Plenarvorträge - DPG-Tagungen

Dielektrische Festkörper Dienstag
DF 3 Gläser I (gemeinsam FV DF/DY)
Zeit: Dienstag 09:30–12:30 Raum: H23
Hauptvortrag DF 3.1 Di 09:30 H23
Theory of the glass transition for systems with trivial statics —
•Rolf Schilling 1 and Grzegorz Szamel 2 — 1 Institut für Physik,
Johannes Gutenberg-Universität, D-55099 Mainz — 2 Department of
Chemistry, Colorado State University, Ft. Collins, CO 80523, USA
The mode coupling theory (MCT) derived and investigated in great
detail by Götze and his coworkers has been the most important step towards
the microscopic understanding of the structural glass transition.
MCT yields a glass transition if the static correlations reach a critical
value. However, there exist systems with trivial statics where static correlations
even vanish. Nevertheless they exhibit a discontinuous glass
transition which can not be described by MCT in its present form. In
a first step we have derived a self-consistent equation for the diffusion
constant which yields a continuous transition, in contrast to simulational
results. We show how this drawback can be eliminated by a modification
of the MCT-approximations. As a result MCT-equations are obtained
which are complementary to the original ones. The glass transition is not
driven by the growth of static correlations but by the increase of collision
events between 2, 3 and 4 particles.
DF 3.2 Di 10:15 H23
Mode coupling equations for molecular crystals — •Michael
Ricker und Rolf Schilling — Universität Mainz, Staudinger Weg 7,
55099 Mainz
We have derived the mode coupling equations for molecular crystals
of axially symmetric particles, describing the time evolution of the tensorial
orientational correlators Slm,l ′ m ′(q, t), where lm and l′ m ′ are pairs
of indices for spherical harmonics. These equations have a similar structure
as those for molecular liquids. Differences arise from the possibility
of umklapp-processes in reciprocal space and from the anisotropy of the
lattice. Because of the latter, the three-point correlation function of orientational
density fluctuations, which occurs due to projections during the
calculations, can not be approximated in such an elegant and insightful
way as for liquid systems. Another difference is that only tensorial orientational
correlators with l, l ′ > 0 are involved, since the l = 0 and/or
l ′ = 0 correlators vanish.
If these mode coupling equations can describe the formation of orientational
glasses is currently tested for hard ellipsoids of revolution on a
simple cubic lattice. The static orientational structure factors Slm,l ′ m ′(q),
which are needed as input, are taken from MC simulations and from the
solution of the Ornstein-Zernike equation using the Percus-Yevick approximation.
DF 3.3 Di 10:30 H23
Das β-peak Phänomen in glasbildenden van-der-Waals
Flüssigkeiten — •Matthias Sperl und Wolfgang Götze —
Physik-Department T37, TU München, 85747 Garching
Messungen der Suszeptibilität mittels Optischem Kerr Effekt an Salol
und Benzophenon zeigen strukturelle Relaxation über bis zu fünf
Grössenordnungen in der Zeit. Dabei entsprechen bis zu drei Dekaden
im Anschluß an die Transiente nicht den universellen Skalengesetzen der
Modenkopplungstheorie. Dieses Problem der Datenanalyse wird durch
den Fit der Daten in einem schematischen Modenkopplungsmodell geklärt.
Der bisher unerklärte Zeitbereich in den Daten kann als Charakteristik
eines β-peak Phänomenes verstanden und analytisch beschrieben
werden.
DF 3.4 Di 10:45 H23
Dielectric relaxation in binary organic glass formers: β process
vs. HF-wing scenario and heterogeneous dynamics — •Thomas
Blochowicz and Ernst Rössler — Universität Bayreuth
We study a series of binary mixtures using 2-picoline in oligomeric
styrenes, in particular tri-styrene. The binary mixtures were characterised
by broad band dielectric spectroscopy in the dynamic range 10 −6 −10 7 Hz.
Although neither of the substances shows a secondary relaxation peak in
the neat phase, there appears a strong β-process in the spectra of the
picoline molecules in the mixture. In fact a careful lineshape analysis of
χ ′′ (ω) with a set of appropriate model functions shows that the same process
that is seen as a high frequency wing at high picoline concentrations
becomes a β relaxation at lower concentrations due to a separation of
time scales. Moreover, in the intermediate concentration range a strong
broadening of the spectra as compared to neat picoline is observed. Nonresonant
dielectric hole burning reveals that this broadening is on the
one hand due to pronounced, long-lived dynamic heterogeneities but to
some part also due to intrinsically non-exponential relaxation.
