7,587 research outputs found
Combinatorial identities for binary necklaces from exact ray-splitting trace formulae
Based on an exact trace formula for a one-dimensional ray-splitting system,
we derive novel combinatorial identities for cyclic binary sequences (P\'olya
necklaces).Comment: 15 page
Nuclear Magnetic Resonance and Hyperfine Structure
Contains reports on two research projects
Parametric correlations versus fidelity decay: the symmetry breaking case
We derive fidelity decay and parametric energy correlations for random matrix
ensembles where time--reversal invariance of the original Hamiltonian is broken
by the perturbation. Like in the case of a symmetry conserving perturbation a
simple relation between both quantities can be established.Comment: 8 pages, 8 figure
The efficacy of halofantrine in the treatment of acute malaria in nonimmune travelers
A multicenter prospective trial was performed to investigate the efficacy and the tolerability of halofantrine in nonimmune patients with malaria imported from areas with drug-resistant falciparum parasites (mainly Africa). Forty-five of the 74 subjects were treated with a one-day regimen (3 x 500 mg) of halofantrine, and the other 29 received the same regimen with an additional treatment on day 7. In the second group, a 100% efficacy rate was demonstrated, but in the group receiving the one-day regimen, four recrudescences were observed in patients with falciparum malaria. Only five mild adverse reactions were seen, which disappeared spontaneously after the end of the treatment. We conclude that halofantrine is highly effective in curing malaria in nonimmune subjects. The treatment scheme for such persons should include an additional treatment on day 7 for nonimmune individuals. This drug was well tolerated in our patients, indicating that halofantrine will be useful in the treatment of multidrug-resistant malaria in nonimmune persons
Star formation environments and the distribution of binary separations
We have carried out K-band speckle observations of a sample of 114 X-ray
selected weak-line T Tauri stars in the nearby Scorpius-Centaurus OB
association. We find that for binary T Tauri stars closely associated to the
early type stars in Upper Scorpius, the youngest subgroup of the OB
association, the peak in the distribution of binary separations is at 90 A.U.
For binary T Tauri stars located in the direction of an older subgroup, but not
closely associated to early type stars, the peak in the distribution is at 215
A.U. A Kolmogorov-Smirnov test indicates that the two binary populations do not
result from the same distibution at a significance level of 98%. Apparently,
the same physical conditions which facilitate the formation of massive stars
also facilitate the formation of closer binaries among low-mass stars, whereas
physical conditions unfavorable for the formation of massive stars lead to the
formation of wider binaries among low-mass stars. The outcome of the binary
formation process might be related to the internal turbulence and the angular
momentum of molecular cloud cores, magnetic field, the initial temperature
within a cloud, or - most likely - a combination of all of these. We conclude
that the distribution of binary separations is not a universal quantity, and
that the broad distribution of binary separations observed among main-sequence
stars can be explained by a superposition of more peaked binary distributions
resulting from various star forming environments. The overall binary frequency
among pre-main-sequence stars in individual star forming regions is not
necessarily higher than among main-sequence stars.Comment: 7 pages, Latex, 4 Postscript figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/pubs/pubs.html ; accepted for
publication in ApJ Letter
Studying Flow Close to an Interface by Total Internal Reflection Fluorescence Cross Correlation Spectroscopy: Quantitative Data Analysis
Total Internal Reflection Fluorescence Cross Correlation Spectroscopy
(TIR-FCCS) has recently (S. Yordanov et al., Optics Express 17, 21149 (2009))
been established as an experimental method to probe hydrodynamic flows near
surfaces, on length scales of tens of nanometers. Its main advantage is that
fluorescence only occurs for tracer particles close to the surface, thus
resulting in high sensitivity. However, the measured correlation functions only
provide rather indirect information about the flow parameters of interest, such
as the shear rate and the slip length. In the present paper, we show how to
combine detailed and fairly realistic theoretical modeling of the phenomena by
Brownian Dynamics simulations with accurate measurements of the correlation
functions, in order to establish a quantitative method to retrieve the flow
properties from the experiments. Firstly, Brownian Dynamics is used to sample
highly accurate correlation functions for a fixed set of model parameters.
Secondly, these parameters are varied systematically by means of an
importance-sampling Monte Carlo procedure in order to fit the experiments. This
provides the optimum parameter values together with their statistical error
bars. The approach is well suited for massively parallel computers, which
allows us to do the data analysis within moderate computing times. The method
is applied to flow near a hydrophilic surface, where the slip length is
observed to be smaller than 10nm, and, within the limitations of the
experiments and the model, indistinguishable from zero.Comment: 18 pages, 12 figure
Gain without inversion in a biased superlattice
Intersubband transitions in a superlattice under homogeneous electric field
is studied within the tight-binding approximation. Since the levels are
equi-populated, the non-zero response appears beyond the Born approximation.
Calculations are performed in the resonant approximation with scattering
processes exactly taken into account. The absorption coefficient is equal zero
for the resonant excitation while a negative absorption (gain without
inversion) takes place below the resonance. A detectable gain in the THz
spectral region is obtained for the low-doped -based superlattice and
spectral dependencies are analyzed taking into account the interplay between
homogeneous and inhomogeneous mechanisms of broadening.Comment: 6 pages, 4 figure
Nuclear Magnetic Resonance and Hyperfine Structure
Contains reports on three research projects
Two-dimensional Dirac fermions in a topological insulator: transport in the quantum limit
Pulsed magnetic fields of up to 55T are used to investigate the transport
properties of the topological insulator Bi_2Se_3 in the extreme quantum limit.
For samples with a bulk carrier density of n = 2.9\times10^16cm^-3, the lowest
Landau level of the bulk 3D Fermi surface is reached by a field of 4T. For
fields well beyond this limit, Shubnikov-de Haas oscillations arising from
quantization of the 2D surface state are observed, with the \nu =1 Landau level
attained by a field of 35T. These measurements reveal the presence of
additional oscillations which occur at fields corresponding to simple rational
fractions of the integer Landau indices.Comment: 5 pages, 4 figure
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