Plenarvorträge - DPG-Tagungen

Metallphysik Montag
cke, Nestler and Stinner that is especially appropriate for modelling
phase transitions in alloy systems with multiple components and multiple
phases. In this work we explore characteristic microsrtuctures occuring
in different regions of a typical binary eutectic phase diagram
using uniform approach based on the generalized phase-field model both
for simulation of single phase growth and for two phase growth structures.
To obtain numerical solution of governing equations we use finite
element method with an adaptive mesh refinement. Simulations include
single primary phase growth and melting of eutectic structure above the
eutectic temperature and eutectic and off-eutectic solidification below.
Components redistribution in the liquid along the solidification front and
along the growth direction for different growing structures, dependence
of two phase lammelar growth on value of surface entropy densities are
analysed and compared with theoretical predictions.
M 4.4 Mo 12:30 H16
A Multiscale 3D Simulator for Dendritic and Eutectic Structure
Formation — •Frank Wendler and Britta Nestler — University
of Applied Sciences Karlsruhe, Germany
Diffuse interface models allow for an effective computational treatment
M 5 Elektronische Eigenschaften
of free boundary problems with complicated interface morphology. Here
different solidification phenomena of pure metals, binary and ternary alloys
are studied relying upon the formerly introduced multi-phase-field
model [1,2].
After an appropriate choice of thermophysical and numerical parameters,
quantitative 2D and 3D calculations of dendritic solidification in a
pure melt (Ni) and in binary (Ni-Cu) and ternary alloys were performed.
The following analysis of the simulation data provides the dynamical and
geometrical parameters. For the dendritic phases these quantities are the
tip speed, the tip radius and the primary arm spacing, which are compared
with theoretical and experimental data. Also, a strong influence
of the interplay between kinetic and surface anisotropy on the dendrite
morphology is observed. Smooth anisotropic and facetted patterns are
simulated and compared. For the examined binary and ternary eutectic
systems, characteristic lamellae spacings are found and related to the
classical theory of Jackson and Hunt and to a new analytical extensions
of this theory for ternary systems.
[1] B. Nestler, A.A. Wheeler, Physica D 138 (2000) 114-133
[2] H. Garcke, B. Nestler und B. Stinner, SIAM J. on Appl. Math., in
print
Zeit: Montag 11:45–12:45 Raum: H4
M 5.1 Mo 11:45 H4
Perfectly transferable many-body potentials and related generalized
cluster expansions not confined to lattices — Ralf
Drautz 1 , •Manfred Fähnle 1 , and Juan M. Sanchez 2 — 1 MPI für
Metallforschung, Heisenbergstr. 3, 70569 Stuttgart, Germany — 2 Texas
Materials Institute, The University of Texas at Austin, Austin, Texas
78712
There are two main streams for the parametrized representation of the
energy of a system of atoms, the expansion into many-body potentials
(which focusses on the positional degrees of freedom) and the conventional
cluster-expansion technique [1] (which focusses on the ordering
degrees of freedom in multicomponent lattice systems). To accelerate
the convergence of the many-body expansion, often empirical effective
and environment-dependent many-body potentials with fitting parameters
are used which have the disadvantage that they are accurate only
for configurations close to certain reference configurations. We demonstrate
how perfectly transferable many-body potentials can be generated
by ab-initio total energy calculations. The so-defined many-body potentials
can be used to compare empirical energy parametrizations on the
basis of the behaviour of their respective potential expansion. It is shown
how the techniques of the many-body expansion and of the conventional
cluster-expansion can be merged to a generalized cluster-expansion not
confined to lattices.
[1] J. M. Sanchez et al., Physica 128A, 334 (1984).
M 5.2 Mo 12:00 H4
A quantum chemical ab-initio calculation of the cohesive energy
of mercury — •Beate Paulus 1 and Krzysztof Rosciszewski 1,2 —
1 Max-Planck-Institut für Physik komplexer Systeme, Nöthnitzer Straße
38, D-01187 Dresden — 2 Institute of Physics, Jagellonian University,
Reymonta 4, Pl 30-059 Krakow, Poland
The cohesive energy of solid mercury in the rhombohedral structure
is determined within an ab-initio many-body expansion for the correlation
part. Very good agreement with the experimental value is obtained.
The self-consistent field ( Hartree-Fock) part is repulsive, the binding is
entirely due to correlations. The correlation of the closed 5d shell contributes
about half the cohesive energy. Relativistic effects are found to
be very important.
M 5.3 Mo 12:15 H4
Initial and final state band contributions in angle resolved
photoemission spectra of Bi(111) — •Christian Ast 1,2 and
Hartmut Höchst 1 — 1 Synchrotron Radiation Center, University of
Wisconsin-Madison, Stoughton, WI, USA — 2 Max-Planck-Institut für
Festkörperforschung, Stuttgart
Establishing the bulk valence band dispersion Ei(k) from angle resolved
photoemission spectra is still an ongoing challenge since the final
state bands Ef(k) are generally not known. However, using a broad range
of photon energies the measurements extend over several bulk Brillouin
zones which can then be used to extract information about the valence
band states. An analysis is presented in which the final and initial state
dispersion along ΓT is extracted from photoemission spectra of Bi(111).
We will show that the free electron final state (FEFS) model is valid
above 60eV. For lower photon energies, we establish an energy dependent
extension of the FEFS’s. Because of direct and umklapp transitions
into multiple final states we observe emission gaps indicating avoided
crossings in certain branches of the final state bands. Once the origin of
these additional features is identified, the true bulk valence band dispersion
can be established.
M 5.4 Mo 12:30 H4
Oktaederverkippung in ACu3Ru4O12 (A=Na, Ca, Sr, La, Nd) —
•Udo Schwingenschlögl, Volker Eyert und Ulrich Eckern —
Theoretische Physik II, Institut für Physik, Universität Augsburg, 86135
Augsburg
Wir haben die elektronischen Eigenschaften der perovskit-artigen Verbindungen
ACu3Ru4O12 (A=Na, Ca, Sr, La, Nd) anhand von Bandstrukturrechnungen
basierend auf der Dichtefunktionaltheorie und der lokalen
Dichtenäherung untersucht [1]. Die elektronische Struktur dieser Materialien
wird in erheblichem Maß von kovalenten Bindungsanteilen zwischen
Übergangsmetal d- und Sauerstoff p-Zuständen beeinflusst. Insbesondere
ist die charakteristische Verkippung der RuO6 Oktaeder auf starke Cu-O
Hybridisierung zurückzuführen. Im Gegensatz dazu werden sowohl die
Ru-O als auch die A-O Bindungslängen durch das Verkippen der Oktaeder
kaum beeinflusst. Die beobachteten ungewöhnlich kleinen A-O Bindungsabstände
und damit das Fehlschlagen des Bond-Valence Modells
sind eine Folge der im Vergleich zu den Cu-O Bindungen nur schwachen
A-O Hybridisierung. Unsere Ergebnisse erlauben ein besseres Verständnis
der Oktaederverkippung als universellen Mechanismus der Optimierung
von Bindungsabständen in perovskit-artigen Materialien. Gerade bei der
Suche nach neuartigen Materialeigenschaften ist dies wegen der Auswirkungen
der strukturellen Verzerrung auf die physikalischen Eigenschaften
von speziellem Interesse.
[1] U. Schwingenschlögl, V. Eyert, and U. Eckern, Chem. Phys. Lett.
370, 719 (2003)