Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Dienstag
DY 25.5 Di 16:00 H3
Fast Crack Growth by Surface Diffusion — •Robert Spatschek
and Efim Brener — Institut für Festkörperforschung, Forschungszentrum
Jülich, 52425 Jülich
Fracture is an intriguing irreversible phenomenon that plays an important
role in our day-to-day-life. Crack propagation is responsible for the
vast field of material failure but is also an interesting subject of physical
research. In particular, it exhibits several peculiarities for higher propagation
velocities. Here we present a continuum theory which describes
the fast growth of a crack by surface diffusion. By introducing a fully
dynamical theory of elasticity, it is possible to obtain a self-consistent
selection of the crack tip radius. This theory describes the complicated
dynamics of a crack tip, the saturation of the steady state velocity appreciably
below the Rayleigh speed, and the blunting of the crack tip.
Furthermore, it includes the possibility of a tip splitting instability for
high applied tensions.
DY 25.6 Di 16:15 H3
Stabilität hexagonaler Erstarrungsmuster — •Mathis Plapp —
Laboratoire de Physique de la Matière Condensée, Ecole Polytechnique,
91128 Palaiseau, Frankreich
DY 26 General Statistical Physics
Hexagonale Muster bilden sich in vielen Nichtgleichgewichtssituationen,
wenn eine zweidimensionale Translationinvarianz durch eine dynamische
Instabilität gebrochen wird. Bei der gerichteten Erstarrung von
Legierungen bilden sich oberhalb einer kritischen Erstarrungsgeschwindigkeit
zelluläre Strukturen, die sich im dreidimensionalen Fall zu hexagonalen
Mustern ordnen. In Dünnschichtexperimenten und numerischen
Arbeiten wurde gezeigt, dass die Stabilität solcher Erstarrungszellen in
zwei Dimensionen erheblich von der Anisotropie der Grenzfläche abhängt.
Hier wird nun mittels Phasenfeld-Simulationen der dreidimensionale Fall
untersucht. Die räumliche Struktur der Instabilitätsmoden wird von der
Symmetrie des Zellenmusters bestimmt; die Stabilitätsgrenzen hängen
stark von der Anisotropie ab. Quadrat- und Streifenmuster sind unstabil.
Ausser Sechsecken wurde ein weiteres stabiles Muster gefunden: Tripletts,
die aus drei assymmetrischen Zellen bestehen. Sie lassen sich durch
zeitlich begrenzte Störungen kontrolliert aus Sechsecken erzeugen.
Zeit: Dienstag 16:45–18:00 Raum: H2
DY 26.1 Di 16:45 H2
Long-term correlations distinguish coarsening mechanisms
in alloys — •Lorenz-M. Stadler 1 , Bogdan Sepiol 1 , Richard
Weinkamer 2 , Markus Hartmann 2 , Peter Fratzl 2 , Jan W.
Kantelhardt 3 , Federico Zontone 4 , Gerhard Grübel 4 , and
Gero Vogl 1 — 1 Institut für Materialphysik, Universität Wien, 1090
Wien, Austria — 2 Max-Planck-Institute of Colloids and Interfaces,
Department of Biophysics, 14424 Potsdam, Germany — 3 Institut
für Theoretische Physik III, Justus-Liebig-Universität Giessen, 35392
Giessen, Germany — 4 ESRF, BP 220, 38043 Grenoble Cedex, France
We determine long-term correlations in the time series of fluctuating
x-ray speckle intensities. A fluctuation analysis of small angle x-ray
scattering data of the two phase-separating alloys Al-6at.%Ag and Al-
9at.%Zn at late stages of phase separation reveals long-term correlations
that are dramatically different for the two systems. From a comparison
with recent Monte Carlo simulations we conclude that two different coarsening
mechanisms are predominant in the two alloys—coarsening either
by diffusion of single atoms or by movement of whole precipitates.
