3,703 research outputs found
Dynamical evolution of the Universe in the quark-hadron phase transition and possible nugget formation
We study the dynamics of first-order phase transition in the early Universe
when it was old with quarks and gluons condensing into hadrons.
We look at how the Universe evolved through the phase transition in small as
well as large super cooling scenario, specifically exploring the formation of
quark nuggets and their possible survival. The nucleation of the hadron phase
introduces new distance scales in the Universe, which we estimate along with
the hadron fraction, temperature, nucleation time etc. It is of interest to
explore whether there is a relic signature of this transition in the form of
quark nuggets which might be identified with the recently observed dark objects
in our galactic halo and account for the Dark Matter in the Universe at
present.Comment: LaTeX file with four postscript figure
Preparation and characterisation of single phase (MoVW)<sub>5</sub>O<sub>14</sub>-type catalyst material
MoVW based materials are highly effective catalysts for partial oxidation reactions such as conversion of acrolein to acrylic acid. They offer a high selectivity, high yields and a good long term stability. Preceding work has identified the catalytically active phase of the MoVW catalyst and characterised it by Raman spectroscopy. The current work has been carried out to synthesise and characterise this active (MoVW)5O14 type structure
Uniformly Accelerated Mirrors. Part 2: Quantum Correlations
We study the correlations between the particles emitted by a moving mirror.
To this end, we first analyze , the
two-point function of the stress tensor of the radiation field. In this we
generalize the work undertaken by Carlitz and Willey. To further analyze how
the vacuum correlations on are scattered by the mirror and redistributed
among the produced pairs of particles, we use a more powerful approach based on
the value of which is conditional to the detection of a given
particle on . We apply both methods to the fluxes emitted by a uniformly
accelerated mirror. This case is particularly interesting because of its strong
interferences which lead to a vanishing flux, and because of its divergences
which are due to the infinite blue shift effects associated with the horizons.
Using the conditional value of , we reveal the existence of
correlations between created particles and their partners in a domain where the
mean fluxes and the two-point function vanish. This demonstrates that the
scattering by an accelerated mirror leads to a steady conversion of vacuum
fluctuations into pairs of quanta. Finally, we study the scattering by two
uniformly accelerated mirrors which follow symmetrical trajectories (i.e. which
possess the same horizons). When using the Davies-Fulling model, the Bogoliubov
coefficients encoding pair creation vanish because of perfectly destructive
interferences. When using regularized amplitudes, these interferences are
inevitably lost thereby giving rise to pair creation.Comment: 30 pages, 9 postscript figure
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Low-index quantum-barrier single-pass tapered semiconductor optical amplifiers for efficient coherent beam combining
The requirements for coherent combination of high power GaAs-based single-pass tapered amplifiers are studied. Changes to the epitaxial layer structure are shown to bring higher beam quality and hence improved combining efficiency for one fixed device geometry. Specifically, structures with large vertical near field and low wave-guiding from the active region show 10% higher beam quality and coherent combining efficiency than reference devices. As a result, coherent combining efficiency is shown to be limited by beam quality, being directly proportional to the power content in the central lobe across a wide range of devices with different construction. In contrast, changes to the in-plane structure did not improve beam quality or combining efficiency. Although poor beam quality does correlate with increased optical intensities near the input aperture, locating monolithically-integrated absorption regions in these areas did not lead to any performance improvement. However, large area devices with subsequently improved cooling do achieve higher output powers. Phase noise can limit coherent combining, but this is shown to be small and independent of device design. Overall, tapered amplifiers are well suited for high power coherent combining applications. © 2020 The Author(s). Published by IOP Publishing Ltd
Thermonuclear Burning Regimes and the Use of SNe Ia in Cosmology
The calculations of the light curves of thermonuclear supernovae are carried
out by a method of multi-group radiation hydrodynamics. The effects of spectral
lines and expansion opacity are taken into account. The predictions for UBVI
fluxes are given. The values of rise time for B and V bands found in our
calculations are in good agreement with the observed values. We explain why our
results for the rise time have more solid physical justification than those
obtained by other authors. It is shown that small variations in the chemical
composition of the ejecta, produced in the explosions with different regimes of
nuclear burning, can influence drastically the light curve decline in the B
band and, to a lesser extent, in the V band. We argue that recent results on
positive cosmological constant Lambda, found from the high redshift supernova
observations, could be wrong in the case of possible variations of the
preferred mode of nuclear burning in the earlier Universe.Comment: 20 pages, 5 figures, presented at the conference "Astronomy at the
Eve of the New Century", Puschino, May 17-22, 1999. A few references and a
table added, typos correcte
Creation of effective magnetic fields in optical lattices: The Hofstadter butterfly for cold neutral atoms
We investigate the dynamics of neutral atoms in a 2D optical lattice which
traps two distinct internal states of the atoms in different columns. Two Raman
lasers are used to coherently transfer atoms from one internal state to the
other, thereby causing hopping between the different columns. By adjusting the
laser parameters appropriately we can induce a non vanishing phase of particles
moving along a closed path on the lattice. This phase is proportional to the
enclosed area and we thus simulate a magnetic flux through the lattice. This
setup is described by a Hamiltonian identical to the one for electrons on a
lattice subject to a magnetic field and thus allows us to study this equivalent
situation under very well defined controllable conditions. We consider the
limiting case of huge magnetic fields -- which is not experimentally accessible
for electrons in metals -- where a fractal band structure, the Hofstadter
butterfly, characterizes the system.Comment: 6 pages, RevTe
Rf-induced transport of Cooper pairs in superconducting single electron transistors in a dissipative environment
We investigate low-temperature and low-voltage-bias charge transport in a
superconducting Al single electron transistor in a dissipating environment,
realized as on-chip high-ohmic Cr microstrips. In our samples with relatively
large charging energy values Ec > EJ, where EJ is the energy of the Josephson
coupling, two transport mechanisms were found to be dominating, both based on
discrete tunneling of individual Cooper pairs: Depending on the gate voltage
Vg, either sequential tunneling of pairs via the transistor island (in the open
state of the transistor around the points Qg = CgVg = e mod(2e), where Cg is
the gate capacitance) or their cotunneling through the transistor (for Qg away
of these points) was found to prevail in the net current. As the open states of
our transistors had been found to be unstable with respect to quasiparticle
poisoning, high-frequency gate cycling (at f ~ 1 MHz) was applied to study the
sequential tunneling mechanism. A simple model based on the master equation was
found to be in a good agreement with the experimental data.Comment: 8 pages, 6 figure
Discretized Diffusion Processes
We study the properties of the ``Rigid Laplacian'' operator, that is we
consider solutions of the Laplacian equation in the presence of fixed
truncation errors. The dynamics of convergence to the correct analytical
solution displays the presence of a metastable set of numerical solutions,
whose presence can be related to granularity. We provide some scaling analysis
in order to determine the value of the exponents characterizing the process. We
believe that this prototype model is also suitable to provide an explanation of
the widespread presence of power-law in social and economic system where
information and decision diffuse, with errors and delay from agent to agent.Comment: 4 pages 5 figure, to be published in PR
Whole genome MBD-seq reveals different CpG methylation patterns in azacytidine-treated Juvenile Myelomonocytic Leukaemia (JMML) patients
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