6,454 research outputs found
Collision-Dependent Atom Tunnelling Rate in Bose-Einstein Condensates
We show that the interaction (cross-collision) between atoms trapped in
distinct sites of a double-well potential can significantly increase the atom
tunneling rate for special trap configurations leading to an effective linear
Rabi regime of population oscillation between the trap wells. The inclusion of
cross-collisional effects significantly extends the validity of the two-mode
model approach allowing it to be alternatively employed to explain the recently
observed increase of tunneling rates due to nonlinear interactions.Comment: 4 pages, 2 figures. Replaced with improved versio
Optimal Conditions for Atomic Homodyne Detection on Bose-Einstein Condensates
The dynamics of a two-mode Bose-Einstein condensate trapped in a double-well
potential results approximately in an effective Rabi oscillation regime of
exchange of population between both wells for sufficiently strong overlap
between the modes functions. Facing this system as a temporal atomic beam
splitter we show that this regime is optimal for a nondestructive atom-number
measurement allowing an atomic homodyne detection, thus yielding indirect
relative phase information about one of the two-mode condensates.Comment: 9 pages, 5 figure
Complete high-precision entropic sampling
Monte Carlo simulations using entropic sampling to estimate the number of
configurations of a given energy are a valuable alternative to traditional
methods. We introduce {\it tomographic} entropic sampling, a scheme which uses
multiple studies, starting from different regions of configuration space, to
yield precise estimates of the number of configurations over the {\it full
range} of energies, {\it without} dividing the latter into subsets or windows.
Applied to the Ising model on the square lattice, the method yields the
critical temperature to an accuracy of about 0.01%, and critical exponents to
1% or better. Predictions for systems sizes L=10 - 160, for the temperature of
the specific heat maximum, and of the specific heat at the critical
temperature, are in very close agreement with exact results. For the Ising
model on the simple cubic lattice the critical temperature is given to within
0.003% of the best available estimate; the exponent ratios and
are given to within about 0.4% and 1%, respectively, of the
literature values. In both two and three dimensions, results for the {\it
antiferromagnetic} critical point are fully consistent with those of the
ferromagnetic transition. Application to the lattice gas with nearest-neighbor
exclusion on the square lattice again yields the critical chemical potential
and exponent ratios and to good precision.Comment: For a version with figures go to
http://www.fisica.ufmg.br/~dickman/transfers/preprints/entsamp2.pd
Optimal experimental design for estimating the kinetic parameters of processes described by the Weibull probability distribution function
The optimum experimental design for determining the kinetic parameters of the
model resulting from the Weibull probability density junction was studied, by
defining the sampling conditions that lead to a minimum confidence region of
the estimates, for a number of observations equal to the number of parameters.
It was found that for one single isothermal experiment the optimum sampling
times corresponded always to fractional concentrations that are irrational
numbers (approximately 0.70 and 0.19) whose product is exactly l/e’. The
experimental determination of the equilibtium conversion (for growth kinetics)
is vety important, but in some situations this is not possible, e.g. due to product
degradation over the length of time required. Sampling times leading to a
maximum precision were determined as a function of the maximum conversion
(or yield) attainable. For studies of kinetic parameters over a range of
temperatures, performed with a minimum of three isothermal experiments, it
was proved that the optimum design consists of two experiments at one limit
temperature with two sampling times (those corresponding to fractional
concentrations of approximate[v 0.70 and 0.19) and another at the other limit
temperature for a sampling time such that the fractional concentration is lie.
Case studies are included for clarijication of the concepts and procedures
Optimal experimental design for estimating the kinetic parameters of processes described by the first-order Arrhenius model under linearly increasing temperature profiles
The optimum experimental design for systems following the ®rst-order Arrhenius model under linearly increasing temperature
pro®les was studied by determining the sampling conditions that lead to a minimum con®dence region of the model parameters. It
was found that experiments should be started at the lowest possible temperature and, for each initial temperature, there is an optimal
heating rate. This heating rate is often too high to be feasible, implying that experiments have to be conducted at a lower practicable
heating rate. In this situation the experiments should be replicated in order to improve the precision of the estimates. If both the
initial temperature and the heating rate are ®xed at their optimal values, the optimal sampling times correspond to fractional
concentrations that are irrational numbers (approximately 0.70 and 0.19) whose product is exactly 1=e2, as earlier found for the
Bigelow model. A case study based on the estimation of the kinetic parameters of the acid hydrolysis of sucrose is also presente
Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures
The occurrence of inhomogeneous spin-density distribution in multilayered
ferromagnetic diluted magnetic semiconductor nanostructures leads to strong
dependence of the spin-polarized transport properties on these systems. The
spin-dependent mobility, conductivity and resistivity in
(Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a
function of temperature, scaled by the average magnetization of the diluted
magnetic semiconductor layers. An increase of the resistivity near the
transition temperature is obtained. We observed that the spin-polarized
transport properties changes strongly among the three materials.Comment: 3 pages, 4 figure
A semiquantitative approach to the impurity-band-related transport properties of GaMnAs nanolayers
We investigate the spin-polarized transport of GaMnAs nanolayers in which a
ferromagnetic order exists below a certain transition temperature. Our
calculation for the self-averaged resistivity takes into account the existence
of an impurity band determining the extended ("metallic" transport) or
localized (hopping by thermal excitation) nature of the states at and near the
Fermi level. Magnetic order and resistivity are inter-related due to the
influence of the spin polarization of the impurity band and the effect of the
Zeeman splitting on the mobility edge. We obtain, for a given range of Mn
concentration and carrier density, a "metallic" behavior in which the transport
by extended carriers dominates at low temperature, and is dominated by the
thermally excited localized carriers near and above the transition temperature.
This gives rise to a conspicuous hump of the resistivity which has been
experimentally observed and brings light onto the relationship between
transport and magnetic properties of this material
Application of D-optimal design for determination of the influence of water content on the thermal degradation kinetics of ascorbic acid at low water contents
The kinetics of thermal degradation of ascorbic acid was studied in a
Maltodextrin matrix at different temperatures between 7.5 and 140°C. Isothermal
experiments were performed with samples earlier equilibrated in environments of
known a, at 4°C enclosed in hermetically sealed vials, with water contents (w)
between 0.17 and 168g water/g solids. Sampling times were selected according
to an optimal experimental design in terms of minimum confidence regions of
the parameters estimated, using the Bigelow model and estimates from
preliminary experiments. The results showed a quadratic dependence of the
reference D-value and linear dependence of the z-value with moisture content:
D r(1400C, = 150*07-290*73w+-269w2 (min); z = 12*14+22.99w (“C) and
showed the applicability of D-optimal designs for determining kinetic
parameters in complex situations, with limited experimental requirements. The
dependence of the rate constants with water content could not be described by
the WLF model in qualitative terms, although in absolute values this model
could be used with constants similar to those expected from glass transition
theory. The thermodynamic analysis of the results showed a good application of
the compensation theory in the whole range of water contents
Fundaments of the ethical acting in university students
It is urgent that the superior Education in the health area develops in the students a pro-active ethical commitment which is translated into the creation of socio-professional and rational and clarified socio-cultural values.info:eu-repo/semantics/publishedVersio
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