22,548 research outputs found
Homogenization and enhancement for the G-equation
We consider the so-called G-equation, a level set Hamilton-Jacobi equation,
used as a sharp interface model for flame propagation, perturbed by an
oscillatory advection in a spatio-temporal periodic environment. Assuming that
the advection has suitably small spatial divergence, we prove that, as the size
of the oscillations diminishes, the solutions homogenize (average out) and
converge to the solution of an effective anisotropic first-order
(spatio-temporal homogeneous) level set equation. Moreover we obtain a rate of
convergence and show that, under certain conditions, the averaging enhances the
velocity of the underlying front. We also prove that, at scale one, the level
sets of the solutions of the oscillatory problem converge, at long times, to
the Wulff shape associated with the effective Hamiltonian. Finally we also
consider advection depending on position at the integral scale
Quantum key distribution and 1 Gbit/s data encryption over a single fibre
We perform quantum key distribution (QKD) in the presence of 4 classical
channels in a C-band dense wavelength division multiplexing (DWDM)
configuration using a commercial QKD system. The classical channels are used
for key distillation and 1 Gbps encrypted communication, rendering the entire
system independent from any other communication channel than a single dedicated
fibre. We successfully distil secret keys over fibre spans of up to 50 km. The
separation between quantum channel and nearest classical channel is only 200
GHz, while the classical channels are all separated by 100 GHz. In addition to
that we discuss possible improvements and alternative configurations, for
instance whether it is advantageous to choose the quantum channel at 1310 nm or
to opt for a pure C-band configuration.Comment: 9 pages, 7 figure
Particle-Based Mesoscale Hydrodynamic Techniques
Dissipative particle dynamics (DPD) and multi-particle collision (MPC)
dynamics are powerful tools to study mesoscale hydrodynamic phenomena
accompanied by thermal fluctuations. To understand the advantages of these
types of mesoscale simulation techniques in more detail, we propose new two
methods, which are intermediate between DPD and MPC -- DPD with a multibody
thermostat (DPD-MT), and MPC-Langevin dynamics (MPC-LD). The key features are
applying a Langevin thermostat to the relative velocities of pairs of particles
or multi-particle collisions, and whether or not to employ collision cells. The
viscosity of MPC-LD is derived analytically, in very good agreement with the
results of numerical simulations.Comment: 7 pages, 2 figures, 1 tabl
Gravitational Radiation from Intermediate-Mass Black Holes
Recent X-ray observations of galaxies with ROSAT, ASCA, and Chandra have
revealed numerous bright off-center point sources which, if isotropic emitters,
are likely to be intermediate-mass black holes, with hundreds to thousands of
solar masses. The origin of these objects is under debate, but observations
suggest that a significant number of them currently reside in young
high-density stellar clusters. There is also growing evidence that some
Galactic globular clusters harbor black holes of similar mass, from
observations of stellar kinematics. In such high-density stellar environments,
the interactions of intermediate-mass black holes are promising sources of
gravitational waves for ground-based and space-based detectors. Here we explore
the signal strengths of binaries containing intermediate-mass black holes or
stellar-mass black holes in dense stellar clusters. We estimate that a few to
tens per year of these objects will be detectable during the last phase of
their inspiral with the advanced LIGO detector, and up to tens per year will be
seen during merger, depending on the spins of the black holes. We also find
that if these objects reside in globular clusters then tens of sources will be
detectable with LISA from the Galactic globular system in a five year
integration, and similar numbers will be detectable from more distant galaxies.
The signal strength depends on the eccentricity distribution, but we show that
there is promise for strong detection of pericenter precession and
Lense-Thirring precession of the orbital plane. We conclude by discussing what
could be learned about binaries, dense stellar systems, and strong gravity if
such signals are detected.Comment: Minor changes, accepted by ApJ (December 10, 2002
Formation and quenching mechanisms of the electron beam pumped (XeRb)+ ionic excimer in different buffer gases
The ionic excimer molecule XeRb+ is formed in an electron beam excited gas mixture of Xe, Rb, and a buffer gas. The formation and quenching mechanisms of ionic excimers are investigated by measuring the XeRb+fluorescence as a function of the gas composition and gas pressure. The formation of XeRb+ is achieved by a three‐body association reaction between Xe+, Rb, and a buffer gas atom. For the buffer gases He, Ne, or Ar the values of the important formationrate constants are determined from the observed fluorescence signal decay
Optical fibers with interferometric path length stability by controlled heating for transmission of optical signals and as components in frequency standards
We present a simple method to stabilize the optical path length of an optical
fiber to an accuracy of about 1/100 of the laser wavelength. We study the
dynamic response of the path length to modulation of an electrically conductive
heater layer of the fiber. The path length is measured against the laser
wavelength by use of the Pound-Drever-Hall method; negative feedback is applied
via the heater. We apply the method in the context of a cryogenic resonator
frequency standard.Comment: Expanded introduction and outlook. 9 pages, 5 figure
Doping dependence of the Neel temperature in Mott-Hubbard antiferromagnets: Effect of vortices
The rapid destruction of long-range antiferromagnetic order upon doping of
Mott-Hubbard antiferromagnetic insulators is studied within a generalized
Berezinskii-Kosterlitz-Thouless renormalization group theory in accordance with
recent calculations suggesting that holes dress with vortices. We calculate the
doping-dependent Neel temperature in good agreement with experiments for
high-Tc cuprates. Interestingly, the critical doping where long-range order
vanishes at zero temperature is predicted to be xc ~ 0.02, independently of any
energy scales of the system.Comment: 4 pages with 3 figures included, minor revisions, to be published in
PR
From the Heart of The Ghoul: C and N Abundances in the Corona of Algol B
Chandra Low Energy Transmission Grating Spectrograph observations of Algol
have been used to determine the abundances of C and N in the secondary star for
the first time. The analysis was performed relative to similar observations of
an adopted "standard" star HR 1099. It is demonstrated that HR 1099 and Algol
are coronal twins in many respects and that their X-ray spectra are very
similar in nearly all details, except for the observed strengths of C and N
lines. The H-like transitions of C and N in the coronae of Algol and HR 1099
demonstrate that the surface abundances of Algol B have been strongly modified
by CN-processing, as shown earlier by Schmitt & Ness (2002). It is found that N
is enhanced in Algol B by a factor of 3 compared to HR 1099. No C lines are
detected in the Algol spectrum, indicating a C depletion relative to HR 1099 by
a factor of 10 or more. These C and N abundances indicate that Algol B must
have lost at least half of its initial mass, and are consistent with
predictions of evolutionary models that include non-conservative mass transfer
and angular momentum loss through magnetic activity. Little or no dredge-up of
material subjected to CN-processing has occurred on the subgiant component of
HR 1099. It is concluded that Fe is very likely depleted in the coronae of both
Algol and HR 1099 relative to their photospheres by 0.5 dex, and C, N and O by
0.3 dex. Instead, Ne is enhanced by up to 0.5 dex.Comment: 17 pages, 4 figures, ApJ accepte
A Detailed Monte-Carlo Simulation for the Belle TOF System
We have developed a detailed Monte Carlo simulation program for the Belle TOF
system. Based on GEANT simulation, it takes account of all physics processes in
the TOF scintillation counters and readout electronics. The simulation
reproduces very well the performance of the Belle TOF system, including the
dE/dx response, the time walk effect, the time resolution, and the hit
efficiency due to beam background. In this report, we will describe the Belle
TOF simulation program in detail.Comment: To be submitted to NI
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