Plenarvorträge - DPG-Tagungen

Oberflächenphysik Montag
many body MD/MC-Corrected Effective Medium potential developed by
De Pristo et al. The calculation of a set of several thousands of trajectories
in the energy range from 1 − 8keV has been carried out for a
microscopic study of nonlinear yield enhancements that have been observed
in recent polyatomic sputtering experiments. We will show that
the application of Ag4 instead of monoatomic silver projectiles at the
same impact velocity, i.e the same energy per projectile atom, leads to
an exponential yield enhancement as a function of the impact velocity.
A detailed analysis of the kinetic energy transfer processes during the
spatial and time evolution of the collision cascade shows that this phenomenon
cannot only be explained by a higher energy density at the
surface due to the strongly localized overlap of collison cascades.
O 10.8 Mo 17:30 H43
Leitfähigkeitsuntersuchungen an freien Silber-Nanodrähten —
•Dieter Wagner 1 , Andreas Graff 2 und Uwe Kreibig 1 — 1 I. Physikalisches
Institut 1A, RWTH Aachen, 52056 Aachen — 2 MPI für Mikrostrukturphysik,
06120 Halle
Es wurden auf chemischem Wege Silber-Nanodrähte hergestellt, an
denen elektrische Leitfähigkeitsmessungen durchgeführt wurden. Diese
Drähte können mit Durchmessern zwischen 15 und 80 nm, und Längen
bis 60 000 nm präpariert werden. Durch TEM-Analysen wurden einkristalline
Struktur und Oberfläche der Drähte eingehend untersucht.
Elektronische Eigenschaften solcher Drähte mit Durchmessern auf der
Nanometerskala weichen erheblich von den Eigenschaften dickerer Drähte
ab. Um den Einfluss von Einbettmedien bzw. Substrat während der elektrischen
Messungen auszuschliessen, war es notwendig, daß die Drähte
freitragend sind. Es werden Ergebnisse vorgestellt die unter angelegtem
äußeren Magnetfeld (≤ 1T) und bei tiefen Temperaturen (∼ 4, 2 K) gewonnen
wurden .
O 11 Elektronische Struktur (Experiment und Theorie) I
Zeit: Montag 15:45–18:30 Raum: H44
O 11.1 Mo 15:45 H44
Electronic Structure Dependence of the Electron-Phonon Interaction
in Ag — •Jens Paggel 1 , Dah-An Luh 2 , Tom Miller 3 ,
and Tai-Chang Chiang 3 — 1 Freie Universität, Institut für Experimentalphysik,
14195 Berlin — 2 Stanford Linear Accelerator Center, Menlo
Park, CA 94015 (USA) — 3 Department of Physics, University of Illinois
at Urbana-Champaign, Urbana, IL 61801 (USA) and Frederick Seitz Materials
Research Laboratory, University of Illinois at Urbana-Champaign,
Urbana, IL 61801 (USA)
The line widths of sp- and d-band derived electronic quantum well
states in thin films of Ag on Fe(100) are measured as a function of temperature
to yield the electron-phonon coupling parameters. The results
vary by a factor of up to 35 among the different states. The origin of
these huge differences is traced to the decay path selection for the various
initial states of the holes created by the photoemission process. The
electron-phonon coupling parameter for the top d-band quantum well
state, 0.015 ± 0.006, is the smallest ever reported.
Supported by the US National Science Foundation, the Petroleum Research
Fund of the American Chemical Society, the US Department of
Energy, and the Deutsche Forschungsgemeinschaft.
O 11.2 Mo 16:00 H44
Electron phonon coupling and electron correlations in the adsorbate
system Cu(100)c(2x2)N — •Thordis Michalke, Jürgen
Hager, and René Matzdorf — Institut für Physik, Universität Kassel
We investigated the self organized nanopatterned surface of nitrogen
covered Cu(100) by means of photoelectron spectroscopy (PES), scanning
tunneling microscopy (STM) and spectroscopy (STS).
With STS we imaged the local density of states of certain eigenstates in
the 5x5 nm 2 islands. The quasi particle lifetime of these states was measured
in dependence of their quantum number, and the effective mass
has been determined to be m ⋆ =1.1 m0.