DF 3.5 Di 11:00 H23
Structure and dynamics of Al–Ni melts: Computer simulations
studies — •Jürgen Horbach, Subir Kumar Das, and Kurt
Binder — Institut für Physik, Johannes Gutenberg-Universität,
Staudingerweg 7, D-55099 Mainz
A combination of Monte Carlo (MC) and Molecular Dynamics (MD)
computer simulation techniques is used to study the structure and dynamics
of Al–Ni melts. As a model to describe the interactions between
the atoms we use an embedded atom potential that was recently proposed
by Mishin et al. [Phys. Rev. B 65, 224114 (2002)]. Monte Carlo
simulations in the semigrandcanonical ensemble yield well–equilibrated
configurations that are used in MD runs to study structural and dynamic
properties. For the case of Al4Ni we demonstrate that our simulation is
in very good agreement with the static structure factor and the diffusion
constants as measured recently with neutron scattering by Meyer et al.
We give an interpretation of the prepeak that appears in the total static
structure factor of Al rich melts around 1.3 ˚A −1 . Furthermore we analyze
the dynamics of Al4Ni by means of mode coupling theory and we discuss
the possibility of a fluid–fluid phase separation in Al–Ni mixtures.
DF 3.6 Di 11:15 H23
The mixed alkali effect in ternary silicates: computer simulation
studies — •Hans Knoth, Jürgen Horbach, and Kurt Binder —
Institut für Physik, Staudinger Weg 7, Johannes Gutenberg–Universität,
D–55099 Mainz
Molecular dynamics computer simulations are used to investigate
ternary alkali silicates (1 − x)Li2O·xK2O·2SiO2 (0 ≤ x ≤ 1). These systems
are typical ion conductors which is due to a high mobility of the
alkali ions. The structure and dynamics is studied for molten systems, as
well as at lower temperatures at which only the alkali ions show a diffusive
motion. Our aim is to understand the origin of the so-called mixed
alkali effect (MAE) which is the phenomenon that the diffusion of the
alkali ions is much slower in ternary systems (e.g. x = 0.5) than in the
corresponding binary systems (i.e. x = 0 and x = 1). Preferable sites for
the alkali diffusion are found that are located in a network of channels. In
the ternary system, each alkali species moves in its own channel network
leading to a stronger localization of alkali sites and the MAE.
DF 3.7 Di 11:30 H23
Nearly constant loss behavior in molecular glass-formers —
•Catalin Gainaru, Alberto Rivera, Thomas Blochowicz,
Christian Tschirwitz und Ernst A. Roessler — Experimentalphysik
II, Universitaet Bayreuth, Germany
We study the dielectric response of molecular glass formers showing no
Johari-Goldstein relaxation (type A systems) below the glass transition
temperature by applying the new Andeen Hagerling ultra-precision capacitance
bridge (50 Hz - 20 KHz). In all glass formers we find a nearly
constant loss behavior extending over several decades in frequency and
exhibiting an exponential temperature dependence. Below say 50 K a
crossover to another relaxation phenomena with a pronounced peak is
observed (5 K - 50 K).
DF 3.8 Di 11:45 H23
Secondary relaxation processes in organic glasses - comparison
between dielectric and NMR spectroscopy — •Sorin Adrian
Lusceac, Peter Medick, Catalin Gainaru, and Ernst A.
Rössler — Experimentalphysik II, Universitaet Bayreuth, Germany
Secondary relaxation processes in glasses were discovered using dielectric
spectroscopy. While dielectric spectroscopy is the preferred technique
to study them for obtaining time constants, NMR spectroscopy is a good
candidate to offer extra-information about their nature, in particular
concerning geometry and hindrance of molecular motion. Several organic
glass formers (polybutadiene Mw= 80000g/mol, mixture of benzene and
polybutadiene Mw= 777 and 2110g/mol) are investigated using multidimensional
NMR spectroscopy and the results are compared with those
provided by dielectric spectroscopy. The main purpose is to find a link