825 research outputs found
Revisiting waterlike network-forming lattice models
In a previous paper [J. Chem. Phys. 129, 024506 (2008)] we studied a 3
dimensional lattice model of a network-forming fluid, recently proposed in
order to investigate water anomalies. Our semi-analytical calculation, based on
a cluster-variation technique, turned out to reproduce almost quantitatively
several Monte Carlo results and allowed us to clarify the structure of the
phase diagram, including different kinds of orientationally ordered phases.
Here, we extend the calculation to different parameter values and to other
similar models, known in the literature. We observe that analogous ordered
phases occur in all these models. Moreover, we show that certain "waterlike"
thermodynamic anomalies, claimed by previous studies, are indeed artifacts of a
homogeneity assumption made in the analytical treatment. We argue that such a
difficulty is common to a whole class of lattice models for water, and suggest
a possible way to overcome the problem.Comment: 13 pages, 12 figure
Cluster-variation approximation for a network-forming lattice-fluid model
We consider a 3-dimensional lattice model of a network-forming fluid, which
has been recently investigated by Girardi and coworkers by means of Monte Carlo
simulations [J. Chem. Phys. \textbf{126}, 064503 (2007)], with the aim of
describing water anomalies. We develop an approximate semi-analytical
calculation, based on a cluster-variation technique, which turns out to
reproduce almost quantitatively different thermodynamic properties and phase
transitions determined by the Monte Carlo method. Nevertheless, our calculation
points out the existence of two different phases characterized by long-range
orientational order, and of critical transitions between them and to a
high-temperature orientationally-disordered phase. Also, the existence of such
critical lines allows us to explain certain ``kinks'' in the isotherms and
isobars determined by the Monte Carlo analysis. The picture of the phase
diagram becomes much more complex and richer, though unfortunately less
suitable to describe real water.Comment: 10 pages, 9 figures, submitted to J. Chem. Phy
Hydration of an apolar solute in a two-dimensional waterlike lattice fluid
In a previous work, we investigated a two-dimensional lattice-fluid model,
displaying some waterlike thermodynamic anomalies. The model, defined on a
triangular lattice, is now extended to aqueous solutions with apolar species.
Water molecules are of the "Mercedes Benz" type, i.e., they possess a D3
(equilateral triangle) symmetry, with three equivalent bonding arms. Bond
formation depends both on orientation and local density. The insertion of inert
molecules displays typical signatures of hydrophobic hydration: large positive
transfer free energy, large negative transfer entropy (at low temperature),
strong temperature dependence of the transfer enthalpy and entropy, i.e., large
(positive) transfer heat capacity. Model properties are derived by a
generalized first order approximation on a triangle cluster.Comment: 9 pages, 5 figures, 1 table; submitted to Phys. Rev.
Perturbative power Q^2-corrections to the structure function g(1)
We prove that regulating infrared divergencies generates power (~1/(Q^2)^k)
corrections to the spin structure function g_1 at small x and large Q^2. At the
same time it leads to the corrections ~(Q^2)^k at small Q^2. We present the
explicit series of such terms as well as the formulae for their resummation.
These contributions are not included in the standard analysis of the
experimental data. We argue that accounting for such terms can sizably change
the impact of the other power corrections conventionally attributed to the
higher twists.Comment: Theoretical grounds for our approach are considered in much more
detailed way than in the previous version; 10 pages, 2 figure
The gluon content of the and mesons and the , electromagnetic transition form factors
We compute power-suppressed corrections to the \eta\gamma and
\eta^{\prime}\gamma transition form factors
Q^2F_{\eta(\eta^{\prime})\gamma}(Q^2) arising from the end point regions x \to
0,1 by employing the infrared-renormalon approach. The contribution to the form
factors from the quark and gluon content of the \eta,\eta^{\prime} mesons is
taken into account using for the \eta-\eta^{\prime} mixing the SU_f(3) singlet
\eta_1 and octet \eta_8 basis. The theoretical predictions obtained this way
are compared with the corresponding CLEO data and restrictions on the input
parameters (Gegenbauer coefficients) B_2^q(\eta_1), B_2^g(\eta_1), and
B_2^q(\eta_8) in the distribution amplitudes for the \eta_1,\eta_8 states with
one nonasymptotic term are deduced. Comparison is made with the results from
QCD perturbation theory.Comment: 25 pages, RevTeX4 used. 9 figures as EPS files. Text significantly
changed to include variation of theoretical parameters. Figures modified.
