4,230 research outputs found
Induced Anticlinic Ordering and Nanophase Segregation of Bow-Shaped Molecules in a Smectic Solvent
Recent experiments indicate that doping low concentrations of bent-core
molecules into calamitic smectic solvents can induce anticlinic and biaxial
smectic phases. We have carried out Monte Carlo (MC) simulations of mixtures of
rodlike molecules (hard spherocylinders with length/breadth ratio ) and bow- or banana-shaped molecules (hard spherocylinder dimers
with length/breadth ratio or 2.5 and opening angle ) to
probe the molecular-scale organization and phase behavior of rod/banana
mixtures. We find that a low concentration (3%) of dimers
induces anticlinic (SmC) ordering in an untilted smectic (SmA) phase for
. For smaller , half of each bow-shaped
molecule is nanophase segregated between smectic layers, and the smectic layers
are untilted. For , no tilted phases are induced. However,
with decreasing we observe a sharp transition from {\sl intralamellar}
nanophase segregation (bow-shaped molecules segregated within smectic layers)
to {\sl interlamellar} nanophase segregation (bow-shaped molecules concentrated
between smectic layers) near . These results demonstrate that
purely entropic effects can lead to surprisingly complex behavior in rod/banana
mixtures.Comment: 5 pages Revtex, 7 postscript figure
Influence of electron correlations on ground-state properties of III-V semiconductors
Lattice constants and bulk moduli of eleven cubic III-V semiconductors are
calculated using an ab initio scheme. Correlation contributions of the valence
electrons, in particular, are determined using increments for localized bonds
and for pairs and triples of such bonds; individual increments, in turn, are
evaluated using the coupled cluster approach with single and double
excitations. Core-valence correlation is taken into account by means of a core
polarization potential. Combining the results at the correlated level with
corresponding Hartree-Fock data, we obtain lattice constants which agree with
experiment within an average error of -0.2%; bulk moduli are accurate to +4%.
We discuss in detail the influence of the various correlation contributions on
lattice constants and bulk moduli.Comment: 4 pages, Latex, no figures, Phys. Rev. B, accepte
Clustering transitions in vibro-fluidized magnetized granular materials
We study the effects of long range interactions on the phases observed in
cohesive granular materials. At high vibration amplitudes, a gas of magnetized
particles is observed with velocity distributions similar to non-magnetized
particles. Below a transition temperature compact clusters are observed to form
and coexist with single particles. The cluster growth rate is consistent with a
classical nucleation process. However, the temperature of the particles in the
clusters is significantly lower than the surrounding gas, indicating a
breakdown of equipartition. If the system is quenched to low temperatures, a
meta-stable network of connected chains self-assemble due to the anisotropic
nature of magnetic interactions between particles.Comment: 4 pages, 5 figure
Updated tests of scaling and universality for the spin-spin correlations in the 2D and 3D spin-S Ising models using high-temperature expansions
We have extended, from order 12 through order 25, the high-temperature series
expansions (in zero magnetic field) for the spin-spin correlations of the
spin-S Ising models on the square, simple-cubic and body-centered-cubic
lattices. On the basis of this large set of data, we confirm accurately the
validity of the scaling and universality hypotheses by resuming several tests
which involve the correlation function, its moments and the exponential or the
second-moment correlation-lengths.Comment: 21 pages, 8 figure
Foreign Investment in Mexico from the Perspective of the Foreign Investor.
A factor contributing to the continuation of the economic revolution in Mexico has been, and will continue to be, foreign investment. The liberalized foreign-investment regulations and the positive attitude of the Foreign Investment Commission (FIC) in approving foreign investment proposals promote a more favorable environment for foreign investors. The Mexican government recently completed negotiating the NAFTA, a proposed free-trade agreement with the United States and Canada. The government is now considering what additional actions may be required to compete successfully with those other nations trying to attract scarce investment funds. Opportunities for foreign investors in Mexico are brighter than they have been in fifty years. Nevertheless, investors will make better decisions by being mindful of the substantial cultural and legal differences between the two countries, as well as the history of foreign-investment regulation in Mexico. This is true in negotiating the establishment of investments, joint ventures, and licensing agreements. With respect to the history of the FIL Regulation, an understanding of these directives is fundamental to predicting what the future may hold in this important area of human activity. Most foreign investors look forward to the day when investment policies in Mexico will be more transparent. Mr. Jaime Serra, the Minister of Trade and Industrial Development, announced the FIL and the FIL Regulations will be amended in order to adopt the NAFTA. These amendments, Mexico’s liberalization of foreign investment controls, and commitments Mexico undertook in the NAFTA, move Mexico closer to a market driven and controlled by economic forces rather than government edict
Renormalized couplings and scaling correction amplitudes in the N-vector spin models on the sc and the bcc lattices
For the classical N-vector model, with arbitrary N, we have computed through
order \beta^{17} the high temperature expansions of the second field derivative
of the susceptibility \chi_4(N,\beta) on the simple cubic and on the body
centered cubic lattices. (The N-vector model is also known as the O(N)
symmetric classical spin Heisenberg model or, in quantum field theory, as the
lattice
O(N) nonlinear sigma model.) By analyzing the expansion of \chi_4(N,\beta) on
the two lattices, and by carefully allowing for the corrections to scaling, we
obtain updated estimates of the critical parameters and more accurate tests of
the hyperscaling relation d\nu(N) +\gamma(N) -2\Delta_4(N)=0 for a range of
values of the spin dimensionality N, including
N=0 [the self-avoiding walk model], N=1 [the Ising spin 1/2 model],
N=2 [the XY model], N=3 [the classical Heisenberg model]. Using the recently
extended series for the susceptibility and for the second correlation moment,
we also compute the dimensionless renormalized four point coupling constants
and some universal ratios of scaling correction amplitudes in fair agreement
with recent renormalization group estimates.Comment: 23 pages, latex, no figure
Automated monitoring of urination events from grazing cattle
AbstractUrine patches deposited by grazing cattle represent ‘hot-spots’ of very high nitrogen (N) loading from which environmentally important losses of N may occur (ammonia and nitrous oxide emissions, nitrate leaching). Information on the quantities of N deposited to grazed pastures as urine, the spatial and temporal distribution of urine patches and how these may be influenced by pasture management practices is limited. The objectives of this study were to assess the potential of recently developed urine sensors for providing data on urination behaviour by grazing cattle and relate this to measurements of ammonia emissions from the grazed paddocks. A total of six trials were conducted across two sites; two on a 1ha paddock at Easter Bush near Edinburgh using beef cattle (c. 630kg live weight) and four on a 0.5ha paddock at North Wyke in Devon using in-calf dairy heifers (c. 450kg live weight). Laboratory calibrations were conducted to provide sensor-specific functions for urine volume and N concentration. The quantity and quality of data from the urine sensors improved with successive trials through modifications to the method of attachment to the cow. The number of urination events per animal per day was greater for the dairy heifers, with a mean value of 11.6 (se 0.70) compared with 7.6 (se 0.76) for the beef cattle. Volume per urination event (mean 1.8, range 0.4–6.4L) and urine N concentration (range 0.6–31.5gL−1, excluding outliers) were similar for the two groups of cattle. Ammonia emission measurements were unsuccessful in most of the trials. The urine sensors have potential to provide useful information on urine N deposition by grazing cattle but suggested improvements including making the sensors lighter, designing a better method of attachment to the cow and including a more reliable location sensor
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