27,253 research outputs found
Why the lowest Landau level approximation works in strongly type II superconductors
Higher than the lowest Landau level contributions to magnetization and
specific heat of superconductors are calculated using Ginzburg - Landau
equations approach. Corrections to the excitation spectrum around solution of
these equations (treated perturbatively) are found. Due to symmetries of the
problem leading to numerous cancellations the range of validity of the LLL
approximation in mean field is much wider then a naive range and extends all
the way down to . Moreover the contribution of higher
Landau levels is significantly smaller compared to LLL than expected naively.
We show that like the LLL part the lattice excitation spectrum at small
quasimomenta is softer than that of usual acoustic phonons. This enhanses the
effect of fluctuations. The mean field calculation extends to third order,
while the fluctuation contribution due to HLL is to one loop. This complements
the earlier calculation of the LLL part to two loop order.Comment: 20 pages, Latex file, three figure
One-dimensional Bose chemistry: effects of non-integrability
Three-body collisions of ultracold identical Bose atoms under tight
cylindrical confinement are analyzed. A Feshbach resonance in two-body
collisions is described by a two-channel zero-range interaction. Elimination of
the closed channel in the three-body problem reduces the interaction to a
one-channel zero-range one with an energy dependent strength. The related
problem with an energy independent strength (the Lieb-Liniger-McGuire model)
has an exact solution and forbids all chemical processes, such as three-atom
association and diatom dissociation, as well as reflection in atom-diatom
collisions. The resonant case is analyzed by a numerical solution of the
Faddeev-Lovelace equations. The results demonstrate that as the internal
symmetry of the Lieb-Liniger-McGuire model is lifted, the reflection and
chemical reactions become allowed and may be observed in experiments.Comment: 5 pages, 4 figure
Resonance ionization spectroscopy of thorium isotopes - towards a laser spectroscopic identification of the low-lying 7.6 eV isomer of Th-229
In-source resonance ionization spectroscopy was used to identify an efficient
and selective three step excitation/ionization scheme of thorium, suitable for
titanium:sapphire (Ti:sa) lasers. The measurements were carried out in
preparation of laser spectroscopic investigations for an identification of the
low-lying Th-229m isomer predicted at 7.6 +- 0.5 eV above the nuclear ground
state. Using a sample of Th-232, a multitude of optical transitions leading to
over 20 previously unknown intermediate states of even parity as well as
numerous high-lying odd parity auto-ionizing states were identified. Level
energies were determined with an accuracy of 0.06 cm-1 for intermediate and
0.15 cm-1 for auto-ionizing states. Using different excitation pathways an
assignment of total angular momenta for several energy levels was possible. One
particularly efficient ionization scheme of thorium, exhibiting saturation in
all three optical transitions, was studied in detail. For all three levels in
this scheme, the isotope shifts of the isotopes Th-228, Th-229, and Th-230
relative to Th-232 were measured. An overall efficiency including ionization,
transport and detection of 0.6 was determined, which was predominantly limited
by the transmission of the mass spectrometer ion optics
Sphaleron Transition Rate in Presence of Dynamical Fermions
We investigate the effect of dynamical fermions on the sphaleron transition
rate at finite temperature for the Abelian Higgs model in one spatial
dimension. The fermion degrees of freedom are included through bosonization.
Using a numerical simulation, we find that massless fermions do not change the
rate within the measurement accuracy. Surprisingly, the exponential dependence
of the sphaleron energy on the Yukawa coupling is not borne out by the
transition rate, which shows a very weak dependence on the fermion mass.Comment: 20 pages, 7 figures, LaTeX, psfi
The Case for a Heat-Pipe Phase of Planet Evolution on the Moon
The prevalence of anorthosite in the lunar highlands is generally attributed to the flotation of less dense plagioclase in the late stages of the solidification of the lunar magma ocean. It is not clear, however, that these models are capable of producing the extremely high plagioclase contents (near 100%) observed in both Apollo samples and remote sensing data, since a mostly solid lithosphere forms (at 60-70% solidification) before plagioclase feldspar reaches saturation (at approximately 80% solidification). Formation as a floating cumulate is made even more problematic by the near uniformity of the alkali composition of the plagioclase, even as the mafic phases record significant variations in Mg/(Mg+Fe) ratios. These problems can be resolved for the Moon if the plagioclase-rich crust is produced and refined through a widespread episode of heat-pipe magmatism rather than a process dominated by density-driven plagioclase flotation. Heat-pipes are an important feature of terrestrial planets at high heat flow, as illustrated by Io's present activity. Evidence for their operation early in Earth's history suggests that all terrestrial bodies should experience an early episode of heat-pipe cooling. As the Moon likely represents the most wellpreserved example of early planetary thermal evolution in our solar system, studies of the lunar surface and of lunar materials provide useful data to test the idea of a universal model of the way terrestrial bodies transition from a magma ocean state into subsequent single-plate, rigid-lid convection or plate tectonic phases
First principles theory of fluctuations in vortex liquids and solids
Consistent perturbation theory for thermodynamical quantities in type II
superconductors in magnetic field at low temperatures is developed. It is
complementary to the existing expansion valid at high temperatures.
Magnetization and specific heat are calculated to two loop order and compare
well to existing Monte Carlo simulations and experiments.Comment: 3 .ps fig. In press Phys. Rev.
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Explanatory debugging: Supporting end-user debugging of machine-learned programs
Many machine-learning algorithms learn rules of behavior from individual end users, such as task-oriented desktop organizers and handwriting recognizers. These rules form a “program” that tells the computer what to do when future inputs arrive. Little research has explored how an end user can debug these programs when they make mistakes. We present our progress toward enabling end users to debug these learned programs via a Natural Programming methodology. We began with a formative study exploring how users reason about and correct a text-classification program. From the results, we derived and prototyped a concept based on “explanatory debugging”, then empirically evaluated it. Our results contribute methods for exposing a learned program's logic to end users and for eliciting user corrections to improve the program's predictions
Attractor Flows from Defect Lines
Deforming a two dimensional conformal field theory on one side of a trivial
defect line gives rise to a defect separating the original theory from its
deformation. The Casimir force between these defects and other defect lines or
boundaries is used to construct flows on bulk moduli spaces of CFTs. It turns
out, that these flows are constant reparametrizations of gradient flows of the
g-functions of the chosen defect or boundary condition. The special flows
associated to supersymmetric boundary conditions in N=(2,2) superconformal
field theories agree with the attractor flows studied in the context of black
holes in N=2 supergravity.Comment: 28 page
Current Results of the EC-sponsored Catchment Modelling (CatchMod) Cluster
To support the Water Framework Directive implementation, much research has been commissioned at both national and European levels. CatchMod is a cluster of these projects, which is focusing on the development of computational catchment models and related tools. This paper presents an overview of the results of the CatchMod cluster to dat
Generating entangled atom-photon pairs from Bose-Einstein condensates
We propose using spontaneous Raman scattering from an optically driven
Bose-Einstein condensate as a source of atom-photon pairs whose internal states
are maximally entangled. Generating entanglement between a particle which is
easily transmitted (the photon) and one which is easily trapped and coherently
manipulated (an ultracold atom) will prove useful for a variety of
quantum-information related applications. We analyze the type of entangled
states generated by spontaneous Raman scattering and construct a geometry which
results in maximum entanglement
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