Corrected typo in equation (34) and trivial mistake in -coefficient.
References added. Conclusions unchange
Effects of genetic and agronomic factors on grain composition in oats
The effects of genetic variability and agronomic practises on the chemical compounds of the grain were evaluated in a set of oat cultivars (16 naked and four husked) grown in different experiments in the years 2004–2008. In the first experiment, carried out for two years, β-glucan and Total Dietary Fibre (TDF) from eight oat cultivars, grown under three different levels of nitrogen fertilization and two seed doses, were considered. The second experiment, involving 12 cultivars for two years, explored the variation of β-glucan solubility and further characterized six cultivars for the content of TDF and arabinoxylans. In both experiments, genotype was found to exert the largest effect on the grain composition; nitrogen levels and seed doses had positive significant effects on protein and β-glucan contents, but did not affect fibre content. Among the naked cultivars, Irina, Abel, Luna, Hendon and Expression showed a good ability to accumulate the examined grain compounds. However, in general, the highest contents of protein and β-glucan were found in the groats of husked cultivars, suggesting that specific breeding programs are a crucial step to identify the suitable naked oat genotypes to produce foods of high nutritional value
Baryon wave function in large-Nc QCD: Universality, nonlinear evolution equation and asymptotic limit
The 1/Nc expansion is formulated for the baryon wave function in terms of a
specially constructed generating functional. The leading order of this 1/Nc
expansion is universal for all low-lying baryons [including the O(1/Nc) and
O(Nc^0) excited resonances] and for baryon-meson scattering states. A nonlinear
evolution equation of Hamilton-Jacobi type is derived for the generating
functional describing the baryon distribution amplitude in the large-Nc limit.
In the asymptotic regime this nonlinear equation is solved analytically. The
anomalous dimensions of the leading-twist baryon operators diagonalizing the
evolution are computed analytically up to the next-to-leading order of the 1/Nc
expansion.Comment: 44 page
On the Nucleon Distribution Amplitude: The Heterotic Solution
We present a new nucleon distribution amplitude which amalgamates features of
the Chernyak-Ogloblin-Zhitnitsky model with those of the Gari-Stefanis model.
This "heterotic" solution provides the possibility to have asymptotically a
small ratio \hbox{}, while fulfilling
most of the sum-rule requirements up to the third order. Using this nucleon
distribution amplitude we calculate the electromagnetic and weak nucleon form
factors, the transition form factor and the decay widths
of the charmonium levels , , and into .
The agreement with the available data is remarkable in all cases.Comment: 15 pages, RUB-TPII-21/92 Preprin
Endpoint behavior of the pion distribution amplitude in QCD sum rules with nonlocal condensates
Starting from the QCD sum rules with nonlocal condensates for the pion
distribution amplitude, we derive another sum rule for its derivative and its
"integral" derivatives---defined in this work. We use this new sum rule to
analyze the fine details of the pion distribution amplitude in the endpoint
region . The results for endpoint-suppressed and flat-top (or
flat-like) pion distribution amplitudes are compared with those we obtained
with differential sum rules by employing two different models for the
distribution of vacuum-quark virtualities. We determine the range of values of
the derivatives of the pion distribution amplitude and show that
endpoint-suppressed distribution amplitudes lie within this range, while those
with endpoint enhancement---flat-type or CZ-like---yield values outside this
range.Comment: 20 pages, 10 figures, 1 table, conclusions update
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