Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Donnerstag
of two-dimensional lattices the site percolation threshold is bound from
above and below by the zero and the inflection point of the Euler characteristic,
respectively. In the case of site percolation on three-dimensional
lattices, bond-site percolation and continuum percolation, the thresholds
are bound from above by the zero of the Euler characteristic.
DY 46.88 Do 16:00 Poster D
Phase transitions in periodic pinning arrays — •Konrad
Mangold 1 , Michael Köppl 1 , Paul Leiderer 1 , and Clemens
Bechinger 2 — 1 Universität Konstanz, Fachbereich Physik —
2 Universität Stuttgart, 2.Physikalisches Institut
We study by means of videomicroscopy the phase behavior of twodimensional
(2D) paramagnetic colloidal systems on periodic pinning arrays,
the latter being created by a hollographic optical tweezer technique.
When the particle interacion is decreased by changing an external magnetic
field the colloidal monolayer shows a melting transition from an
incommensurable solid to a liquid phase. An undisturbed 2D system exhibits
a two-stage melting as described by the KTHNY theory. With
an additional pinning array the 2D system shows a rich phase behavior.
The details of the diffusion of interstitial particles through a matrix
of pinned particles depend among other things on the geometry of the
pinning lattice (square or hexagonal) and on density ratio of pinned to
diffusing particles. We compare our results with numerical studies on
vortex melting in periodic pinning arrays.
We also present first results where the particle transport through a
channel under non-equilibrium conditions is studied. A topological channel
is created by means of electron lithography and the motion of the
particles are studied for different particle interactions and channel geometries.
DY 46.89 Do 16:00 Poster D
Short-Time Dynamics — •Eric Lorenz and Wolfhard Janke
— Institut für Theoretische Physik, Universität Leipzig, Augustusplatz
10/11, 04109 Leipzig, Germany
The 2d Ising model on a square lattice is quenched from a state at
high temperature to the critical temperature and its dynamic relaxation
is investigated with Monte Carlo methods. In the case of a small initial
magnetization, a critical initial increase of the magnetization is observed
during early time of the dynamic evolution. With power laws derived
from the scaling relation being valid already in non-equilibrium, one can
determine estimates for the critical exponents θ, z, and β/ν. This procedure
of observing quantities in non-equilibrium prevents critical slowing
down caused by the divergence of the spatial correlation length at the
critical temperature in equilibrium. In this work special attention is paid
to a comparison of algorithms. The dynamics produced by heat-bath,
Metropolis and Glauber updates are found to be universal, despite surprising
differences during the very early time evolution, which are investigated
in some detail.
DY 46.90 Do 16:00 Poster D
Damage Spreading in Ising Lattice — •Rodrigo Megaides and
Wolfhard Janke — Institut für Theoretische Physik, Universität
Leipzig, Augustusplatz 10/11, 04109 Leipzig, Germany
We study the “damage spreading phenomenon” in a two-dimensional
Ising lattice of different sizes. The phenomenon consists in the propagation
of a small modification, in the initial state of the lattice, to the
whole system without changing the random noise. The method is first
equilibrating the system and then performing a time average over final
“Hamming distances”(a measure of the number of spins in opposition in
two similar systems) of initial pairs of lattices (the first one taken for such
average and the second one being identical to the former except for the
individual state of several spins, the so called “damage”). In every individual
measurement, we find one of these three types of “final”Hamming
distances (HD) : HD = 0 or healing (the damage disappears at the first
sweeps), HD ≃ 0.5 or 0.5-spreading (the damage spreads until it reaches
one half of the lattices, except for some fluctuations), HD = 1 or infinite
spreading (the damage fills the lattice). The probabilities of such cases
are obtained (for different temperatures and lattice sizes) from the time
average, as well as a mean value of the Hamming distance which is easily
calculated from the mentioned probabilities. Some scaling properties are
proposed.
