281 research outputs found
Integration and convergence: Lessons from greece's experience in the European Community
The date of entry of Spain and Portugal into the European Community is drawing ever nearer. What are the likely economic consequences of the integration into the EC of these Southern European countries, whose level of development is quite different from that of the other members? The experience of Greece, the first Southern European country to join the EC, can provide some valuable lessons here
On the origin of the -transition in liquid Sulphur
Developing a novel experimental technique, we applied photon correlation
spectroscopy using infrared radiation in liquid Sulphur around ,
i.e. in the temperature range where an abrupt increase in viscosity by four
orders of magnitude is observed upon heating within few degrees. This allowed
us - overcoming photo-induced and absorption effects at visible wavelengths -
to reveal a chain relaxation process with characteristic time in the ms range.
These results do rehabilitate the validity of the Maxwell relation in Sulphur
from an apparent failure, allowing rationalizing the mechanical and
thermodynamic behavior of this system within a viscoelastic scenario.Comment: 5 pages, 4 eps figures, accepted in Phys. Rev. Let
High frequency dynamics in a monatomic glass
The high frequency dynamics of glassy Selenium has been studied by Inelastic
X-ray Scattering at beamline BL35XU (SPring-8). The high quality of the data
allows one to pinpoint the existence of a dispersing acoustic mode for
wavevectors () of nm, helping to clarify a previous
contradiction between experimental and numerical results. The sound velocity
shows a positive dispersion, exceeding the hydrodynamic value by 10%
at nm. The dependence of the sound attenuation
, reported for other glasses, is found to be the low- limit of a
more general law which applies also to the
higher region, where no longer holds.Comment: Phys. Rev. Lett. (Accepted
Dynamics of proteins: Light scattering study of dilute and dense colloidal suspensions of eye lens homogenates
We report a dynamic light scattering study on protein suspensions of bovine
lens homogenates at conditions (pH and ionic strength) similar to the
physiological ones. Light scattering data were collected at two temperatures,
20 oC and 37 oC, over a wide range of concentrations from the very dilute limit
up to the dense regime approaching to the physiological lens concentration. A
comparison with experimental data from intact bovine lenses was advanced
revealing differences between dispersions and lenses at similar concentrations.
In the dilute regime two scattering entities were detected and identified with
the long-time, self-diffusion modes of alpha-crystallins and their aggregates,
which naturally exist in lens nucleus. Self-diffusion coefficients are
temperature insensitive, whereas the collective diffusion coefficient depends
strongly on temperature revealing a reduction of the net repulsive
interparticle forces with lowering temperature. While there are no rigorous
theoretical approaches on particle diffusion properties for multi-component,
non-ideal hard-sphere, polydispersed systems, as the suspensions studied here,
a discussion of the volume fraction dependence of the long-time, self-diffusion
coefficient in the context of existing theoretical approaches was undertaken.
This study is purported to provide some insight into the complex light
scattering pattern of intact lenses and the interactions between the
constituent proteins that are responsible for lens transparency. This would
lead to understand basic mechanisms of specific protein interactions that lead
to lens opacification (cataract) under pathological conditions.Comment: To appear in J. Chem. Phy
On the analysis of the vibrational Boson peak and low-energy excitations in glasses
Implications of reduction procedures applied to the low energy part of the
vibrational density of states in glasses and supercooled liquids are considered
by advancing a detailed comparison between the excess - over the Debye limit -
vibrational density of states g(w) and the frequency-reduced representation
g(w)/w^2 usually referred to as the Boson peak. Analyzing representative
experimental data from inelastic neutron and Raman scattering we show that
reduction procedures distort to a great extent the otherwise symmetric excess
density of states. The frequency of the maximum and the intensity of the excess
experience dramatic changes; the former is reduced while the latter increases.
The frequency and the intensity of the Boson peak are also sensitive to the
distribution of the excess. In the light of the critical appraisal between the
two forms of the density of states (i.e. the excess and the frequency-reduced
one) we discuss changes of the Boson peak spectral features that are induced
under the presence of external stimuli such as temperature (quenching rate,
annealing), pressure, and irradiation. The majority of the Boson peak changes
induced by the presence of those stimuli can be reasonably traced back to
simple and expected modifications of the excess density of states and can be
quite satisfactorily accounted for the Euclidean random matrix theory.
