Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Donnerstag
DY 46.95 Do 16:00 Poster D
Monte Carlo Study of the Bond-Diluted 3D Ising Model —
Pierre Emmanuel Berche 1 , Christophe Chatelain 2 , Bertrand
Berche 2 , and •Wolfhard Janke 3 — 1 Groupe de Physique des
Matériaux, Université de Rouen, 76821 Mont Saint-Aignan Cedex,
France — 2 Laboratoire de Physique des Matériaux, Université Henri
Poincaré, Nancy 1, BP 239, 54506 Vandœuvre les Nancy Cedex, France
— 3 Institut für Theoretische Physik, Universität Leipzig, Augustusplatz
10/11, 04109 Leipzig, Germany
We study by Monte Carlo simulations the influence of bond dilution on
the three-dimensional Ising model. This paradigmatic model in its pure
version displays a second-order phase transition with a positive specific
heat critical exponent α. According to the Harris criterion the influence
of disorder should hence lead to a new fixed point governed by new critical
exponents. We have determined the phase diagram of the diluted
model, starting from the pure model limit down to the neighbourhood
of the percolation threshold. For the estimation of critical exponents, we
have first performed a finite-size scaling study, where we concentrated on
three different dilutions to check the stability of the disorder fixed point.
Particular emphasis is placed on cross-over phenomena between the pure,
disorder and percolation fixed points which lead to effective critical exponents
dependent on the concentration. Furthermore, the temperature
behaviour of physical quantities has been studied in order to characterize
the disorder fixed point more accurately. The question of non-selfaveraging
at the disorder fixed point is also investigated and compared
with recent results for the bond-diluted q = 4 Potts model.
DY 46.96 Do 16:00 Poster D
Photoangeregte Nanopartikel im starken Nichtgleichgewicht —
•Vassilios Kotaidis 1 , Samuel Gresillion 2 , Gero von Plessen 3
und Anton Plech 1 — 1 Fachbereich Physik der Universität Konstanz,
Universitätsstr. 10, 78457 Konstanz — 2 ESPCI 10, rue Vauquelin, F-
75005 Paris — 3 RWTH Aachen, I. Physikalisches Institut A, D-52074
Aachen
Metallische Nanopartikel stellen ideale Systeme dar, um elementare
dynamische Prozesse im Nichtgleichgewicht zu untersuchen. Durch optische
Anregung mit Femtosekundenlasern kann innerhalb einer Pikosekunde
dem System geung Energie zugeführt werden, um Gittertemperaturen
von einigen Tausend Kelvin zu erreichen. Mit der Kombination
aus Laseranregung mit 150 fs Pulsen und Röntgenpulsabfrage werden
irreversible Strukturänderungen in den metallischen Nanopartikeln aus
Silber und Gold untersucht.
Schmelzprozesse bzw. Fragmentationsprozesse werden auf der 100 ps
Zeitskala aufgezeichnet und in Beziehung zu der Ankopplung an das Medium,bzw.
eine glatte Oberfläche gesetzt.
DY 50 Entanglement and Decoherence
DY 46.97 Do 16:00 Poster D
2-D Crystals on Curved Surfaces — •Peter Lipowsky, Hans-
Georg von Ribbeck, Michael G. Nikolaides, and Andreas
R. Bausch — TU Muenchen, Physik-Department E22, James-Franck-
Strasse, D-85747 Garching
Determining the minimum-energy configuration of repulsive particles
on spherical surfaces is a rather difficult problem. Euler’s theorem states
that twelve pentagons are required to close a hexagonal network. Theory
and computer simulation have shown that, with rising system size, the
strain induced by single pentagonal disclinations will be lowered by the
introduction of additional pairs of bound 5-7 defects. An experimental
system to test the theoretical predictions has been developed. We use
surface-modified microspheres self-assembled onto the surface of water
droplets in an organic solvent. Crystalline structures are observed as soon
as the area density of particles at the interface is high enough. As predicted
by theory, we find that these crystals form distinctive high-angle
grain boundaries. The number of excess defects in these scars grows linearly
with the system size. Using digital video microscopy, particle tracking
algorithms, and triangulation routines, the dynamical behaviour of
the defects and the movements of single particles in the spherical lattice
are explored. The elastic potential of the lattice, the Young modulus, and
the diffusion constant are determined. First results demonstrating typical
phase transitions between fluid and crystalline phases are presented. Binary
mixtures formed by beads of two different diameters at the interface
were investigated.
