157 research outputs found
Vibrational States of Glassy and Crystalline Orthotherphenyl
Low-frequency vibrations of glassy and crystalline orthoterphenyl are studied
by means of neutron scattering. Phonon dispersions are measured along the main
axes of a single crystal, and the corresponding longitudinal and transversal
sound velocities are obtained. For glassy and polycrystalline samples, a
density of vibrational states is determined and cross-checked against other
dynamic observables. In the crystal, low-lying zone-boundary modes lead to an
excess over the Debye density of states. In the glass, the boson peak is
located at even lower frequencies. With increasing temperature, both glass and
crystal show anharmonicity.Comment: 7 pages of LaTeX (svjour), 2 tables, 10 figures accepted in Eur.
Phys. J.
Wavenumber dependence of structural alpha relaxation in a molecular liquid
Structural alpha relaxation in liquid orthoterphenyl is studied by means of
coherent neutron time-of-flight and backscattering spectroscopy over a large
temperature range. Not only amplitude and relaxation time but also the spectral
line shape show a significant variation with wavenumber. As expected from mode
coupling theory, these variations are correlated with the static structure
factor. Even far above the melting point, alpha relaxation remains
non-exponential.Comment: 6 pages of LaTeX, 4 figure
Excitation lines and the breakdown of Stokes-Einstein relations in supercooled liquids
By applying the concept of dynamical facilitation and analyzing the
excitation lines that result from this facilitation, we investigate the origin
of decoupling of transport coefficients in supercooled liquids. We illustrate
our approach with two classes of models. One depicts diffusion in a strong
glass former, and the other in a fragile glass former. At low temperatures,
both models exhibit violation of the Stokes-Einstein relation,
, where is the self diffusion constant and is the
structural relaxation time. In the strong case, the violation is sensitive to
dimensionality , going as for , and as for . In the fragile case, however, we argue that
dimensionality dependence is weak, and show that for , . This scaling for the fragile case compares favorably with the
results of a recent experimental study for a three-dimensional fragile glass
former.Comment: 7 pages, 7 figures, submitted to Phys. Rev.
Connected Network of Minima as a Model Glass: Long Time Dynamics
A simple model to investigate the long time dynamics of glass-formers is
presented and applied to study a Lennard-Jones system in supercooled and glassy
phases. According to our model, the point representing the system in the
configurational phase space performs harmonic vibrations around (and activated
jumps between) minima pertaining to a connected network. Exploiting the model,
in agreement with the experimental results, we find evidence for: i) stretched
relaxational dynamics; ii) a strong T-dependence of the stretching parameter;
iii) breakdown of the Stokes-Einstein law.Comment: 4 pages (Latex), 4 eps figure
All Polymer Diffusion Regimes Covered by Combining Field-Cycling and Field-Gradient <sup>1</sup>H NMR
© 2015 American Chemical Society. Field-cycling and field-gradient 1H NMR experiments were combined to reveal the segmental mean-square displacement as a function of time for polydimethylsiloxane (PDMS) and polybutadiene (PB). Together, more than 10 decades in time are covered, and all four power-law regimes of the tube-reptation (TR) model are identified with exponents rather close to the predicted ones. Characteristic polymer properties like the tube diameter a0, the Kuhn length b, the mean-square end-to-end distance 02>, the segmental correlation time τs(T), the entanglement time τe(T), and the disengagement time τd(T) are estimated from the measurements and compared to results from literature. Concerning τd(T), fair agreement is found. In the case of τe, agreement with rheological data is achieved when the time constant is extracted from the minimum in the shear modulus G″(ω). Concerning the TR predictions the molar mass (M) dependence of τd is essentially reproduced. Yet, calculating τe from τd for PDMS yields agreement with experimental data while for PB it gets by 2 orders of magnitude too short. In no case τe is correctly reproduced from τs(T). Segmental and shortest Rouse times appear to coincide for PB, while in the case of PDMS the latter turns out to be longer by 1 decade
Crystal-like high frequency phonons in the amorphous phases of solid water
The high frequency dynamics of low- (LDA) and high-density amorphous-ice
(HDA) and of cubic ice (I_c) has been measured by inelastic X-ray Scattering
(IXS) in the 1-15 nm^{-1} momentum transfer (Q) range. Sharp phonon-like
excitations are observed, and the longitudinal acoustic branch is identified up
to Q = 8nm^{-1} in LDA and I_c and up to 5nm^{-1} in HDA. The narrow width of
these excitations is in sharp contrast with the broad features observed in all
amorphous systems studied so far. The "crystal-like" behavior of amorphous
ices, therefore, implies a considerable reduction in the number of decay
channels available to sound-like excitations which is assimilated to low local
disorder.Comment: 4 pages, 3 figure
Potential Energy Landscape and Long Time Dynamics in a Simple Model Glass
We analyze the properties of a Lennard-Jones system at the level of the
potential energy landscape. After an exhaustive investigation of the
topological features of the landscape of the systems, obtained studying small
size sample, we describe the dynamics of the systems in the multi-dimensional
configurational space by a simple model. This consider the configurational
space as a connected network of minima where the dynamics proceeds by jumps
described by an appropriate master equation. Using this model we are able to
reproduce the long time dynamics and the low temperature regime. We investigate
both the equilibrium regime and the off-equilibrium one, finding those typical
glassy behavior usually observed in the experiments such as: {\it i)} stretched
exponential relaxation, {\it ii)} temperature-dependent stretching parameter,
{\it iii)} breakdown of the Stokes-Einstein relation, and {\it iv)} appearance
of a critical temperature below which one observes deviation from the
fluctuation-dissipation relation as consequence of the lack of equilibrium in
the system.Comment: 11 pages (Latex), 9 ps figure
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