We acknowledge financial support from the Austrian Federal Ministry
for Education, Science and Culture (project GZ 45.529/2-VI/B/7a/2002)
and the Hahn-Meitner-Institute Berlin in cooperation with the University
of Potsdam.
DY 26.2 Di 17:00 H2
Domain-Wall Energy Analysis of exact Ground States for
±J SG model in D=2 — •Amoruso Carlo and Alexander
K.Hartmann — Institute for Theoretical Physics, Göttingen
Computing ground states of Spin Glasses is a NP-hard problem, this
means that only algorithms are known, where the running time in the
worst case increases exponentially with the system size. For the special
case of two-dimensional spin glasses without an external field and with
periodic boundary conditions in at most one direction, efficient polynomial
algorithms for the calculation of exact ground states are available.
By using a matching algorithm, we computed exact ground states of two
dimensional Ising Spin Glasses with a certain concentration of antiferromagnetic
bonds p up to size L = 700. We calculated with high precision
the critical concentration of pc at which the ferromagnetic phase ceases to
exist, obtaining pc = 0.103(1). If the Nishimori point pN is located on the
phase boundary (as believed), the phase diagram has a small reentrance,
since pN ∼ 0.110. Besides we show that there is no spin-glass phase at
finite temperature.
DY 26.3 Di 17:15 H2
Ground-state structure of the vertex-cover problem —
•Wolfgang Barthel and Alexander K. Hartmann — Institute
for Theoretical Physics, Göttingen
Hard combinatorical optimization problems play an important role in
Theoretical Computer Science. One is especially interested in the time
complexity of such problems, i.e. how fast a typical instance can be solved
depending on its size. It is expected that this is strongly connected to
the landscape of the ground state structure of this problems. Here we
consider the vertex-cover problem: Take a random graph consisting of
undirected edges that meet at vertices and put guards on the vertices
such that there is one at at least one endpoint of every edge. Solutions
are covers which need a minimal number of guards and usually there is
an exponential number of them. We numerically analyze the landscape
of the solution space. A change in the ground state structure is observed
at the point where all known fast algorithms fail.
DY 26.4 Di 17:30 H2
High-Loop Variational Calculation of the Effective Potential —
•Sebastian Brandt and Axel Pelster — Institut für Theoretische
Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany
The thermodynamic properties of a quantum mechanical point particle
moving in a one-dimensional potential V (x) follow from its effective potential
Veff(X) where X denotes the path average. For zero temperature
we present an efficient high-loop calculation of Veff(X) based on algebraic
recursion relations. In case of an anharmonic oscillator with x 3 and x 4
interactions we resum the divergent loop expansion via variational perturbation
theory. Using both frequency and position of a trial oscillator
as variational parameters, we determine the ground-state energy of the
above anharmonic oscillator for arbitrary coupling strength. For pure x 4
interaction, we furthermore extend our approach to D spatial dimensions
and investigate, in particular, the large D-limit.
DY 26.5 Di 17:45 H2
Three-phase contact line interface profiles in electric fields
— •Juergen Buehrle, Stephan Herminghaus, and Frieder
Mugele — University of Ulm; Applied Physics Department; D-89069
Ulm
Conductive or electrolytic liquid droplets in electric fields assume an
apparent contact angle (the angle assumed by the droplet shape far away
from the contact line) which is different from Young’s angle. This effect
was investigated in detail and new quantitative expressions for the contact
angle deviation are derived. In the vicinity of the contact line longrange
electric fields deform the liquid interface profile. We have investigated
the equilibrium profiles by balancing electrostatic and capillary
forces locally at the liquid vapor interface. Numerical results suggest that
the contact angle at the contact line is equal to Young’s angle. Simultaneously,
the local curvature displays a weak algebraic divergence. We
present an asymptotic analytical model, which confirms these results and
elucidates the scaling behavior of the profile close to the contact line. (see
also J. Buehrle, S. Herminghaus, and F. Mugele, Phys. Rev. Lett., 2003,
91, 86101)