342,838 research outputs found
Multiband Emission from Pulsar Wind Nebulae: A Possible Injection Spectrum
A recent research shows that particles with a spectrum of a relativistic
Maxwellian plus a high-energy tail can be accelerated by relativistic
collisionless shocks. We investigate the possibility of the high-energy
particles with this new spectrum injected in pulsar wind nebulae (PWNe) from
the terminate shock based on the study of multiwavelength emission from PWNe.}
{The dynamics of a supernova remnant (SNR) and multiband nonthermal emission
from the PWN inside the remnant are investigated using a dynamical model with
electrons/positrons injected with the new spectrum. In this model, the
dynamical and radiative evolution of a pulsar wind nebula in a non-radiative
supernova remnant can be self-consistently described.} {This model is applied
to the three composite SNRs, G0.9+0.1, MSH 15-52, G338.3-0.0, and the multiband
observed emission from the three PWNe can be well reproduced.} {Our studies on
the three remnant provide evidence for the new spectrum of the particles, which
are accelerated by the terminate shock, injected into a PWN.Comment: 9 pages, 9 figures, accepted by A&
Diffractionless image propagation and frequency conversion via four-wave mixing exploiting the thermal motion of atoms
A setup to frequency-convert an arbitrary image encoded in the spatial
profile of a probe field onto a signal field using four-wave mixing in a
thermal atom vapor is proposed. The atomic motion is exploited to cancel
diffraction of both signal and probe fields simultaneously. We show that an
incoherent probe field can be used to enhance the transverse momentum bandwidth
which can be propagated without diffraction, such that smaller structures with
higher spatial resolution can be transmitted. It furthermore compensate linear
absorption with non-linear gain, to improve the four-wave mixing performance
since the propagation dynamics of the various field intensities is favorably
modified.Comment: 12 pages, 7 figure
The Galactic distribution of magnetic fields in molecular clouds and HII regions
{Magnetic fields exist on all scales in our Galaxy. There is a controversy
about whether the magnetic fields in molecular clouds are preserved from the
permeated magnetic fields in the interstellar medium (ISM) during cloud
formation. We investigate this controversy using available data in the light of
the newly revealed magnetic field structure of the Galactic disk obtained from
pulsar rotation measures (RMs).} % {We collected measurements of the magnetic
fields in molecular clouds, including Zeeman splitting data of OH masers in
clouds and OH or HI absorption or emission lines of clouds themselves.} % {The
Zeeman data show structures in the sign distribution of the line-of-sight
component of the magnetic field. Compared to the large-scale Galactic magnetic
fields derived from pulsar RMs, the sign distribution of the Zeeman data shows
similar large-scale field reversals. Previous such examinations were flawed in
the over-simplified global model used for the large-scale magnetic fields in
the Galactic disk.} % {We conclude that the magnetic fields in the clouds may
still ``remember'' the directions of magnetic fields in the Galactic ISM to
some extent, and could be used as complementary tracers of the large-scale
magnetic structure. More Zeeman data of OH masers in widely distributed clouds
are required.}Comment: Typo fixed in this new versio
First-principles investigation of transient current of molecular devices by using complex absorbing potential
Based on the non-equilibrium Green's function (NEGF) coupled with density
function theory (DFT), namely, NEGF-DFT quantum transport theory, we propose an
efficient formalism to calculate the transient current of molecular devices
under a step-like pulse from first principles. By combining NEGF-DFT with the
complex absorbing potential (CAP), the computational complexity of our
formalism (NEGF-DFT-CAP) is proportional to \emph{O}(N) where is the
number of time steps in the time-dependent transient calculation. Compared with
state-of-the-art algorithm of first principles time-dependent calculation that
scales with at least , this order N technique drastically reduces the
computational burden making it possible to tackle realistic molecular devices.
To ensure the accuracy of our method, we carry out the benchmark calculation
compared with exact NEGF-TDDFT formalism and they agree well with each other.
As an illustration, we investigate the transient current of molecular device
Al-C-Al from first principles
Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios
In the widely-studied two-color laser scheme for terahertz (THz) radiation
from a gas, the frequency ratio of the two lasers is usually fixed at
1:2. We investigate THz generation with uncommon frequency
ratios. Our experiments show, for the first time, efficient THz generation with
new ratios of 1:4 and 2:3. We observe that the THz
polarization can be adjusted by rotating the longer-wavelength laser
polarization and the polarization adjustment becomes inefficient by rotating
the other laser polarization; the THz energy shows similar scaling laws with
different frequency ratios. These observations are inconsistent with multi-wave
mixing theory, but support the gas-ionization model. This study pushes the
development of the two-color scheme and provides a new dimension to explore the
long-standing problem of the THz generation mechanism.Comment: 6 pages, 3 figure
An effective quasi-one-dimensional description of a spin-1 atomic condensate
Within the mean field theory we extend the effective quasi-1D non-polynomial
Schr\"{o}dinger equation (NPSE) approach to the description of a spin-1 atomic
condensate in a tight radial confinement geometry for both weak and strong
atom-atom interactions. Detailed comparisons with full time dependent 3D
numerical simulations show excellent agreement as in the case of a single
component scalar condensate, demonstrating our result as an efficient and
effective tool for the understanding of spin-1 condensate dynamics observed in
several recent experiments.Comment: 5 pages, 3 eps figures, to appear in Phys. Rev. A. Small typoes
corrections. Updated Reference
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Electricity pricing in China and the role of the state
Theoretical and empirical evidence is presented to show that inefficient power firms in China are subsidised by the state through the ability to charge high electricity prices and the creation of a “soft price constraint” on cost. This cost inefficiency challenges the merits of planned power supply
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