2,390 research outputs found
Two-photon fluorescence coincidence analysis: Rapid measurements of enzyme kinetics
Dual-color fluorescence cross-correlation analysis is a powerful tool for probing interactions of different fluorescently labeled molecules in aqueous solution. The concept is the selective observation of coordinated spontaneous fluctuations in two separate detection channels that unambiguously reflect the existence of physical or chemical linkages among the different fluorescent species. It has previously been shown that the evaluation of cross-correlation amplitudes, i.e., coincidence factors, is sufficient to extract essential information about the kinetics of formation or cleavage of chemical or physical bonds. Confocal fluorescence coincidence analysis (CFCA) (Winkler et al., Proc. Natl. Acad. Sci. U.S.A. 96:13751378, 1999) emphasizes short analysis times and simplified data evaluation and is thus particularly useful for screening applications or measurements on live cells where small illumination doses need to be applied. The recent use of two-photon fluorescence excitation has simplified dual- or multicolor measurements by enabling the simultaneous excitation of largely different dye molecules by a single infra-red laser line (Heinze et al., Proc. Natl. Acad. Sci. U.S.A. 97:10377- 10382, 2000). It is demonstrated here that a combination of CFCA with two-photon excitation allows for minimization of analysis times for multicomponent systems down to some hundreds of milliseconds, while preserving all known advantages of two- photon excitation. By introducing crucial measurement parameters, experimental limits for the reduction of sampling times are discussed for the special case of distinguishing positive from negative samples in an endonucleolytic cleavage assay
Direct imaging constraints on planet populations detected by microlensing
Results from gravitational microlensing suggested the existence of a large
population of free-floating planetary mass objects. The main conclusion from
this work was partly based on constraints from a direct imaging survey. This
survey determined upper limits for the frequency of stars that harbor giant
exoplanets at large orbital separations. Aims. We want to verify to what extent
upper limits from direct imaging do indeed constrain the microlensing results.
We examine the current derivation of the upper limits used in the microlensing
study and re-analyze the data from the corresponding imaging survey. We focus
on the mass and semi-major axis ranges that are most relevant in context of the
microlensing results. We also consider new results from a recent M-dwarf
imaging survey as these objects are typically the host stars for planets
detected by microlensing. We find that the upper limits currently applied in
context of the microlensing results are probably underestimated. This means
that a larger fraction of stars than assumed may harbor gas giant planets at
larger orbital separations. Also, the way the upper limit is currently used to
estimate the fraction of free-floating objects is not strictly correct. If the
planetary surface density of giant planets around M-dwarfs is described as
df_Planet ~ a^beta da, we find that beta ~ 0.5 - 0.6 is consistent with results
from different observational studies probing semi-major axes between ~0.03 - 30
AU. Having a higher upper limit on the fraction of stars that may have gas
giant planets at orbital separations probed by the microlensing data implies
that more of the planets detected in the microlensing study are potentially
bound to stars rather than free-floating. The current observational data are
consistent with a rising planetary surface density for giant exoplanets around
M-dwarfs out to ~30 AU.Comment: Accepted for publication in A&A as Research Note, 3 page
Evolution of spectral properties along the O(6)-U(5) transition in the interacting boson model. I. Level dynamics
We investigate the evolution of quantal spectra and the corresponding wave
functions along the [O(6)-U(5)]O(5) transition of the interacting
boson model. The model is integrable in this regime and its ground state passes
through a second-order structural phase transition. We show that the whole
spectrum as a function of the Hamiltonian control parameter, as well as
structures of all excited states, exhibit rather organized and correlated
behaviors, that provide deeper insight into the nature of this transitional
path.Comment: 10 pages, 8 figure
Magnetic Scanning Tunneling Microscopy with a Two-Terminal Non-Magnetic Tip: Quantitative Results
We report numerical simulation result of a recently proposed \{P. Bruno,
Phys. Rev. Lett {\bf 79}, 4593, (1997)\} approach to perform magnetic scanning
tunneling microscopy with a two terminal non-magnetic tip. It is based upon the
spin asymmetry effect of the tunneling current between a ferromagnetic surface
and a two-terminal non-magnetic tip. The spin asymmetry effect is due to the
spin-orbit scattering in the tip. The effect can be viewed as a Mott scattering
of tunneling electrons within the tip. To obtain quantitative results we
perform numerical simulation within the single band tight binding model, using
recursive Green function method and Landauer-B\"uttiker formula for
conductance. A new model has been developed to take into account the spin-orbit
scattering off the impurities within the single-band tight-binding model. We
show that the spin-asymmetry effect is most prominent when the device is in
quasi-ballistic regime and the typical value of spin asymmetry is about 5%.Comment: 5 pages, Late
First On-Sky High Contrast Imaging with an Apodizing Phase Plate
We present the first astronomical observations obtained with an Apodizing
Phase Plate (APP). The plate is designed to suppress the stellar diffraction
pattern by 5 magnitudes from 2-9 lambda/D over a 180 degree region. Stellar
images were obtained in the M' band (4.85 microns) at the MMTO 6.5m telescope,
with adaptive wavefront correction made with a deformable secondary mirror
designed for low thermal background observations. The measured PSF shows a halo
intensity of 0.1% of the stellar peak at 2 lambda/D (0.36 arcsec), tapering off
as r^{-5/3} out to radius 9 lambda/D. Such a profile is consistent with
residual errors predicted for servo lag in the AO system.
We project a 5 sigma contrast limit, set by residual atmospheric
fluctuations, of 10.2 magnitudes at 0.36 arcsec separation for a one hour
exposure. This can be realised if static and quasi-static aberrations are
removed by differential imaging, and is close to the sensitivity level set by
thermal background photon noise for target stars with M'>3. The advantage of
using the phase plate is the removal of speckle noise caused by the residuals
in the diffraction pattern that remain after PSF subtraction. The APP gives
higher sensitivity over the range 2-5 lambda/D compared to direct imaging
techniques.Comment: 22 pages, 5 figures, 1 table, ApJ accepte
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