16,228 research outputs found
Macrophage-sensory neuronal interaction in HIV-1 gp120-induced neurotoxicity
Acknowledgements We thank Dr Jim Perkins of University College London for his help with the statistical analysis of our gene array data. We thank Prof. Maria Papathanasopoulos from the University of the Witwatersrand, Johannesburg, for the gift of gp120Bal.Peer reviewedPublisher PD
Astrometric signatures of self-gravitating protoplanetary discs
We use high resolution numerical simulations to study whether gravitational
instabilities within circumstellar discs can produce astrometrically detectable
motion of the central star. For discs with masses of M_disc = 0.1 M_star, which
are permanantly stable against fragmentation, we find that the magnitude of the
astrometric signal depends upon the efficiency of disc cooling. Short cooling
times produce prominent filamentary spiral structures in the disc, and lead to
stellar motions that are potentially observable with future high precision
astrometric experiments. For a disc that is marginally unstable within radii of
\~10 au, we estimate astrometric displacements of 10-100 microarcsec on decade
timescales for a star at a distance of 100 pc. The predicted displacement is
suppressed by a factor of several in more stable discs in which the cooling
time exceeds the local dynamical time by an order of magnitude. We find that
the largest contribution comes from material in the outer regions of the disc
and hence, in the most pessimistic scenario, the stellar motions caused by the
disc could confuse astrometric searches for low mass planets orbiting at large
radii. They are, however, unlikely to present any complications in searches for
embedded planets orbiting at small radii, relative to the disc size, or Jupiter
mass planets or greater orbiting at large radii.Comment: 6 pages, 9 figures, accepted for publication in MNRA
Zeeman splittings of the 5D0â7F2 transitions of Eu3+ ions implanted into GaN
We report the magnetic field splittings of emission lines assigned to the 5D0â7F2 transitions of Eu3+ centres in GaN. The application of a magnetic field in the c-axis direction (B||c) leads to a splitting of the major lines at 621 nm, 622 nm and 622.8 nm into two components. The Zeeman splitting is linear with magnetic field up to 5 Tesla for each line. In contrast, a magnetic field applied in the growth plane (BâŽc) does not influence the photoluminescence spectra. The estimated g-factors vary slightly from sample to sample with mean values of g|| ~2.8, ~1.5 and ~2.0 for the emission lines at 621 nm, 622 nm and 622.8 nm respectively
Enhanced Coherence of Antinodal Quasiparticles in a Dirty d-wave Superconductor
Recent ARPES experiments show a narrow quasiparticle peak at the gap edge
along the antinodal [1,0]-direction for the overdoped cuprate superconductors.
We show that within weak coupling BCS theory for a d-wave superconductor the
s-wave single-impurity scattering cross section vanishes for energies of the
gap edge. This coherence effect occurs through multiple scattering off the
impurity. For small impurity concentrations the spectral function has a
pronounced increase of the (scattering) lifetime for antinodal quasiparticles
but shows a very broad peak in the nodal direction, in qualitative agreement
with experiment and in strong contrast to the behavior observed in underdoped
cuprates.Comment: 4 pages, 3 figures, submitte
Planetesimal Formation In Self-Gravitating Discs
We study particle dynamics in local two-dimensional simulations of
self-gravitating accretion discs with a simple cooling law. It is well known
that the structure which arises in the gaseous component of the disc due to a
gravitational instability can have a significant effect on the evolution of
dust particles. Previous results using global simulations indicate that spiral
density waves are highly efficient at collecting dust particles, creating
significant local over-densities which may be able to undergo gravitational
collapse. We expand on these findings, using a range of cooling times to mimic
the conditions at a large range of radii within the disc. Here we use the
Pencil Code to solve the 2D local shearing sheet equations for gas on a fixed
grid together with the equations of motion for solids coupled to the gas solely
through aerodynamic drag force. We find that spiral density waves can create
significant enhancements in the surface density of solids, equivalent to 1-10cm
sized particles in a disc following the profiles of Clarke (2009) around a
solar mass star, causing it to reach concentrations several orders of magnitude
larger than the particles mean surface density. We also study the velocity
dispersion of the particles, finding that the spiral structure can result in
the particle velocities becoming highly ordered, having a narrow velocity
dispersion. This implies low relative velocities between particles, which in
turn suggests that collisions are typically low energy, lessening the
likelihood of grain destruction. Both these findings suggest that the density
waves that arise due to gravitational instabilities in the early stages of star
formation provide excellent sites for the formation of large,
planetesimal-sized objects.Comment: 11 pages, 8 figures, accepted for publication in MNRA
Convergence of SPH simulations of self-gravitating accretion discs: Sensitivity to the implementation of radiative cooling
Recent simulations of self-gravitating accretion discs, carried out using a
three-dimensional Smoothed Particle Hydrodynamics (SPH) code by Meru and Bate,
have been interpreted as implying that three-dimensional global discs fragment
much more easily than would be expected from a two-dimensional local model.
Subsequently, global and local two-dimensional models have been shown to
display similar fragmentation properties, leaving it unclear whether the
three-dimensional results reflect a physical effect or a numerical problem
associated with the treatment of cooling or artificial viscosity in SPH. Here,
we study how fragmentation of self-gravitating disc flows in SPH depends upon
the implementation of cooling. We run disc simulations that compare a simple
cooling scheme, in which each particle loses energy based upon its internal
energy per unit mass, with a method in which the cooling is derived from a
smoothed internal energy density field. For the simple per particle cooling
scheme, we find a significant increase in the minimum cooling time scale for
fragmentation with increasing resolution, matching previous results. Switching
to smoothed cooling, however, results in lower critical cooling time scales,
and tentative evidence for convergence at the highest spatial resolution
tested. We conclude that precision studies of fragmentation using SPH require
careful consideration of how cooling (and, probably, artificial viscosity) is
implemented, and that the apparent non-convergence of the fragmentation
boundary seen in prior simulations is likely a numerical effect. In real discs,
where cooling is physically smoothed by radiative transfer effects, the
fragmentation boundary is probably displaced from the two-dimensional value by
a factor that is only of the order of unity.Comment: 9 pages, 11 figures, MNRAS in pres
Optimal Control of Quantum Dynamics : A New Theoretical Approach
A New theoretical formalism for the optimal quantum control has been
presented. The approach stems from the consideration of describing the
time-dependent quantum system in terms of the real physical observables, viz.,
the probability density rho(x,t) and the quantum current j(x,t) which is well
documented in the Bohm's hydrodynamical formulation of quantum mechanics. The
approach has been applied for manipulating the vibrational motion of HBr in its
ground electronic state under an external electric field.Comment: 4 figure
Quasiparticles in the Pseudogap Phase of Underdoped Cuprate
Recent angle resolved photoemission \cite{yang-nature-08} and scanning
tunneling microscopy \cite{kohsaka-nature-08} measurements on underdoped
cuprates have yielded new spectroscopic information on quasiparticles in the
pseudogap phase. New features of the normal state such as particle-hole
asymmetry, maxima in the energy dispersion and accompanying drops in the
spectral weight of quasiparticles agree with the ansatz of Yang \textit{et al.}
for the single particle propagator in the pseudogap phase. The coherent
quasiparticle dispersion and reduced asymmetry in the tunneling density of
states in the superconducting state can also be described by this propagator.Comment: updated version, 6 pages, 7 figures, 1 table, EPL 86 (2009) 37002
(https://www.epletters.net
- âŠ