23,825 research outputs found
Spinodal fractionation in a polydisperse square well fluid
Using Kinetic Monte Carlo simulation, we model gas-liquid spinodal
decomposition in a size-polydisperse square well fluid, representing a
'near-monodisperse' colloidal dispersion. We find that fractionation (demixing)
of particle sizes between the phases begins asserting itself shortly after the
onset of phase ordering. Strikingly, the direction of size fractionation can be
reversed by a seemingly trivial choice between two inter-particle potentials
which, in the monodisperse case, are identical -- we rationalise this in terms
of a perturbative, equilibrium theory of polydispersity. Furthermore, our
quantitative results show that Kinetic Monte Carlo simulation can provide
detailed insight into the role of fractionation in real colloidal systems.Comment: 7 pages, 7 figures, to be published in Phys. Rev.
Radio-wave propagation in the non-Gaussian interstellar medium
Radio waves propagating from distant pulsars in the interstellar medium
(ISM), are refracted by electron density inhomogeneities, so that the intensity
of observed pulses fluctuates with time. The theory relating the observed pulse
time-shapes to the electron-density correlation function has developed for 30
years, however, two puzzles have remained. First, observational scaling of
pulse broadening with the pulsar distance is anomalously strong; it is
consistent with the standard model only when non-uniform statistics of electron
fluctuations along the line of sight are assumed. Second, the observed pulse
shapes are consistent with the standard model only when the scattering material
is concentrated in a narrow slab between the pulsar and the Earth.
We propose that both paradoxes are resolved at once if one assumes stationary
and uniform, but non-Gaussian statistics of the electron-density distribution.
Such statistics must be of Levy type, and the propagating ray should exhibit a
Levy flight. We propose that a natural realization of such statistics may be
provided by the interstellar medium with random electron-density
discontinuities. We develop a theory of wave propagation in such a non-Gaussian
random medium, and demonstrate its good agreement with observations. The
qualitative introduction of the approach and the resolution of the
anomalous-scaling paradox was presented earlier in [PRL 91, 131101 (2003); ApJ
584, 791 (2003)].Comment: 27 pages, changes to match published versio
An exploratory randomised controlled trial comparing telephone and hospital follow-up after treatment for colorectal cancer
Aim:  Following treatment for colorectal cancer it is common practice for patients to attend hospital clinics at regular intervals for routine monitoring, although debate persists on the benefits of this approach. Nurse-led telephone follow-up is effective in meeting information and psycho-social needs in other patient groups. We explored the potential benefits of nurse-led telephone follow-up for colorectal cancer patients.
Method:  Sixty-five patients were randomised to either telephone or hospital follow-up in an exploratory randomised trial.
Results:  The telephone intervention was deliverable in clinical practice and acceptable to patients and health professionals. Seventy-five percent of eligible patients agreed to randomization. High levels of satisfaction were evident in both study groups. Appointments in the hospital group were shorter (median 14.0 minutes) than appointments in the telephone group (median 28.9 minutes). Patients in the telephone arm were more likely to raise concerns during consultations.
Conclusion:  Historical approaches to follow-up unsupported by evidence of effectiveness and efficiency are not sustainable. Telephone follow-up by specialist nurses may be a feasible option. A main trial comparing hospital and telephone follow-up is justified although consideration needs to be given to trial design and practical issues related to the availability of specialist nurses at study locations
A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses
Multipath propagation in the interstellar medium distorts radio pulses, an
effect predominant for distant pulsars observed at low frequencies. Typically,
broadened pulses are analyzed to determine the amount of propagation-induced
pulse broadening, but with little interest in determining the undistorted pulse
shapes. In this paper we develop and apply a method that recovers both the
intrinsic pulse shape and the pulse broadening function that describes the
scattering of an impulse. The method resembles the CLEAN algorithm used in
synthesis imaging applications, although we search for the best pulse
broadening function, and perform a true deconvolution to recover intrinsic
pulse structre. As figures of merit to optimize the deconvolution, we use the
positivity and symmetry of the deconvolved result along with the mean square
residual and the number of points below a given threshold. Our method makes no
prior assumptions about the intrinsic pulse shape and can be used for a range
of scattering functions for the interstellar medium. It can therefore be
applied to a wider variety of measured pulse shapes and degrees of scattering
than the previous approaches. We apply the technique to both simulated data and
data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
Anomalous Radio-Wave Scattering from Interstellar Plasma Structures
This paper considers scattering screens that have arbitrary spatial
variations of scattering strength transverse to the line of sight, including
screens that are spatially well confined, such as disks and filaments. We
calculate the scattered image of a point source and the observed pulse shape of
a scattered impulse. The consequences of screen confinement include: (1) Source
image shapes that are determined by the physical extent of the screen rather
than by the shapes of much-smaller diffracting microirregularities. These
include image elongations and orientations that are frequency dependent. (2)
Variation with frequency of angular broadening that is much weaker than the
trademark \nu^{-2} scaling law (for a cold, unmagnetized plasma), including
frequency-independent cases; and (3) Similar departure of the pulse broadening
time from the usually expected \nu^{-4} scaling law. We briefly discuss
applications that include scattering of pulses from the Crab pulsar by
filaments in the Crab Nebula; image asymmetries from Galactic scattering of the
sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active
galactic nuclei by intervening galaxies. We also address the consequences for
inferences about the shape of the wavenumber spectrum of electron density
irregularities, which depend on scaling laws for the image size and the pulse
broadening. Future low-frequency (< 100 MHz) array observations will also be
strongly affected by the Galactic structure of scattering material. Our
formalism is derived in the context of radio scattering by plasma density
fluctuations. It is also applicable to optical, UV and X-ray scattering by
grains in the interstellar medium.Comment: 21 pages, LaTeX2e with AASTeX-4.0, 6 PostScript figures, accepted by
ApJ, revised version has minor changes to respond to referee comments and
suggestion
Diffusion Quantum Monte Carlo Calculations of Excited States of Silicon
The band structure of silicon is calculated at the Gamma, X, and L wave
vectors using diffusion quantum Monte Carlo methods. Excited states are formed
by promoting an electron from the valence band into the conduction band. We
obtain good agreement with experiment for states around the gap region and
demonstrate that the method works equally well for direct and indirect
excitations, and that one can calculate many excited states at each wave
vector. This work establishes the fixed-node DMC approach as an accurate method
for calculating the energies of low lying excitations in solids.Comment: 5 pages, 1 figur
Non-Gaussian Radio-Wave Scattering in the Interstellar Medium
It was recently suggested by Boldyrev & Gwinn that the characteristics of
radio scintillations from distant pulsars are best understood if the
interstellar electron-density fluctuations that cause the time broadening of
the radio pulses obey non-Gaussian statistics. In this picture the density
fluctuations are inferred to be strong on very small scales (). We argue that such density structures could correspond to the ionized
boundaries of molecular regions (clouds) and demonstrate that the power-law
distribution of scattering angles that is required to match the observations
arises naturally from the expected intersections of our line of sight with
randomly distributed, thin, approximately spherical ionized shells of this
type. We show that the observed change in the time-broadening behavior for
pulsar dispersion measures is consistent
with the expected effect of the general ISM turbulence, which should dominate
the scattering for nearby pulsars. We also point out that if the clouds are
ionized by nearby stars, then their boundaries may become turbulent on account
of an ionization front instability. This turbulence could be an alternative
cause of the inferred density structures. An additional effect that might
contribute to the strength of the small-scale fluctuations in this case is the
expected flattening of the turbulent density spectrum when the eddy sizes
approach the proton gyroscale.Comment: 15 pages, 3 figures, accepted to Ap
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