1,487 research outputs found
Acceleration radiation, transition probabilities, and trans-Planckian physics
An important question in the derivation of the acceleration radiation, which
also arises in Hawking's derivation of black hole radiance, is the need to
invoke trans-Planckian physics for the quantum field that originates the
created quanta. We point out that this issue can be further clarified by
reconsidering the analysis in terms of particle detectors, transition
probabilities, and local two-point functions. By writing down separate
expressions for the spontaneous- and induced-transition probabilities of a
uniformly accelerated detector, we show that the bulk of the effect comes from
the natural (non trans-Planckian) scale of the problem, which largely
diminishes the importance of the trans-Planckian sector. This is so, at least,
when trans-Planckian physics is defined in a Lorentz invariant way. This
analysis also suggests how to define and estimate the role of trans-Planckian
physics in the Hawking effect itself.Comment: 19 page
Scheduling data flow program in xkaapi: A new affinity based Algorithm for Heterogeneous Architectures
Efficient implementations of parallel applications on heterogeneous hybrid
architectures require a careful balance between computations and communications
with accelerator devices. Even if most of the communication time can be
overlapped by computations, it is essential to reduce the total volume of
communicated data. The literature therefore abounds with ad-hoc methods to
reach that balance, but that are architecture and application dependent. We
propose here a generic mechanism to automatically optimize the scheduling
between CPUs and GPUs, and compare two strategies within this mechanism: the
classical Heterogeneous Earliest Finish Time (HEFT) algorithm and our new,
parametrized, Distributed Affinity Dual Approximation algorithm (DADA), which
consists in grouping the tasks by affinity before running a fast dual
approximation. We ran experiments on a heterogeneous parallel machine with six
CPU cores and eight NVIDIA Fermi GPUs. Three standard dense linear algebra
kernels from the PLASMA library have been ported on top of the Xkaapi runtime.
We report their performances. It results that HEFT and DADA perform well for
various experimental conditions, but that DADA performs better for larger
systems and number of GPUs, and, in most cases, generates much lower data
transfers than HEFT to achieve the same performance
Anomalous transport in Charney-Hasegawa-Mima flows
Transport properties of particles evolving in a system governed by the
Charney-Hasegawa-Mima equation are investigated. Transport is found to be
anomalous with a non linear evolution of the second moments with time. The
origin of this anomaly is traced back to the presence of chaotic jets within
the flow. All characteristic transport exponents have a similar value around
, which is also the one found for simple point vortex flows in the
literature, indicating some kind of universality. Moreover the law
linking the trapping time exponent within jets to the transport
exponent is confirmed and an accumulation towards zero of the spectrum of
finite time Lyapunov exponent is observed. The localization of a jet is
performed, and its structure is analyzed. It is clearly shown that despite a
regular coarse grained picture of the jet, motion within the jet appears as
chaotic but chaos is bounded on successive small scales.Comment: revised versio
Amorphization kinectics under swift heavy ion irradiation: a cumulative overlapping-track approach
A simple illustrative physical model is presented to describe the kinetics of damage and amorphization by swiftheavyions (SHI) in LiNbO3. The model considers that every ion impact generates initially a defective region (halo) and a full amorphous core whose relative size depends on the electronic stopping power. Below a given stopping power threshold only a halo is generated. For increasing fluences the amorphized area grows monotonically via overlapping of a fixed number N of halos. In spite of its simplicity the model, which provides analytical solutions, describes many relevant features of the kinetic behaviour. In particular, it predicts approximate Avrami curves with parameters depending on stopping power in qualitative accordance with experiment that turn into Poisson laws well above the threshold valu
Detailed black hole state counting in loop quantum gravity
We give a complete and detailed description of the computation of black hole
entropy in loop quantum gravity by employing the most recently introduced
number-theoretic and combinatorial methods. The use of these techniques allows
us to perform a detailed analysis of the precise structure of the entropy
spectrum for small black holes, showing some relevant features that were not
discernible in previous computations. The ability to manipulate and understand
the spectrum up to the level of detail that we describe in the paper is a
crucial step towards obtaining the behavior of entropy in the asymptotic (large
horizon area) regime
Nanoscale piezoelectric response across a single antiparallel ferroelectric domain wall
Surprising asymmetry in the local electromechanical response across a single
antiparallel ferroelectric domain wall is reported. Piezoelectric force
microscopy is used to investigate both the in-plane and out-of- plane
electromechanical signals around domain walls in congruent and
near-stoichiometric lithium niobate. The observed asymmetry is shown to have a
strong correlation to crystal stoichiometry, suggesting defect-domain wall
interactions. A defect-dipole model is proposed. Finite element method is used
to simulate the electromechanical processes at the wall and reconstruct the
images. For the near-stoichiometric composition, good agreement is found in
both form and magnitude. Some discrepancy remains between the experimental and
modeling widths of the imaged effects across a wall. This is analyzed from the
perspective of possible electrostatic contributions to the imaging process, as
well as local changes in the material properties in the vicinity of the wall
Large-Eddy Simulations of Fluid and Magnetohydrodynamic Turbulence Using Renormalized Parameters
In this paper a procedure for large-eddy simulation (LES) has been devised
for fluid and magnetohydrodynamic turbulence in Fourier space using the
renormalized parameters. The parameters calculated using field theory have been
taken from recent papers by Verma [Phys. Rev. E, 2001; Phys. Plasmas, 2001]. We
have carried out LES on grid. These results match quite well with direct
numerical simulations of . We show that proper choice of parameter is
necessary in LES.Comment: 12 pages, 4 figures: Proper figures inserte
Sampling the diffusion paths of a neutral vacancy in Silicon with quantum mechanical calculations
We report a first-principles study of vacancy-induced self-diffusion in
crystalline silicon. Starting form a fully relaxed configuration with a neutral
vacancy, we proceed to search for local diffusion paths. The diffusion of the
vacancy proceeds by hops to first nearest neighbor with an energy barrier of
0.40 eV in agreement with experimental results. Competing mechanisms are
identified, like the reorientation, and the recombination of dangling bonds by
Wooten-Winer-Weaire process.Comment: 10 pages, 5 figures, accepted for publication in Pysical review
Dy3+- and Pr3+-doped phosphate glass optical fibres for laser emission at visible wavelengths
Visible lasers find applications in many fields such as medicine, materials processing, display and entertainment technology, microscopy and scientific research
The SU(2) black hole entropy revisited
We study the state-counting problem that arises in the SU(2) black hole entropy calculation in loop quantum gravity. More precisely, we compute the leading term and the logarithmic correction of both the spherically symmetric and the distorted SU( 2) black holes. Contrary to what has been done in previous works, we have to take into account "quantum corrections" in our framework in the sense that the level k of the Chern-Simons theory which describes the black hole is finite and not sent to infinity. Therefore, the new results presented here allow for the computation of the entropy in models where the quantum group corrections are important
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