DY 46.98 Do 16:00 Poster D
First-principle investigations of phonon spectra of crystalline
ice — •Waheed Adeniyi Adeagbo 1 , Peter Entel 1 und Jürgen
Hafner 2 — 1 Institute of Physics, University of Duisburg-Essen, 47048
Duisburg, Germany. — 2 Material Physics Institute, University of Vienna,
Austria
Lattice dynamical calculations have been carried out for a tetrahedrally
coordinated crystalline ice structure using Vienna Ab-initio Simulation
Package (VASP). A one directional periodically replicated unit cell
containing 24 molecules of water, whose constituents molecules are arranged
according to Bernal-Fowler ice rules is employed in our phonon
calculations. Dynamical properties were determined within the harmonic
approximations by finite difference evaluation of the dynamical matrix
from atomic forces. We investigated the effect of polarization arising from
dipole-dipole interactions on our phonon spectra by carrying out two separate
calculations with and without effect of induced polarization. The
polariazation was taken into account by switching on the evaluation of the
usual Berry phase expression for the electronic polarization of an insulating
ground state system in the VASP code. This produces a significant
effect in the splitting of longitudinal and transverse optic modes. Our
results are discussed with respect to the neutron diffraction scattering
data.
Zeit: Freitag 10:15–12:15 Raum: H2
Hauptvortrag DY 50.1 Fr 10:15 H2
How can entanglement between three or more qubits be measured?
— •Jens Siewert 1 and Andreas Osterloh 2 — 1 Institut
fuer Theoretische Physik, Universitaet Regensburg, D-93040 Regensburg,
Germany — 2 Dipartimento di Metodologie Fisiche e Chimiche per
l’Ingegneria, Universita di Catania, I-95125 Catania, Italy
Entanglement is, apart from its importance as a fundamental concept
in quantum mechanics, a key resource for quantum information processing.
While it is easy to say whether or not a given pure state of N qubits
is factorizable, it is a much more delicate question to quantify how much
it is entangled, even more so for mixed states. A well-known entanglement
measure for two qubits is the so-called concurrence (or 2-tangle)
[1,2] which is applicable for both pure and mixed states. While there exists
an analogous measure for pure states of three qubits [2], no N-tangle
is known for N = 3 mixed states or any state with N > 3. In this contribution,
the question is discussed what the “underlying principles” for
the N-tangle are, and how they can be generalized to N > 3.
[1] S. Hill and W.K. Wootters, Phys. Rev. Lett. 80, 2245 (1998).
[2] V. Coffman, J. Kundu, and W.K. Wootters, Phys. Rev. A 61,
052306 (2000).
DY 50.2 Fr 10:45 H2
Probing the decoherence of an oscillator with light — •Carsten
Henkel 1 , Mathias Nest 2 , Peter Domokos 3 , and Ron Folman 4
— 1 Institut für Physik, Universität Potsdam, Germany — 2 Institut für
Chemie, Universität Potsdam, Germany — 3 Institute for Solid State
Physics and Optics, Budapest, Hungary — 4 Physics Department, Ben
Gurion University, Beer Sheva, Israel
The decoherence of an oscillating particle due to the coupling to its
environment can be modelled in the framework of quantum Brownian
motion. From the average motion of the particle, some of the relevant
parameters (damping rate, equilibrium distribution) can be extracted,
but many details of the environmental bath remain arbitrary. We suggest
that experiments using an optical interferometer can obtain more
detailed information [1,2,3]. We show that the optical signal can differentiate
between master equations for different models for the environmental
coupling [4]. The scaling with particle mass and possible enhancements
using an optical cavity are discussed.
[1] S. Bose et al., Phys. Rev. A 56 (1997) 4175
[2] S. Mancini et al., Phys. Rev. Lett. 88 (2002) 120401
[3] W. Marshall et al., Phys. Rev. Lett. 91 (2003) 130401
[4] quant-ph/0310160