538 research outputs found
Dynamic Transitions in a Two Dimensional Associating Lattice Gas Model
Using Monte Carlo simulations we investigate some new aspects of the phase
diagram and the behavior of the diffusion coefficient in an associating lattice
gas (ALG) model on different regions of the phase diagram. The ALG model
combines a two dimensional lattice gas where particles interact through a soft
core potential and orientational degrees of freedom. The competition between
soft core potential and directional attractive forces results in a high density
liquid phase, a low density liquid phase, and a gas phase. Besides anomalies in
the behavior of the density with the temperature at constant pressure and of
the diffusion coefficient with density at constant temperature are also found.
The two liquid phases are separated by a coexistence line that ends in a
bicritical point. The low density liquid phase is separated from the gas phase
by a coexistence line that ends in tricritical point. The bicritical and
tricritical points are linked by a critical -line. The high density
liquid phase and the fluid phases are separated by a second critical
line. We then investigate how the diffusion coefficient behaves on different
regions of the chemical potential-temperature phase diagram. We find that
diffusivity undergoes two types of dynamic transitions: a fragile-to-strong
trans ition when the critical -line is crossed by decreasing the
temperature at a constant chemical potential; and a strong-to-strong transition
when the -critical line is crossed by decreasing the temperature at a
constant chemical potential.Comment: 22 page
Temperature Evolution of Sodium Nitrite Structure in a Restricted Geometry
The NaNO nanocomposite ferroelectric material in porous glass was
studied by neutron diffraction. For the first time the details of the crystal
structure including positions and anisotropic thermal parameters were
determined for the solid material, embedded in a porous matrix, in ferro- and
paraelectric phases. It is demonstrated that in the ferroelectric phase the
structure is consistent with bulk data but above transition temperature the
giant growth of amplitudes of thermal vibrations is observed, resulting in the
formation of a "premelted state". Such a conclusion is in a good agreement with
the results of dielectric measurements published earlier.Comment: 4 pages, 4 figure
Nouveau regard sur l’intelligence économique au Maroc : les enjeux de la cyber-sécurité dans la nouvelle économie mondiale
L’article analyse comment le Maroc a su utiliser l’Intelligence Economique depuis les années 2000 pour effectuer un rattrapage économique et s’insérer de façon active dans l’économie mondiale. Le travail propose d’étudier également quelles seraient les pistes à explorer pour que le Maroc puisse poursuivre son développement économique en utilisant tous les domaines de l’Intelligence Economique, et en particulier en se concentrant sur l’analyse prospective des informations qui permet de nourrir le processus d’innovation dans l’économie marocaine et de conforter la place que ce pays a su obtenir dans l’économie mondiale grâce à la mobilisation de techniques d’influence, de lobbying et d’image adaptées.
Dans une première partie, nous étudierons les facteurs de la croissance économique et du développement du Maroc en identifiant le rôle joué par les pratiques de veille et d’intelligence économique dans son insertion dans l’économie mondiale. Dans une seconde partie, nous proposerons au Maroc des pistes pour développer une stratégie de création de connaissances et d’innovation qui pourrait compléter de façon utile la stratégie choisie en matière d’Intelligence Economique Stratégique sur les marchés internationaux. La prise en compte simultané du long terme et du court terme devrait permettre de mieux structurer une politique publique d’Intelligence Economique au Maroc dans un schéma plus global permettant au Royaume de mieux se développer à l’intérieur du pays tout en maintenant sa place essentielle au sein de l’économie mondiale
High-Pressure Amorphous Nitrogen
The phase diagram and stability limits of diatomic solid nitrogen have been
explored in a wide pressure--temperature range by several optical spectroscopic
techniques. A newly characterized narrow-gap semiconducting phase has
been found to exist in a range of 80--270 GPa and 10--510 K. The vibrational
and optical properties of the phase produced under these conditions
indicate that it is largely amorphous and back transforms to a new molecular
phase. The band gap of the phase is found to decrease with pressure
