28,189 research outputs found
Stability of small-scale baryon perturbations during cosmological recombination
In this paper, we study small-scale fluctuations (baryon pressure sound
waves) in the baryon fluid during recombination. In particular, we look at
their evolution in the presence of relative velocities between baryons and
photons on large scales (), which are
naturally present during the era of decoupling. Previous work concluded that
the fluctuations grow due to an instability of sound waves in a recombining
plasma, but that the growth factor is small for typical cosmological models.
These analyses model recombination in an inhomogenous universe as a
perturbation to the parameters of the homogenous solution. We show that for
relevant wavenumbers the dynamics are
significantly altered by the transport of both ionizing continuum (
eV) and Lyman- photons between crests and troughs of the density
perturbations. We solve the radiative transfer of photons in both these
frequency ranges and incorporate the results in a perturbed three-level atom
model. We conclude that the instability persists at intermediate scales. We use
the results to estimate a distribution of growth rates in random
realizations of large-scale relative velocities. Our results indicate that
there is no appreciable growth; out of these realizations, the maximum
growth factor we find is less than at wavenumbers of . The instability's low growth factors are due to the
relatively short duration of the recombination epoch during which the electrons
and photons are coupled.Comment: 20 pages, 13 figures, updated with published versio
Bose Gases Near Unitarity
We study the properties of strongly interacting Bose gases at the density and
temperature regime when the three-body recombination rate is substantially
reduced. In this regime, one can have a Bose gas with all particles in
scattering states (i.e. the "upper branch") with little loss even at unitarity
over the duration of the experiment. We show that because of bosonic
enhancement, pair formation is shifted to the atomic side of the original
resonance (where scattering length ), opposite to the fermionic case. In
a trap, a repulsive Bose gas remains mechanically stable when brought across
resonance to the atomic side until it reaches a critical scattering length
. For , the density consists of a core of
upper branch bosons surrounded by an outer layer of equilibrium phase. The
conditions of low three-body recombination requires that the particle number
in a harmonic trap with frequency , where
is a constant.Comment: 4 pages, 4 figure
Optical Study of GaAs quantum dots embedded into AlGaAs nanowires
We report on the photoluminescence characterization of GaAs quantum dots
embedded into AlGaAs nano-wires. Time integrated and time resolved
photoluminescence measurements from both an array and a single quantum
dot/nano-wire are reported. The influence of the diameter sizes distribution is
evidenced in the optical spectroscopy data together with the presence of
various crystalline phases in the AlGaAs nanowires.Comment: 5 page, 5 figure
Diffusion controlled initial recombination
This work addresses nucleation rates in systems with strong initial
recombination. Initial (or `geminate') recombination is a process where a
dissociated structure (anion, vortex, kink etc.) recombines with its twin
brother (cation, anti-vortex, anti-kink) generated in the same nucleation
event. Initial recombination is important if there is an asymptotically
vanishing interaction force instead of a generic saddle-type activation
barrier. At low temperatures, initial recombination strongly dominates
homogeneous recombination. In a first part, we discuss the effect in one-,
two-, and three-dimensional diffusion controlled systems with spherical
symmetry. Since there is no well-defined saddle, we introduce a threshold which
is to some extent arbitrary but which is restricted by physically reasonable
conditions. We show that the dependence of the nucleation rate on the specific
choice of this threshold is strongest for one-dimensional systems and decreases
in higher dimensions. We discuss also the influence of a weak driving force and
show that the transport current is directly determined by the imbalance of the
activation rate in the direction of the field and the rate against this
direction. In a second part, we apply the results to the overdamped sine-Gordon
system at equilibrium. It turns out that diffusive initial recombination is the
essential mechanism which governs the equilibrium kink nucleation rate. We
emphasize analogies between the single particle problem with initial
recombination and the multi-dimensional kink-antikink nucleation problem.Comment: LaTeX, 11 pages, 1 ps-figures Extended versio
Author's reply to "Comments on "Noise source modeling for cyclostationary noise analysis in large-signal device operation""
HIV-infected sex workers with beneficial HLA-variants are potential hubs for selection of HIV-1 recombinants that may affect disease progression
Cytotoxic T lymphocyte (CTL) responses against the HIV Gag protein are associated with lowering viremia; however, immune control is undermined by viral escape mutations. The rapid viral mutation rate is a key factor, but recombination may also contribute. We hypothesized that CTL responses drive the outgrowth of unique intra-patient HIV-recombinants (URFs) and examined gag sequences from a Kenyan sex worker cohort. We determined whether patients with HLA variants associated with effective CTL responses (beneficial HLA variants) were more likely to carry URFs and, if so, examined whether they progressed more rapidly than patients with beneficial HLA-variants who did not carry URFs. Women with beneficial HLA-variants (12/52) were more likely to carry URFs than those without beneficial HLA variants (3/61) (pâ<â0.0055; odds ratioâ=â5.7). Beneficial HLA variants were primarily found in slow/standard progressors in the URF group, whereas they predominated in long-term non-progressors/survivors in the remaining cohort (pâ=â0.0377). The URFs may sometimes spread and become circulating recombinant forms (CRFs) of HIV and local CRF fragments were over-represented in the URF sequences (pâ<â0.0001). Collectively, our results suggest that CTL-responses associated with beneficial HLA variants likely drive the outgrowth of URFs that might reduce the positive effect of these CTL responses on disease progression
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