Parallels to the heat capacity Boson peak are also briefly discussed.Comment: To appear in J. Non-Cryst. Solids (Proceedings of the 5th IDMRCS,
Lille, July 2005
Stress distribution and the fragility of supercooled melts
We formulate a minimal ansatz for local stress distribution in a solid that
includes the possibility of strongly anharmonic short-length motions. We
discover a broken-symmetry metastable phase that exhibits an aperiodic,
frozen-in stress distribution. This aperiodic metastable phase is characterized
by many distinct, nearly degenerate configurations. The activated transitions
between the configurations are mapped onto the dynamics of a long range
classical Heisenberg model with 6-component spins and anisotropic couplings. We
argue the metastable phase corresponds to a deeply supercooled non-polymeric,
non-metallic liquid, and further establish an order parameter for the
glass-to-crystal transition. The spin model itself exhibits a continuous range
of behaviors between two limits corresponding to frozen-in shear and uniform
compression/dilation respectively. The two regimes are separated by a
continuous transition controlled by the anisotropy in the spin-spin
interaction, which is directly related to the Poisson ratio of the
material. The latter ratio and the ultra-violet cutoff of the theory determine
the liquid configurational entropy. Our results suggest that liquid's fragility
depends on the Poisson ratio in a non-monotonic way. The present ansatz
provides a microscopic framework for computing the configurational entropy and
relaxational spectrum of specific substances.Comment: 11 pages, 5 figures, Final version published in J Phys Chem
Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous-to-crystal transition
We report on an inelastic (Raman) light scattering study of the local
structure of amorphous GeTe films. A detailed analysis of the
temperature-reduced Raman spectra has shown that appreciable structural changes
occur as a function of temperature. These changes involve modifications of
atomic arrangements such as to facilitate the rapid amorphous-to-crystal
transformation, which is the major advantage of phase-change materials used in
optical data storage media. A particular structural model, supported by
polarization analysis, is proposed being compatible with the experimental data
as regards both the structure of a-GeTe and the crystallization transition. The
remarkable difference between the Raman spectrum of the crystal and the glass
can thus naturally be accounted for.Comment: Published in: J. Phys. Condens. Matter. 18, 965-979 (2006
EXPERIMENTAL STUDY ON THE EFFECTIVENESS OF AN ENERGY DISSIPATION CONFIGURATION IN AN OPEN CHANNEL OF STEEP SLOPE
Στην παρούσα εργασία μελετάται πειραματικά η ροή σε σήραγγα υπό συνθήκες ελεύθερης επιφάνειας και έντονης κλίσης πυθμένα 1:10. Διερευνάται η δυνατότητα μείωσης της ταχύτητας ροής μέσω συνδυασμού κατακόρυφων, πλευρικών στοιχείων τραχύτητας και λεκανών καταστροφής ενέργειας με οδοντώσεις. Η μελετώμενη διάταξη στηρίζεται στην ύπαρξη επαναλαμβανομένων τμημάτων (modules) εντός των οποίων επιτυγχάνεται επαναληψιμότητα της ροής. Η αποτελεσματικότητα του σχεδιασμού διερευνήθηκε σε υδραυλικό ομοίωμα κλίμακας 1:12,5 που βασίσθηκε σε συνθήκες δυναμικής ομοιότητας κατά Froude για χαρακτηριστικές τιμές παροχής. Η επεξεργασία των μετρήσεων έδειξε ότι με κατάλληλη διάταξη πλευρικών στοιχείων τραχύτητας και διαμόρφωση της λεκάνης καταστροφής ενέργειας ελέγχεται η τιμή της ταχύτητας, ικανοποιείται η απαίτηση μεγίστου βάθους ροής σε σχέση με τις διαστάσεις της σήραγγας και επιτυγχάνεται επαναληψιμότητα της ροής σε κάθε module.The free-surface flow in a tunnel of steep bed slope 1:10 is studied experimentally. The effectiveness of vertical roughness elements on the side walls and energy dissipation basins with blocks is investigated, with the aim to reduce flow velocity in the tunnel. The design is based on the concept of repeated modules in order to achieve flow repeatability. The scale of the physical model was 1:12.5 under Froude similarity conditions. The analysis of measurements indicates that a suitable arrangement of vertical roughness elements on the side walls and a suitable design of the energy dissipation basin can control the flow velocity magnitude, satisfy the maximum flow depth requirement with respect to the tunnel dimensions, and achieve flow repeatability in each modul
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