In photoelectron spectra one can observe several adsorbate induced
states. Temperature dependent measurements show that these two dimensional
states couple strongly to phonons. The mass enhancement
factor was calculated with the Einstein model and known phonon energy
of 40 meV to be between λ = 0.5 and λ = 1.4 for different states. One
of the adsorbate states shows exceptional behaviour when approaching
the Fermi energy. Here the band renormalization due to electron phonon
coupling may be visible.
O 11.3 Mo 16:15 H44
Global electronic structure and charge-density-wave mechanism
in 2H-TaSe2 — •Kai Rossnagel 1,2 , Eli Rotenberg 1 , N.V. Smith 1 ,
O. Seifarth 2 , and L. Kipp 2 — 1 Advanced Light Source, Lawrence
Berkeley National Laboratory, Berkeley, CA 94720, USA — 2 Institut
für Experimentelle und Angewandte Physik, Universität Kiel, D–24098
Kiel, Germany
The simple layered charge-density-wave system 2H-TaSe2 has received
renewed interest recently because it may share important physical properties
with the high-temperature superconducting cuprates, such as quasi-
two-dimensionality, qualitatively similar resisitivity curves and optical
responses, saddle bands close to the chemical potential, and a possible
correlation between the opening of a gap on parts of the Fermi surface
and the occurence of a strong energy renormalization on ungapped parts.
We present here a detailed angle-resolved photoelectron spectroscopy
study of the near-EF electronic structure of 2H-TaSe2, focusing on Fermi
surface topology, energy gaps, and band renormalization effects. Our results
provide important clues as to the similarities to the electronic structure
of cuprates and the origin of the still-debated charge-density-wave
mechanism in 2H-TaSe2.
The experiments were carried out at the Electronic Structure Factory
at beamline 7 of the Advanced Light Source in Berkeley. K.R. gratefully
acknowledges support by the Alexander von Humboldt Foundation. Work
at the University of Kiel is supported by DFG Forschergruppe FOR 353.
O 11.4 Mo 16:30 H44
The electronic structure of Pb films on Si(111) — •Ad Ettema 1 ,
Anton Mans 1 , Hugo Dil 2 , JeongWon Kim 2 , and Karsten
Horn 2 — 1 TUDelft (NL) — 2 Fritz Haber Institute, Berlin
The Pb/Si(111) system shows a rich variety of interesting physical
properties and phenomena, such as the magic height of islands, vertical
Friedel oscillations [1] , a competition between classical and quantum mechanical
effects in the shape relaxation [2], anomalous optical absorption
[3] and anomalous behaviour in the Hall effect as a function of thickness.
In this contribution we present the results of photoemission experiments
carried out at the synchrotron radiation facilities of MAX-lab
in Lund (Sweden) and BESSY in Berlin. The results show an anomalously
large in-plane effective mass of the 2D quantum well state bands,
whereas the original bulk band has a strong dispersion and a free electron
behaviour. Moreover, band structure calculations for bulk and thin
film slabs predict bands with predominantly free electron character. Our
analysis suggests that this phenomenon may be due to a strong electron
correlation effect in the 2D bands of the Pb quantum well states.
[1] W.B. Jian, W.B. Su, C.S. Chang and T.T.Tsong, Physical Review
Letters 90, 6603 (2003). [2] H Okamoto, D. Chen, T. Yamada , Physical
Review Letters 89, 6101 (2002). [3]M Jalochowski et al, Physical Review
B 66, 205417 (2002).
O 11.5 Mo 16:45 H44
Sub-Monolayer growth of Pb on Ag(111) — •Gero Wittich,
Lucia Vitali, Peter Wahl, M. Alexander Schneider, Christian
Ast, Lars Diekhöner und Klaus Kern — Max-Planck-Institut
für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany
The growth of ultrathin Pb films deposited on Ag(111) surfaces is studied
by low temperature scanning tunneling microscopy (STM). Pb is
known to form surface alloys with e.g. Cu although it is immiscible with
noble metals in the bulk. In the system Pb/Ag(111) we observe, that the
Pb atoms are incorporated in the first Ag layer and repel each other.
With increasing coverage a long range ordered ( √ 3 x √ 3 )R30 ◦ surface
alloy forms. Above 0.3 monolayers, dealloying starts and hexagonal clo-