indicating possible metallization by band overlap above 280 GPa.Comment: 5 pages, 4 figure
Frequency dependent specific heat of viscous silica
We apply the Mori-Zwanzig projection operator formalism to obtain an
expression for the frequency dependent specific heat c(z) of a liquid. By using
an exact transformation formula due to Lebowitz et al., we derive a relation
between c(z) and K(t), the autocorrelation function of temperature fluctuations
in the microcanonical ensemble. This connection thus allows to determine c(z)
from computer simulations in equilibrium, i.e. without an external
perturbation. By considering the generalization of K(t) to finite wave-vectors,
we derive an expression to determine the thermal conductivity \lambda from such
simulations. We present the results of extensive computer simulations in which
we use the derived relations to determine c(z) over eight decades in frequency,
as well as \lambda. The system investigated is a simple but realistic model for
amorphous silica. We find that at high frequencies the real part of c(z) has
the value of an ideal gas. c'(\omega) increases quickly at those frequencies
which correspond to the vibrational excitations of the system. At low
temperatures c'(\omega) shows a second step. The frequency at which this step
is observed is comparable to the one at which the \alpha-relaxation peak is
observed in the intermediate scattering function. Also the temperature
dependence of the location of this second step is the same as the one of the
peak, thus showing that these quantities are intimately connected to
each other. From c'(\omega) we estimate the temperature dependence of the
vibrational and configurational part of the specific heat. We find that the
static value of c(z) as well as \lambda are in good agreement with experimental
data.Comment: 27 pages of Latex, 8 figure
Structural study of alpha-Bi2O3 under pressure
An experimental and theoretical study of the structural properties of monoclinic bismuth oxide (alpha-(BiO3)-O-2) under high pressures is here reported. Both synthetic and mineral bismite powder samples have been compressed up to 45 GPa and their equations of state have been determined with angle-dispersive x-ray diffraction measurements. Experimental results have been also compared with theoretical calculations which suggest the possibility of several phase transitions below 10 GPa. However, experiments reveal only a pressure-induced amorphization between 15 and 25 GPa, depending on sample quality and deviatoric stresses. The amorphous phase has been followed up to 45 GPa and its nature discussed.Financial support from the Spanish Consolider Ingenio 2010 Program (MALTA Project No. CSD2007-00045) is acknowledged. This work was also supported by Brazilian Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) under project 201050/2012-9, Spanish MICINN under projects MAT2010-21270-C04-01/03/04, Spanish MINECO under project CTQ2012-36253-C03-02, by Generalitat Valenciana through project GVA-ACOMP2013- 012 and from Vicerrectorado de Investigaci on dePereira, ALJ.; Errandonea, D.; Beltrán, A.; Gracia, L.; Gomis Hilario, O.; Sans, JA.; García-Domene, B.... (2013). Structural study of alpha-Bi2O3 under pressure. Journal of Physics: Condensed Matter. 25(47):475402-1-475402-12. https://doi.org/10.1088/0953-8984/25/47/475402S475402-1475402-12254
EUSEDcollab: a network of data from European catchments to monitor net soil erosion by water
As a network of researchers we release an open-access database (EUSEDcollab) of water discharge and suspended sediment yield time series records collected in small to medium sized catchments in Europe. EUSEDcollab is compiled to overcome the scarcity of open-access data at relevant spatial scales for studies on runoff, soil loss by water erosion and sediment delivery. Multi-source measurement data from numerous researchers and institutions were harmonised into a common time series and metadata structure. Data reuse is facilitated through accompanying metadata descriptors providing background technical information for each monitoring station setup. Across ten European countries, EUSEDcollab covers over 1600 catchment years of data from 245 catchments at event (11 catchments), daily (22 catchments) and monthly (212 catchments) temporal resolution, and is unique in its focus on small to medium catchment drainage areas (median = 43 km(2), min = 0.04 km(2), max = 817 km(2)) with applicability for soil erosion research. We release this database with the aim of uniting people, knowledge and data through the European Union Soil Observatory (EUSO)
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