Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Montag
Fachsitzungen
– Haupt-, Kurzvorträge und Posterbeiträge –
DY 10 Critical Phenomena
Zeit: Montag 09:30–11:15 Raum: H2
Hauptvortrag DY 10.1 Mo 09:30 H2
Critical Dynamics in Pure Fluids and Binary Mixtures —
•Reinhard Folk — Institut für Theoretische Physik, Universität Linz,
A-4040 Linz, Österreich
The critical behavior of transport coefficients (e.g. thermal conductivity,
mass diffusion, shear viscosity) near continuous phase transitions
(PT) is still of great interest. In the last years progress has been made on
the experimental as well as on the theoretical side, which allows a quantitative
comparison between theory and experiment. Such a comparison
is presented for the gas-liquid PT, the demixing PT as well as for the
suprafluid PT in He4 and He4-He3 mixtures. Especially for this last case
the recent field theoretic two loop calculations are presented. In addition
light scattering experiments, sound velocity and sound absorption
measurements will be discussed shortly.
DY 10.2 Mo 10:00 H2
Phase Diagram of the Generalized Complex |ψ| 4 Model — •E.
Bittner und W. Janke — Institut für Theoretische Physik, Universität
Leipzig, Augustusplatz 10/11, 04109 Leipzig, Germany
Using Monte Carlo simulations, we analyze the critical behaviour of
the complex |ψ| 4 theory in the presence of an additional fugacity term.
With this modification we can show that the complex |ψ| 4 theory can
be tuned to undergo a first-order transition by varying the strength of
the new term in the generalized Hamiltonian. From a finite-size scaling
analysis we determine the critical endpoint of the line of first-order phase
transitions for several values of the strength of the new term in the generalized
Hamiltonian. Based on these results we sketch the phase diagram
of the generalized complex |ψ| 4 model.
DY 10.3 Mo 10:15 H2
Quantum Phase Diagram for Bose Gases — •Hagen Kleinert 1 ,
Axel Pelster 1 , and Sebastian Schmidt 2 — 1 Institut für Theoretische
Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany
— 2 Department of Physics, Yale University, P.O. Box 208120, New
Haven, CT 06520-8120, USA
We locate the quantum phase transition for a dilute homogeneous Bose
gas in the plane of s-wave scattering length as and temperature T. This
is done by improving a lowest-order result for the thermodynamic potential
with the help of recent high-order perturbative calculations on
the leading shift of the critical temperature due to a weak atomic repul-
sion using variational perturbation theory. The quantum phase diagram
shows a nose above the interaction-free critical temperature T (0)
c , so that
we predict the existence of a reentrant transition above T (0)
c , where an
increasing repulsion leads to the formation of a condensate. Furthermore,
we obtain a similar quantum phase diagram for a bose gas trapped in an
optical lattice as a function of effective scattering length aeff and temperature
T.
[1] H. Kleinert, S. Schmidt, and A. Pelster, cond-mat/0307412
DY 10.4 Mo 10:30 H2
Critical Dynamics: theory for time-dependent intensity correlations
measured by X-ray microdiffraction — •Klaus Mecke 1 ,
Cristian Mocuta 2 , and Harald Reichert 1 — 1 MPI für Metallforschung,
D-70569 Stuttgart, Germany — 2 ESRF, F-38043 Grenoble,
France
We studied the dynamics of critical fluctuations in Fe3Al, which exhibits
an order-disorder phase transition (DO3-B2, TC =780 K). In the
experiments we used a well focused beam to probe a sub-micron sized
single crystalline sample, so that the size of fluctuating domains is large
compared to the probed volume due to the diverging correlation length
close to TC. Pronounced effects on the time-course of the intensity of
a superstructure peak (characteristic for the chemical order) were measured.
Starting from a stochastic Langevin equation for the fluctuations
of a non-conserved order-parameter (model A) the four-point structure
function can be calculated and directly related to the measured timedependent
intensity-intensity correlation function. The analytical expression
fits very well the experimental data sets fixing the correlation time
τ as only free parameter of the theory. We find a strong dependence on
temperature, in particular an increase of τ close to the critical point. The
agreement of theory and experiments - even for various spatial and time
resolutions - demonstrate the potential of partially coherent x-ray microbeams
for the direct observation of fluctuation phenomena at critical
phase transition.
DY 10.5 Mo 10:45 H2
Exact renormalization group analysis of the O(2) theory including
all marginal and relevant coupling parameters — •Nils Hasselmann,
Sascha Ledowski, and Peter Kopietz — Institut für
Theoretische Physik, Universität Frankfurt, Robert-Mayer-Str. 8, 60054
Frankfurt/Main
We use the exact renormalization group formalism to study the critical
regime of the O(2) theory in dimensions 4 > D ≥ 3 analytically. Besides
the two relevant parameters, the mass parameter and the constant part of
the 4-point vertex, our analysis tracks also the flow of all three marginal
parameters at D = 3. Two marginal parameters characterize the lowest
order momentum dependence of the 4-point vertex and the third is the
momentum-independent part of the 6-point vertex. We find competitive
results for the critical exponents ν and η.
DY 10.6 Mo 11:00 H2
Non-Equilibrium Relaxation method for the critical Analysis of
the Hubbard Model — •Hans-Georg Matuttis 1 and Nobuyasu
Ito 2 — 1 The University of Electrocommunications, Department of Mechanical
and Control Engineering, Chofu Chofugaoka 1-5-1,Tokyo 182-
8585,Japan — 2 The University of Tokyo, Department of Applied Physics,
7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8565 Japan
The non-equilibrium relaxation (NER) analysis for critical phenomena
goes back to work by M. Suzuki in the 1970’s. One can show that the
relaxation from a non-equilibrium configuration (e.g. ordered state) to
equilibrium (e.g. disordered state) does not only tell whether the temperature
is below, above or at the critical temperature, but it is even
possible to extract the critical exponents from the time series of the relaxation.
In recent years, the NER-method has been applied to the computational
analysis of critical phenomena ranging from discrete over continous
spin models, Kosterlitz-Thouless-Transisions and spin-glas models up to
transitions in molecular dynamics systems. In this talk, the application
of the NER-analysis to auxiliary field quantum Monte-Carlo simulations
will be explained and results for the transition to the anti-ferromagnetic
phase in the groundstate at halffilling will be presented.