DY 46.91 Do 16:00 Poster D
Desorption behaviour of binary quench-condensed noble gas
mixtures — •M. Layer, A. Netsch, A. Fleischmann, and S. Hunklinger
— Kirchhoff-Institut für Physik, Universität Heidelberg, Im
Neuenheimer Feld 227, D-69120 Heidelberg
Due to their simple van der Waals interaction noble gas crystals are
commonly considered as model system for condensed matter. This holds
in particular for binary mixtures. Freezing theories as well as molecular
dynamical simulations predict the existence of structural ordered
phases in good agreement with each other. We studied the desorption behaviour
of quench-condensed noble gas films by means of high-frequency
surface acoustic waves. In contrast to theoretical investigations quenchcondensed
films are in a state far away from thermal equilibrium. By annealing,
these films are relaxing into thermodynamically more favorable
phases. Over a wide range of preparation conditions (including substrate
temperature, gas temperature, composition of the mixture) we found evidence
for the formation of ordered film structures. Furthermore it turned
out that in particular the ratio of diameters of the two components is relevant
to the occurence of ordered film structures. Desorption processes of
several condensed binary mixtures (Ne/Ar, Ne/Kr, Ar/Kr) are presented
and the forming of possible van der Waals compounds is discussed.
DY 46.92 Do 16:00 Poster D
Computer Simulations of the 3–Dimensional Abelian Higgs
Model — •Sandro Wenzel, Elmar Bittner, Wolfhard Janke,
and Arwed Schiller — Institut für Theoretische Physik, Universität
Leipzig, Augustusplatz 10/11, 04109 Leipzig, Germany
Using multicanonical and heat–bath Monte Carlo algorithms we
investigated the 3–dimensional Abelian Higgs model [1] on the lattice
with compact gauge fields and fluctuating Higgs amplitudes (λ �= ∞).
Special consideration was given to the first–order regime of the
confinement–deconfinement phase transition. The existence and location
of a (tri–)critical point was explored using reweighting techniques.
[1] S.Mo, J.Hove, A. Sudbø, Phys. Rev. B 65 (2002) 104501.
DY 46.93 Do 16:00 Poster D
3-Zustands Potts-Modell auf Voronoi-Delaunay Zufallsgraphen
in 2 Dimensionen — •Goetz Kähler, Martin Weigel und Wolfhard
Janke — Institut für Theoretische Physik, Universität Leipzig,
Augustusplatz 10/11, 04109 Leipzig, Germany
Im Rahmen der Diplomarbeit erstellten wir Voronoi-Delaunay Zufallsgitter
in verschiedenen 2D-Topologien (Torus, Kugeloberfläche). Auf
diesen wurde dann ein modifiziertes, reichweitenabhängiges 3-Zustands
Potts-Modell mit Hilfe des Wolff’schen Clusteralgorithmus auf vielen
Realisierungen des Zufallsgitters simuliert. Ergebnisse dieser Arbeit, insbesondere
in Bezug auf das kritische Verhalten von Magnetisierung und
Suszeptibilität [1] werden hier vorgestellt.
[1] F.W.S. Lima, U.M.S. Costa, M.P. Almeida und J.S. Andrade Jr., Eur.
Phys. J. B17, 111 (2000).
DY 46.94 Do 16:00 Poster D
The Bragg glass - Vortex glass transition — •Roland Schorr,
Ludger Santen, and Heiko Rieger — Universität des Saarlandes,
FR 7.1 Theoretische Physik, 66041 Saarbrücken
Many phase diagrams of high Tc superconductors obtained experimentally,
e.g. in Bi2Sr2CaCu2O8+y (BSCCO) and Y Ba2Cu3O7−δ (YBCO),
reveal a very complicated landscape of various phases. The most significant
modifications applied to a naive idea of a type II superconductor,
which discriminates only between Meissner (H < Hc1) and Abrikosov
(Hc1 < H < Hc2) phase, have to take into account the impact of termal
fluctuations and disorder. This leads to a water-like melting phenomenon
in the regime of the Abrikosov phase with a melting line terminating at
fields that are much lower than Hc2. The phase below the melting line
is called Bragg glass whereas above the line one has to distinguish between
Vortex glass and Vortex liquid. We focus on the phase transition
between Bragg glass and Vortex liquid and Vortex glass, respectively,
induced by the disorder at finite temperatures. As a starting point of
our simulation we choose an elastic description of fluxlines fluctuating
in a random potential. We perform a Monte Carlo Simulation applying
cluster updates in order to sample the configuration space. Whereas local
updating schemes seem to be problematic due to slowing down and a
dynamical critical exponent z = 2, our cluster algorithm works well with
z ≈ 0. This enables us to realize the three phases for system sizes up to
L = 32.