268 research outputs found
Casimir piston for massless scalar fields in three dimensions
We study the Casimir piston for massless scalar fields obeying Dirichlet
boundary conditions in a three dimensional cavity with sides of arbitrary
lengths and where is the plate separation. We obtain an exact
expression for the Casimir force on the piston valid for any values of the
three lengths. As in the electromagnetic case with perfect conductor
conditions, we find that the Casimir force is negative (attractive) regardless
of the values of , and . Though cases exist where the interior
contributes a positive (repulsive) Casimir force, the total Casimir force on
the piston is negative when the exterior contribution is included. We also
obtain an alternative expression for the Casimir force that is useful
computationally when the plate separation is large.Comment: 19 pages,3 figures; references updated and typos fixed to match
published versio
Competitive Priorities and Competitive Advantage in Jordanian Manufacturing
The purpose of this research was to explore and predict the relationship between the competitive priorities (quality, cost, flexibility and delivery) and the competitive advantage of firms in the Jordanian Industrial Sector. A population of 88 Jordanian manufacturing firms, registered on the Amman Stock Exchange, was targeted using a cross-sectional survey employing a questionnaire method of data collection. The results of the data analysis indicate a significant relationship between competitive priorities and competitive advantage. The research suggests that recognising and nurturing this relationship provides the master key for a firm to survive in a turbulent environment. Therefore, operational and marketing strategies should place emphasis on competitive priorities such as quality, cost, flexibility and delivery to achieve, develop and maintain competitive advantage. This study is one of the first to examine the relationship between the competitive priorities of Jordanian manufacturing firms and their competitive advantage
Self-Duality in Superconductor-Insulator Quantum Phase Transitions
It is argued that close to a Coulomb interacting quantum critical point, the
interaction between two vortices in a disordered superconducting thin film
separated by a distance changes from logarithmic in the mean-field region
to in the region dominated by quantum critical fluctuations. This gives
support to the charge-vortex duality picture of the observed reflection
symmetry in the current-voltage characteristics on both sides of the
transition.Comment: 4 pages, no figures, 2nd version: title (slightly) changed and text
accordingl
Preliminary investigations on sunburn in Chardonnay grapevine variety
The aim of this investigation was to determine if a temperature response curve can be used to describe sunburn in grape berries. Trials were carried out at the Viticulture and Enology Department (University of California, Davis) on cv Chardonnay (clone 29) grown under both field and greenhouse conditions.
Greenhouse plants were two years old, grown in 5 L pots, and watered daily with a modified Hoagland’s nutrient solution. The vines were pruned to two shoots with one or two clusters per shoot, and the shoots were vertically trained to approximately 1.5 m. Field-grown vines were clone 29 grafted onto 101-14 rootstock, planted at 2.5 x 3.7 m spacing, cane pruned, and VSP trained. Rows were north-south oriented.
In order to increase the temperature of the berry surface, solar radiation was concentrated using a normal reading lens with different magnifications degrees. Temperature was measured with a copper-constantan thermocouples attached to the berry surface. Experiments were performed just before harvest. Sunburn was caused by using different ranges of temperatures held constant for 2 or 5 minutes in the case of greenhouse plants and 5, 10, and 15 minutes in the case of field-grown plants. The effects of treatments were rated on visual basis by a panel of 3 people at one day intervals for three or four consecutive days after the treatments. On the last day, treated berries were harvested and analyzed for cell viability and membrane integrity using the fluorescein diacetate (FDA) technique.
In greenhouse grown vines, a temperature of 38-40 °C for 5 minutes was sufficient to cause visual symptoms of sunburn two days after the treatments, even if no cells were permanently damaged. In field-grown vines, 5 minutes at 40-43 °C caused 12.4% cell mortality and permanent surface deformation.
