID32 at the ESRF.
O 28.13 Mi 16:00 Bereich C
Mechanisms and capability of Steam Laser Cleaning — •F.
Lang 1 , M. Mosbacher 1 , S. Georgiou 2 , and P. Leiderer 1 —
1 University of Konstanz, Fach M676, 78457 Konstanz, Germany —
2 FORTH - IESL, P.O. Box 1527, 71110 Heraklion, Greece
Steam Laser Cleaning is a powerful approach to remove submicron
particles from surfaces. In the first step a liquid is applied onto the contaminated
surface. Afterwards a laser pulse heats part of the fluid to
a superheated state resulting in bubble nucleation and growth. Thereby
strong forces are exerted on the contaminants, which are able to overcome
the adhesion of the particles to the surface.
Our measurements covering a broad range of particle sizes demonstrated
the existence of a universal cleaning threshold for the laser fluence.
In addition the influence of different liquid layer thicknesses was
explored. Investigations of the surfaces after cleaning verified for the first
time that damage free removal can be achieved, if the experimental parameters
are chosen properly. Moreover fundamental studies of the bubble
nucleation and growth in the superheated liquid were conducted to
gain a more detailed understanding of the phase transition dynamics.
O 28.14 Mi 16:00 Bereich C
In-situ Video-STM Untersuchung der Dynamik von Sulfidadsorbaten
auf Cu(100)-Elektroden — •Tunay Tansel und Olaf
Magnussen — Institut für Experimentelle und Angewandte Physik
Christian-Albrechts-Universität Kiel
Die Diffusion und Wechselwirkung von Adsorbaten an Elektrodenoberflächen
ist von großer Bedeutung für das Verständnis der elektrochemischen
Phasengrenze. Solche dynamischen Prozesse wurden hier am Beispiel
von Sulfidadsorbaten (Bedeckungen 0,01 bis 0,03ML) auf Cu(100) in
0,01M HCl mittels elektrochemischer Hochgeschwindigkeits- Rastertunnelmikroskopie
(Video-STM) direkt auf der atomaren Skala untersucht.
Eine statistische Analyse der Diffusion isolierter Sad ergab Sprungfrequenzen
im Bereich von ≈ 2 · 10 −1 Hz. An den Domänengrenzen c(2x2)-
Phase ist die Sad Mobilität jedoch stark erhöht. Dies zeigt, dass die
Gegenwart des c(2x2)-Cl Koadsorbatgitters die effektive Diffusionsbarriere
von Sad deutlich vergrößert. Weiterhin weisen die Beobachtungen
auf attraktive Sad-Sad Wechselwirkungen und eine erhöhte Mobilität von
Clustern an mehreren, benachbarten Sad hin.