50 research outputs found
Microscopic Dynamics of Hard Ellipsoids in their Liquid and Glassy Phase
To investigate the influence of orientational degrees of freedom onto the
dynamics of molecular systems in its supercooled and glassy regime we have
solved numerically the mode-coupling equations for hard ellipsoids of
revolution. For a wide range of volume fractions and aspect ratios
we find an orientational peak in the center of mass spectra
and about one decade
below a high frequency peak. This orientational peak is the counterpart of a
peak appearing in the quadrupolar spectra and
. The latter peak is almost insensitive on
for close to one, i.e. for weak steric hindrance, and broadens strongly
with increasing . Deep in the glass we find an additional peak between
the orientational and the high frequency peak. We have evidence that this
intermediate peak is the result of a coupling between modes with and
, due to the nondiagonality of the static correlators.Comment: 6 figures, 12 page
Dynamical precursor of nematic order in a dense fluid of hard ellipsoids of revolution
We investigate hard ellipsoids of revolution in a parameter regime where no
long range nematic order is present but already finite size domains are formed
which show orientational order. Domain formation leads to a substantial slowing
down of a collective rotational mode which separates well from the usual
microscopic frequency regime. A dynamic coupling of this particular mode into
all other modes provides a general mechanism which explains an excess peak in
spectra of molecular fluids. Using molecular dynamics simulation on up to 4096
particles and on solving the molecular mode coupling equation we investigate
dynamic properties of the peak and prove its orientational origin.Comment: RevTeX4 style, 7 figure
Structural relaxation in a system of dumbbell molecules
The interaction-site-density-fluctuation correlators, the dipole-relaxation
functions, and the mean-squared displacements of a system of symmetric
dumbbells of fused hard spheres are calculated for two representative
elongations of the molecules within the mode-coupling theory for the evolution
of glassy dynamics. For large elongations, universal relaxation laws for states
near the glass transition are valid for parameters and time intervals similar
to the ones found for the hard-sphere system. Rotation-translation coupling
leads to an enlarged crossover interval for the mean-squared displacement of
the constituent atoms between the end of the von Schweidler regime and the
beginning of the diffusion process. For small elongations, the superposition
principle for the reorientational -process is violated for parameters
and time intervals of interest for data analysis, and there is a strong
breaking of the coupling of the -relaxation scale for the diffusion
process with that for representative density fluctuations and for dipole
reorientations.Comment: 15 pages, 14 figures, Phys. Rev. E in pres
Dynamik unterkĂĽhlter molekularer FlĂĽssigkeiten in Modenkopplungsapproximation - die Rolle der Orientierungsfreiheitsgrade fĂĽr den Bosonenpeak
Die von Schilling et al. 1997 aufgestellten molekularenModenkopplungsgleichungen sind für zwei Systeme, dipolareharte Kugeln undharte Rotationsellipsoide, numerisch gelöst worden.In den Kompressibilitätsspektrenund den entsprechenden Korrelationsspektren der beidenSysteme fiel im Vergleich mit den Spektren einfacherFlüssigkeiten einzusätzlicher Peak (bp) auf.Die Position des zusätzlichen Peaks ist etwa eine Dekade unterhalb der Position derHochfrequenzpeaks, also im mesoskopischen Frequenzbereich.In molekularen Flüssigkeiten sind Phasenübergänge derOrientierung derglqq Molekülegrqq möglich.Bei dipolaren harten Kugeln kann ferroelektrische Ordnung,bei hartenEllipsoiden nematische Ordnung auftreten. Als Vorläuferphänomen der Orientierungsordnung bilden sich in den molekularenFlüssigkeiten orientierungsgeordnete Domänen.
Die Kurzzeitdynamik dieser Domänen findetauf mesoskopischer Zeitskala statt. In der unterkühlten Flüssigkeit ist die Dynamik aller Modenan die Dynamikdieser Orientierungsmoden gekoppelt. Unsere Untersuchungen zeigen starke Hinweise darauf, dassdiese Kopplung denbp erzeugt.Demnach ist die Streuung nichtlokalisierter longitudinalerakustischer Modenan lokalisierten Orientierungsmoden die Ursache für den bp.hat nahezu keine q-Abhängigkeit und zeigt einenIsotopeneffekt. Weiterhin verglichen wir die Eigenschaften des bp mit denEigenschaften des Bosonenpeaks (BP).Der BP befindet sich in Spektren realer Systeme ebenfallseine Dekadeunterhalb der Hochfrequenzpeaks.Dieser Vergleich und der Vergleich mit den Ergebnissen einerMD-Simulation desHE-Systems ließ uns vermuten, dass es sich bei dem bp und bei dem BP in Spektrenmolekularer Systeme um dasselbe Phänomen handelt.In einem schematischen Modenkopplungsmodell konnte dasVerhalten des bpzusätzlich untersucht werden.Dynamics of supercooled molecular liquids in mode-couplingapproximation - the role of orientational degrees of freedomfor theBoson-Peak
The molecular mode-coupling equations for twosystems, dipolar hard spheres (DHS) and hard ellipsoids (HE) were solved numerically.A peak, about one decade belowthe high frequency peak, exists in the spectra of bothsystems.Its characteristics are similar to those of the so calledBoson-Peak.As a consequence of mode-coupling effectsthe coupling of quite slow orientational to thetranslational dynamics leads to a simple mechanism as an origin of thisorientational induced peak (op).
This mechanism and the characteristics of this peak comparedto that of the Boson-Peak for both systems is discussed.Furthermore its dependence on the variation of the packingfraction and thedipole moment (DHS) or the aspect ratio (HE) of themoleculesis presented.The position of the op exhibits an ''isotope''-effect. We give evidencethat the
existence of this peak is related to the occurrenceof the mediumranged orientational order. It is shown that some of thesefeature alsoexist for schematic mode coupling models