In conclusion, exposure of berries to a surface temperature of 40-43 °C appears to be effective in causing sunburn in greenhouse and field-grown plants. The radiation regime experienced by the cluster during the growing season may be important to determine the critical level of temperature causing sunburn
Induced quantum numbers in the (2+1)-dimensional electron gas
A gas of electrons confined to a plane is examined in both the relativistic
and nonrelativistic case. Using a (0+1)-dimensional effective theory, a
remarkably simple method is proposed to calculate the spin density induced by
an uniform magnetic background field. The physical properties of possible
fluxon excitations are determined. It is found that while in the relativistic
case they can be considered as half-fermions (semions) in that they carry half
a fermion charge and half the spin of a fermion, in the nonrelativistic case
they should be thought of as fermions, having the charge and spin of a fermion.Comment: 19 pages, REVTE
Superconductor-insulator transition driven by local dephasing
We consider a system where localized bound electron pairs form an array of
"Andreev"-like scattering centers and are coupled to a fermionic subsystem of
uncorrelated electrons. By means of a path-integral approach, which describes
the bound electron pairs within a coherent pseudospin representation, we derive
and analyze the effective action for the collective phase modes which arise
from the coupling between the two subsystems once the fermionic degrees of
freedom are integrated out. This effective action has features of a quantum
phase model in the presence of a Berry phase term and exhibits a coupling to a
field which describes at the same time the fluctuations of density of the bound
pairs and those of the amplitude of the fermion pairs. Due to the competition
between the local and the hopping induced non-local phase dynamics it is
possible, by tuning the exchange coupling or the density of the bound pairs, to
trigger a transition from a phase ordered superconducting to a phase disordered
insulating state. We discuss the different mechanisms which control this
occurrence and the eventual destruction of phase coherence both in the weak and
strong coupling limit.Comment: 16 pages, 5 figures, submitted to PRB (05-Feb04
The Evolution of roommate networks : a comment on Jackson and Watts JET (2002)
We extend Jackson and Watts's (2002) result on the coincidence of S-stochastically stable and core stable networks from marriage problems to roommate problems. In particular, we show that the existence of a side-optimal core stable network, on which the proof of Jackson and Watts (2002) hinges, is not crucial for their result
La investigaciĂłn activa como herramienta para mejorar la enseñanza de la quĂmica : nuevos enfoques
In many European countries, chemistry education faces a number of important recurrent difficulties. For example, at the secondary school level, students' interest in chemistry is decreasing and teachers complain that repeated explanation and demonstration are not very effective, which frustrates them. At university level, lecture courses and laboratory courses are often disconnected and many student experiments are considered as boring «cookbook» problems. These and other recurrent difficulties have evoked the need to improve the quality of chemistry education at both levels. In this article, the contribution of educational research to an improvement of chemistry education is discussed. This research is considered as a tool for analysing teaching and learning problems, for elucidating backgrounds of these difficulties and for developing and evaluating new approaches in chemistry education. Special attention is focused on two promising new research approaches. First, a new research instrument is presented: protocols of discussions during real classroom and laboratory sessions. Second, a new research design is presented: the developmental research approach. Both research approaches can contribute to the improvement of chemistry classroom/laboratory practices and chemistry teacher training
Fractal Structure of Spin Clusters and Domain Walls in 2D Ising Model
The fractal structure of spin clusters and their boundaries in the critical
two-dimensional (2D) Ising model is investigated numerically. The fractal
dimensions of these geometrical objects are estimated by means of Monte Carlo
simulations on relatively small lattices through standard finite-size scaling.
The obtained results are in excellent agreement with theoretical predictions
and partly provide significant improvements in precision over existing
numerical estimates.Comment: 8 pages, 8 figures; v2: minor changes in text, various plots are put
in one figur
Geometric invariant theory approach to the determination of ground states of D-wave condensates in isotropic space
A complete and rigorous determination of the possible ground states for
D-wave pairing Bose condensates is presented, using a geometrical invariant
theory approach to the problem. The order parameter is argued to be a vector,
transforming according to a ten dimensional real representation of the group
{\bf O}{\bf U}. We determine the equalities
and inequalities defining the orbit space of this linear group and its symmetry
strata, which are in a one-to-one correspondence with the possible distinct
phases of the system. We find 15 allowed phases (besides the unbroken one),
with different symmetries, that we thoroughly determine. The group-subgroup
relations between bordering phases are pointed out. The perturbative sixth
degree corrections to the minimum of a fourth degree polynomial -invariant
free energy, calculated by Mermin, are also determined.Comment: 27 revtex pages, 2 figures, use of texdraw; minor changes in the
bibliography and in Table II
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