15,278 research outputs found
Single-molecule stochastic resonance
Stochastic resonance (SR) is a well known phenomenon in dynamical systems. It
consists of the amplification and optimization of the response of a system
assisted by stochastic noise. Here we carry out the first experimental study of
SR in single DNA hairpins which exhibit cooperatively folding/unfolding
transitions under the action of an applied oscillating mechanical force with
optical tweezers. By varying the frequency of the force oscillation, we
investigated the folding/unfolding kinetics of DNA hairpins in a periodically
driven bistable free-energy potential. We measured several SR quantifiers under
varied conditions of the experimental setup such as trap stiffness and length
of the molecular handles used for single-molecule manipulation. We find that
the signal-to-noise ratio (SNR) of the spectral density of measured
fluctuations in molecular extension of the DNA hairpins is a good quantifier of
the SR. The frequency dependence of the SNR exhibits a peak at a frequency
value given by the resonance matching condition. Finally, we carried out
experiments in short hairpins that show how SR might be useful to enhance the
detection of conformational molecular transitions of low SNR.Comment: 11 pages, 7 figures, supplementary material
(http://prx.aps.org/epaps/PRX/v2/i3/e031012/prx-supp.pdf
Computing the phase diagram of binary mixtures: a patchy particle case study
We investigate the phase behaviour of 2D mixtures of bi-functional and
three-functional patchy particles and 3D mixtures of bi-functional and
tetra-functional patchy particles by means of Monte Carlo simulations and
Wertheim theory. We start by computing the critical points of the pure systems
and then we investigate how the critical parameters change upon lowering the
temperature. We extend the Successive Umbrella Sampling method to mixtures to
make it possible to extract information about the phase behaviour of the system
at a fixed temperature for the whole range of densities and compositions of
interest.Comment: 13 pages, 13 figure
The intrinsic three-dimensional shape of galactic bars
We present the first statistical study on the intrinsic three-dimensional
(3D) shape of a sample of 83 galactic bars extracted from the CALIFA survey. We
use the galaXYZ code to derive the bar intrinsic shape with a statistical
approach. The method uses only the geometric information (ellipticities and
position angles) of bars and discs obtained from a multi-component photometric
decomposition of the galaxy surface-brightness distributions. We find that bars
are predominantly prolate-triaxial ellipsoids (68%), with a small fraction of
oblate-triaxial ellipsoids (32%). The typical flattening (intrinsic C/A
semiaxis ratio) of the bars in our sample is 0.34, which matches well the
typical intrinsic flattening of stellar discs at these galaxy masses. We
demonstrate that, for prolate-triaxial bars, the intrinsic shape of bars
depends on the galaxy Hubble type and stellar mass (bars in massive S0 galaxies
are thicker and more circular than those in less massive spirals). The bar
intrinsic shape correlates with bulge, disc, and bar parameters. In particular
with the bulge-to-total (B/T) luminosity ratio, disc g-r color, and central
surface brightness of the bar, confirming the tight link between bars and their
host galaxies. Combining the probability distributions of the intrinsic shape
of bulges and bars in our sample we show that 52% (16%) of bulges are thicker
(flatter) than the surrounding bar at 1 level. We suggest that these
percentages might be representative of the fraction of classical and disc-like
bulges in our sample, respectively.Comment: 18 pages, 11 figures, accepted for publication in MNRA
The Dirichlet Obstruction in AdS/CFT
The obstruction for a perturbative reconstruction of the five-dimensional
bulk metric starting from the four-dimensional metric at the boundary,that is,
the Dirichlet problem, is computed in dimensions and some
comments are made on its general structure and, in particular, on its
relationship with the conformal anomaly, which we compute in dimension .Comment: 13 pages, references added (this paper supersedes hep-th/0206140, "A
Note on the Bach Tensor in AdS/CFT", which has been withdrawn
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Optimization, evaluation, and comparison of standard algorithms for image reconstruction with the VIP-PET
A novel positron emission tomography (PET) scanner design based on a room-temperature pixelated CdTe solid-state detector is being developed within the framework of the Voxel Imaging PET (VIP) Pathfinder project []. The simulation results show a great potential of the VIP to produce high-resolution images even in extremely challenging conditions such as the screening of a human head []. With unprecedented high channel density (450 channels/cm 3) image reconstruction is a challenge. Therefore optimization is needed to find the best algorithm in order to exploit correctly the promising detector potential. The following reconstruction algorithms are evaluated: 2-D Filtered Backprojection (FBP), Ordered Subset Expectation Maximization (OSEM), List-Mode OSEM (LM-OSEM), and the Origin Ensemble (OE) algorithm. The evaluation is based on the comparison of a true image phantom with a set of reconstructed images obtained by each algorithm. This is achieved by calculation of image quality merit parameters such as the bias, the variance and the mean square error (MSE). A systematic optimization of each algorithm is performed by varying the reconstruction parameters, such as the cutoff frequency of the noise filters and the number of iterations. The region of interest (ROI) analysis of the reconstructed phantom is also performed for each algorithm and the results are compared. Additionally, the performance of the image reconstruction methods is compared by calculating the modulation transfer function (MTF). The reconstruction time is also taken into account to choose the optimal algorithm. The analysis is based on GAMOS [] simulation including the expected CdTe and electronic specifics
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