2,193 research outputs found
Spatial Compactification and Decay-Rate Behavior
The pahse transition from instanton-dominated quantum tunneling regime to
sphaleron-dominated classical crossover regime is explored in (1+1)-dimensional
scalar field theory when spatial coordinate is compactified. It is shown that
the type of sphaleron transition is critically dependent on the circumference
of the spatial coordinate.Comment: 12 pages, 6 postscript figures included, accepted version in PL
Winding Number Transitions in the Mottola-Wipf Model on a Circle
Winding number transitions from quantum to classical behavior are studied in
the case of the {1+1} dimensional Mottola-Wipf model with the space coordinate
on a circle for exploring the possibility of obtaining transitions of second
order. The model is also studied as a prototype theory which demonstrates the
procedure of such investigations. In the model at hand we find that even on a
circle the transitions remain those of first order.Comment: 17pages + 5 ps figures, final version to appear in Nucl. Phys.
Nucleation at finite temperature beyond the superminispace model
The transition from the quantum to the classical regime of the nucleation of
the closed Robertson-Walker Universe with spacially homogeneous matter fields
is investigated with a perturbation expansion around the sphaleron
configuration. A criterion is derived for the occurrence of a first-order type
transition, and the related phase diagram for scalar and vector fields is
obtained. For scalar fields both the first and second order transitions can
occur depending on the shape of the potential barrier. For a vector field, here
that of an O(3) nonlinear -model, the transition is seen to be only of
the first order.Comment: 15 pages, 3 figure
Hidden functional relation in Large-N Quark-Monopole system at finite temperature
The quark-monopole potential is computed at finite temperature in the context
of correspondence. It is found that the potential is invariant under
and . As in the quark-quark case there exists a
maximum separation between quark and monopole, and -dependence of the
potential exhibits a bifurcation behavior. We find a functional relation
which is responsible for the bifurcation.
The remarkable property of this relation is that it makes a relation between
physical quantities defined at the boundary through a quantity defined at
the bulk. The physical implication of this relation for the existence of the
extra dimension is speculated.Comment: 22 pages, 3 figures, v1 one more reference added v2 version to appear
in NP
Fermi Edge Singularities and Backscattering in a Weakly Interacting 1D Electron Gas
The photon-absorption edge in a weakly interacting one-dimensional electron
gas is studied, treating backscattering of conduction electrons from the core
hole exactly. Close to threshold, there is a power-law singularity in the
absorption, , with where is the forward scattering
phase shift of the core hole. In contrast to previous theories, is
finite (and universal) in the limit of weak core hole potential. In the case of
weak backscattering , the exponent in the power-law dependence of
absorption on energy crosses over to a value above an energy scale , where is a dimensionless measure of the
electron-electron interactions.Comment: 8 pages + 1 postscript figure, preprint TPI-MINN-93/40-
3D chemical characterization of frozen hydrated hydrogels using ToF-SIMS with argon cluster sputter depth profiling
Hydrogels have been used extensively in bioengineering as artificial cell culture supports. Investigation of the interrelationship between cellular response to the hydrogel and its chemistry ideally requires methods that allow characterization without labels and can map species in three dimensional to follow biomolecules adsorbed to, and absorbed into, the open structure before and during culture. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has the potential to be utilized for through thickness characterization of hydrogels. The authors have established a simple sample preparation procedure to successfully achieve analysis of frozen hydrated hydrogels using ToF-SIMS without the need for dry glove box entry equipment. They demonstrate this on a poly(2-hydroxyethyl methacrylate) (pHEMA) film where a model protein (lysozyme) is incorporated using two methods to demonstrate how protein distribution can be determined. A comparison of lysozyme incorporation is made between the situation where the protein is present in a polymer dip coating solution and where lysozyme is in an aqueous medium in which the film is incubated. It is shown that protonated water clusters H(H2O)nĂŸ where n ÂŒ 5â11 that are indicative of ice are detected through the entire thickness of the pHEMA. The lysozyme distribution through the pHEMA hydrogel films can be determined using the intensity of a characteristic amino acid secondary ion fragment
MRI size assessment of cerebral microvasculature using diffusion-time-dependent stimulated-echo acquisition: A feasibility study in rodent
In this study, a stimulated-echo (STE) method was employed to robustify the cerebral vessel size estimation nearair-tissue, bone-tissue interfaces, and large vessels. The proposed solution is to replace the relaxation rate changefrom gradient-echo (GRE) with that from STE with long diffusion time after the injection of an intravascularcontrast agent, superparamagnetic iron oxide nanoparticles. The corresponding diffusion length of STE is shorterthan the length over which the unwanted macroscopicfield inhomogeneities but is still longer than the corre-lation length of thefields induced by small vessels. Therefore, the unwantedfield inhomogeneities are refocused,while preserving microscopic susceptibility contrast from cerebral vessels. The mean vessel diameter (dimen-sionless) derived from the diffusion-time-varying STE method was compared to the mean vessel diameter ob-tained by a conventional spin-echo (SE) and GRE combination based on Monte-Carlo proton diffusion simulationsand in vivo rat experiments at 7 T. The in vivo mean vessel diameter from the MRI experiments was directlycompared to available reference mouse brain vasculature obtained by a knife-edge scanning microscope (KESM),which is considered to be the gold standard. Monte-Carlo simulation revealed that SE and GRE-based MRrelaxation rate changes (??R2and??R2*, respectively) can be enhanced using single STE-based MR relaxation ratechange (??RSTE) by regulating diffusion time, especially for small vessels. The in vivo mean vessel diameter fromthe STE method demonstrated a closer agreement with that from the KESM compared to the combined SE andGRE method, especially in the olfactory bulb and cortex. This study demonstrates that STE relaxation rate changescan be used as consistent measures for assessing small cerebral microvasculature, where macroscopicfield in-homogeneity is severe and signal contamination from adjacent large vessels is significant
Conductance Fluctuations in PbTe Wide Parabolic Quantum Wells
We report on conductance fluctuations which are observed in local and
non-local magnetotransport experiments. Although the Hall bar samples are of
macroscopic size, the amplitude of the fluctuations from the local measurements
is close to e^2/h. It is shown that the fluctuations have to be attributed to
edge channel effects.Comment: postscript file including 3 figs, 3 pages, Paper presented at 3rd
Int. Symposium on "New Phenomena in Mesoscopic Structures" in Maui, Hawaii
199
Control Theory Forecasts of Optimal Training Dosage to Facilitate Childrenâs Arithmetic Learning in a Digital Educational Application
Education can be viewed as a control theory problem in which students seek ongoing exogenous inputâeither through traditional classroom teaching or other alternative training resourcesâto minimize the discrepancies between their actual and target (reference) performance levels. Using illustrative data from n= 784 Dutch elementary school students as measured using the Math Garden, a web-based computer adaptive practice and monitoring system, we simulate and evaluate the outcomes of using off-line and finite memory linear quadratic controllers with constraints to forecast studentsâ optimal training durations. By integrating population standards with each studentâs own latent change information, we demonstrate that adoption of the control theory-guided, person- and time-specific training dosages could yield increased training benefits at reduced costs compared to studentsâ actual observed training durations, and a fixed-duration training scheme. The control theory approach also outperforms a linear scheme that provides training recommendations based on observed scores under noisy and the presence of missing data. Design-related issues such as ways to determine the penalty cost of input administration and the size of the control horizon window are addressed through a series of illustrative and empirically (Math Garden) motivated simulations
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