Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Dienstag
DY 28.10 Di 12:15 H23
Relating the macroscopic dynamics of liquid silica to its potential
energy landscape — •Aimorn Saksaengwijit and Andreas
Heuer — Westfälische Wilhelms–Universität, Institut für Physikalische
Chemie, D-48149 Münster
We have studied the dynamics of liquid silica in terms of the potential
energy landscape (PEL) defined in configuration space. The concept
of superstructures of PEL minima, denoted metabasins, is used to extract
the relevant properties of the PEL. On the level of metabasins it
DY 29 Glasses II (joint session DF/DY)
turns out that the average residence time 〈τ〉 is inversely proportional
to the diffusion constant, i.e. D(T) = c/〈τ(T)〉. The temperature dependence
of the residence time can be obtained from the analysis of local
escape processes out of individual metabasins. Furthermore we show that
the fragile-to-strong transition of silica can be reproduced by using the
PEL analysis. This allows us to obtain an improved understanding of the
underlying nature of this transition. A comparison to a fragile system
(Lennard-Jones liquid) is included.
Zeit: Dienstag 14:30–17:45 Raum: H23
Hauptvortrag DY 29.1 Di 14:30 H23
Dynamische Heterogenität ungeordneter Festkörper —
•Roland Böhmer 1 , Manfred Winterlich 1 , Sven Berndt 1 ,
Gregor Diezemann 2 und Ken R. Jeffrey 3 — 1 Experimentelle
Physik III, Universität Dortmund, 44221 Dortmund — 2 Institut für
Physikalische Chemie, Johannes Gutenberg-Universität, 55099 Mainz —
3 Department of Physics, University of Guelph, Canada
Die Natur der nichtexponentiellen Relaxation von Flüssigkeiten und
Polymerschmelzen ist in letzter Zeit sowohl theoretisch als auch experimentell
intensiv erforscht worden [1]. Im Bereich der Festkörper gab es
bislang aber nur wenige Aktivitäten. Neuere Untersuchungen mit der dielektrischen
Spektroskopie und mit der kernmagnetischen Resonanz konzentrieren
sich hier auf die Rotationsdynamik ungeordneter plastischer
Kristalle [2] und auf die Translationsdynamik von festen Ionenleitern
[3,4].
[1] H. Sillescu, J. Non-Cryst. Solids 243, 81 (1999)
[2] M. Winterlich, G. Diezemann, H. Zimmermann, R. Böhmer, Phys.
Rev. Lett. 91, (2003)
[3] R. Richert, R. Böhmer, Phys. Rev. Lett. 83, 4337 (1999)
[4] M. Vogel, C. Brinkmann, H. Eckert, A. Heuer, cond-mat/0310256
DY 29.2 Di 15:00 H23
Rotationsdynamik einzelner Moleküle in unterkühlten
Flüssigkeiten — •Gerald Hinze — Institut für Physikalische
Chemie, Johannes Gutenberg-Universität, 55099 Mainz
Die Dynamik in unterkühlten Flüssigkeiten ist durch eine große Bandbreite
an vorkommenden Zeitskalen, Amplituden und Bewegungsmechanismen
gekennzeichnet. Neben der sehr signifikanten Temperaturabhängigkeit
des primären Relaxationsprozesses sind nichtexponentielle
Korrelationsfunktionen typisch und werden in praktisch allen Glasbildnern
bzw. unterkühlten Flüssigkeiten gefunden. Eine damit verknüpfte
und zur Zeit sehr kontrovers diskutierte Fragestellung betrifft die dynamische
Heterogenität in der Nähe der kalorischen Glastemperatur. Während
einige Experimente Hinweise dafür liefern, dass die Moleküle einer Probe
relativ einheitlich reorientieren, bzw. Unterschiede sich auf der Zeitskala
der Rotation herausmitteln, ergeben andere Experimente deutlich
getrennte Zeitskalen für die Rotation und die Änderung der Rotationskorrelationszeiten.
Mit Hilfe der konfokalen Fluoreszenzmikroskopie ist es
möglich, die Rotationsdynamik einzelner Moleküle zu verfolgen. Damit
bietet sie prinzipiell die Möglichkeit, dynamische Heterogenitäten und
deren Lebensdauern zu bestimmen, da keine Ensemble-Mittelungen auftreten.
Wir stellen Einzelmolekülexperimente in der amorphen Matrix
PDE (Phenolphthalein-dimethylether) vor. Die erhaltenen Rotationskorrelationsfunktionen
werden mit einfachen Rotationsmodellen verglichen.
