2,995 research outputs found
Timescape cosmology with radiation fluid
The timescape cosmology represents a potentially viable alternative to the
standard homogeneous cosmology, without the need for dark energy. Although
average cosmic evolution in the timescape scenario only differs substantially
from that of Friedmann-Lemaitre model at relatively late epochs when the
contribution from the energy density of radiation is negligible, a full
solution of the Buchert equations to incorporate radiation is necessary to
smoothly match parameters to the epoch of photon decoupling and to obtain
constraints from cosmic microwave background data. Here we extend the
matter-dominated solution found in earlier work to include radiation, providing
series solutions at early times and an efficient numerical integration strategy
for generating the complete solution. The numerical solution is used to
directly calculate the scale of the sound horizon at decoupling, and at the
baryon drag epoch. The constraints on these scales from the Planck satellite
data yield bounds on the timescape cosmological parameters, which are found to
also agree with the best-fit values from a recent analysis of SDSS-II supernova
data, while avoiding the problem of a primordial lithium-7 abundance anomaly.Comment: 21 pages, 6 figures; v2: small additions, accepted by Class. Quantum
Gra
Creating excitonic entanglement in quantum dots through the optical Stark effect
We show that two initially non-resonant quantum dots may be brought into
resonance by the application of a single detuned laser. This allows for control
of the inter-dot interactions and the generation of highly entangled excitonic
states on the picosecond timescale. Along with arbitrary single qubit
manipulations, this system would be sufficient for the demonstration of a
prototype excitonic quantum computer.Comment: 4 pages, 3 figures; published version, figure 3 improved, corrections
to RWA derive
Matter collineations of Spacetime Homogeneous G\"odel-type Metrics
The spacetime homogeneous G\"odel-type spacetimes which have four classes of
metrics are studied according to their matter collineations. The obtained
results are compared with Killing vectors and Ricci collineations. It is found
that these spacetimes have infinite number of matter collineations in
degenerate case, i.e. det, and do not admit proper matter
collineations in non-degenerate case, i.e. det. The degenerate
case has the new constraints on the parameters and which characterize
the causality features of the G\"odel-type spacetimes.Comment: 12 pages, LaTex, no figures, Class. Quantum.Grav.20 (2003) 216
High Ferromagnetic Transition Temperature (172K) in Mn delta-doped GaAs with p-type Selective Doping
We have found high ferromagnetic transition temperature in Mn delta-doped
GaAs-based heterostructures grown on GaAs(001) substrates by molecular beam
epitaxy. A 0.3 ML Mn d-doped GaAs samples showed high resistivity at low
temperature and did not show a ferromagnetic behavior. However, in a
selectively doped heterostructure (Mn delta-doped GaAs / Be-doped AlGaAs),
where holes were supplied from the Be-doped AlGaAs layer, clear ferromagnetic
order was observed. The ferromagnetic transition temperature of the selectively
doped heterostructure was as high as 172K with suitable low-temperature (LT)
annealing treatment
Nuclear structure and reaction studies at SPIRAL
The SPIRAL facility at GANIL, operational since 2001, is described briefly.
The diverse physics program using the re-accelerated (1.2 to 25 MeV/u) beams
ranging from He to Kr and the instrumentation specially developed for their
exploitation are presented. Results of these studies, using both direct and
compound processes, addressing various questions related to the existence of
exotic states of nuclear matter, evolution of new "magic numbers", tunnelling
of exotic nuclei, neutron correlations, exotic pathways in astrophysical sites
and characterization of the continuum are discussed. The future prospects for
the facility and the path towards SPIRAL2, a next generation ISOL facility, are
also briefly presented.Comment: 48 pages, 27 figures. Accepted for publication in Journal of Physics
Aspects of Quantum Coherence in Nanosystems
Coherence is a familiar concept in physics: It is the driving force behind
wavelike phenomena such as the diffraction of light. Moreover, wave-particle
duality implies that all quantum objects can exhibit coherence, and this
quantum coherence is crucial to understanding the behaviour of a plethora of
systems.
In this article, which is written at an undergraduate level, we shall briefly
introduce what is meant by coherence in a well-known classical setting, before
going on to describe its quantum version. We will show that coherence is
important in describing the properties of solid-state nanosystems, and
especially quantum dots. Simple experiments that reveal the coherent nature of
matter - and how this leads to some very powerful applications - will be
described. Finally, we shall discuss the fragility of coherence and shall
introduce a method for describing decoherence in open quantum systems.Comment: Invited review paper at an introductory level, aimed at an
undergraduate audience. To appear in European Journal of Physics, Special
feature on Nanotechnology. 14 pages, 4 figure
Effect of Fin Geometry on Flow-Induced Vibration Response of a Finned Tube in a Tube Bundle
An experimental study is carried out on a parallel triangular finned tube array with P/Deff ratio 1.62 to examine the effect of fin geometry on flow-induced vibration response. Fins on a tube increase the heat transfer rate but these also affect the fluid dynamics around the tube. The flow pattern across the finned tubes is complex as compared to bare tube arrays. There are numerous parameters that affect the finned tube vibration subjected to air cross-flow in a tube array. In the current study, some of these parameters i.e. fin thickness and fin density are focused and their effects on flow-induced vibration response are analyzed in different rows of fin tube array. The current experimentation is performed in a subsonic wind tunnel using a single flexible Aluminum finned tube in a rigid array. Seven tubes with similar specifications but distinct fin thickness and fin density are used for the testing purpose. Their amplitude response suggests that the flow-induced vibration behavior is greatly affected by changing the finned tube parameters. It has also been observed during spectral analysis that the Strouhal number is independent of fin geometry since it remained constant in different rows of the array for finned tubes under study. It suggests that the vortex shedding has also contributed towards the finned tube vibration predominantly in the first, second and the fourth row of tube array
MachiBase: a Drosophila melanogaster 5′-end mRNA transcription database
MachiBase (http://machibase.gi.k.u-tokyo.ac.jp/) provides a comprehensive and freely accessible resource regarding Drosophila melanogaster 5′-end mRNA transcription at different developmental states, supporting studies on the variabilities of promoter transcriptional activities and gene-expression profiles in the fruitfly. The data were generated in conjunction with the recently developed high-throughput genome sequencer Illumina/Solexa using a newly developed 5′-end mRNA collection method
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