DY 29.3 Di 15:15 H23
Dynamical heterogeneities in simulated amorphous Ni0.5Zr0.5:
correlations with density and order parameter fluctuations —
•Imad Ladadwa and Helmar Teichler — Institut für Materialphysik,
and SFB 602, Universität Göttingen
Dynamical heterogeneity is a fundamental aspect of particle motion in
supercooled liquids and glasses. For metallic alloy system, a great part of
the information on dynamical heterogeneity comes from computer simulations,
leaving open so far, however, a detailed answer concerning the
origin of heterogeneity. In order to approach this question, we have carried
out molecular dynamics simulations for an amorphous Ni0.5Zr0.5 model,
generating well relaxed structures at 700, 760, and 810 K, that means
well below the dynamical glass temperature Tc = 1120K. From analyzing
long-time simulations over 0.7µs for medium size systems (N = 5184,
periodic boundary conditions), we find striking interrelationship between
spatial fluctuations of the atomic mobility, that means dynamical heterogeneity,
and of the fluctuation of topological order and of the chemical
composition in the system. The results indicate an anti-correlation between
local Ni content and Ni atoms mobility, on the one hand, and
a correlation between the local order and the Ni mobility on the other
hand. The talk will present the definitions of suitable measuring tools for
these quantities and quantitative estimates for the inherent correlations.
DY 29.4 Di 15:30 H23
Short-range order fluctuations and the boson peak in simulated
amorphous NiZr — •M. Guerdane and H. Teichler — Institut
für Materialphysik, Universität Göttingen
The boson peak is a striking feature in the vibration spectrum of glassy
systems. We here present results from molecular dynamics simulations
for Ni30Zr70, relating the boson peak of the system to local fluctuations
in the short-range order. Analysis of the topology around Ni atoms in
the system shows that Ni in the amorphous state on average has a coordination
number < Z >= 9, where in detail a mixture of perfect or
slightly distorted trigonal prisms (TP, Z = 9) or archimedean antiprisms
(AAP, Z = 10) and environments with Z = 8 predominate. Both TP and
AAP represent the basic structural units in the crystalline intermetallic
compounds NiZr (B33-Type) and NiZr2 (C16-Type) respectively. We
show that the boson peak in the amorphous NiZr system is related to
fluctuations in time between local environments of different coordination
numbers Z.
DY 29.5 Di 15:45 H23
Molecular Dynamics in Phosphate Glasses — •Oliver Baldus
and Ernst Rössler — Lehrstuhl für Experimentalphysik II, Universität
Bayreuth, Universitätsstr. 30, 95440 Bayreuth
The molecular dynamics of the glass formers tricresylphosphate (TCP),
triphenylphosphite (TPP) and mixtures with polystyrene are investigated
by NMR methods. The observed deviations from the KWWbehaviour
of the relaxation function obtained in stimulated echo measurements
are explained. Further it will be discussed whether those deviations
are a sign of the so called wing phenomenon that is seen in
dielectric spectra and how far the measurements are influenced by limitations
of the stimulated echo method. Restrictions of the stimulated
echo method are shown by simulations.
DY 29.6 Di 16:00 H23
Ionic transport in alkali-borate glasses — •F. Berkemeier,
Á.W. Imre, S. Voss und H. Mehrer — Institut für Materialphysik,
Wilhelm–Klemm–Str. 10, 48149 Münster, Westfählische–Wilhelms–
Universität Münster, Sonderforschungsbereich 458
We studied the ionic transport in binary and ternary alkali-borate glasses
of the compositions y Na2O·(1 − y) B2O3, y Rb2O ·(1 − y) B2O3 and
0.3 [x Na2O·(1−x) Rb2O]·0.7 B2O3. The total alkali content of the binary
glasses ranges from y = 0.04 up to y = 0.30 while the relative composition
of the mixed glasses varied in between x = 0 and x = 1. First, the ionic
transport was observed by temperature dependent measurements of the
dc conductivity which is connected to the long range transport of the cations.
For each glass composition we observed an Arrhenius behaviour of
the dc conductivity and determined an activation enthalpy ∆H. For the
binary systems we found decreasing values of ∆H with increasing alkali
content while for the mixed glasses we observed a maximum of the activation
enthalpy at the composition range around x ≈ 0.4. Conductivity
measurements only reveal the sum of the contributions of the different