9,487 research outputs found
Strong energy enhancement in a laser-driven plasma-based accelerator through stochastic friction
Conventionally, friction is understood as an efficient dissipation mechanism
depleting a physical system of energy as an unavoidable feature of any
realistic device involving moving parts, e.g., in mechanical brakes. In this
work, we demonstrate that this intuitive picture loses validity in nonlinear
quantum electrodynamics, exemplified in a scenario where spatially random
friction counter-intuitively results in a highly directional energy flow. This
peculiar behavior is caused by radiation friction, i.e., the energy loss of an
accelerated charge due to the emission of radiation. We demonstrate
analytically and numerically how radiation friction can enhance the performance
of a specific class of laser-driven particle accelerators. We find the
unexpected directional energy boost to be due to the particles' energy being
reduced through friction whence the driving laser can accelerate them more
efficiently. In a quantitative case we find the energy of the laser-accelerated
particles to be enhanced by orders of magnitude.Comment: 14 pages, 3 figure
Superconductivity and Phase Diagram in (LiFe)OHFeSeS
A series of (LiFe)OHFeSeS (0 x 1)
samples were successfully synthesized via hydrothermal reaction method and the
phase diagram is established. Magnetic susceptibility suggests that an
antiferromagnetism arising from (LiFe)OH layers coexists with
superconductivity, and the antiferromagnetic transition temperature nearly
remains constant for various S doping levels. In addition, the lattice
parameters of the both a and c axes decrease and the superconducting transition
temperature T is gradually suppressed with the substitution of S for Se,
and eventually superconductivity vanishes at = 0.90. The decrease of T
could be attributed to the effect of chemical pressure induced by the smaller
ionic size of S relative to that of Se, being consistent with the effect of
hydrostatic pressure on (LiFe)OHFeSe. But the detailed
investigation on the relationships between and the crystallographic
facts suggests a very different dependence of on anion height from
the Fe2 layer or -Fe2- angle from those in FeAs-based superconductors.Comment: 6 pages, 6 figure
A 1.3 cm line survey toward IRC +10216
IRC +10216 is the prototypical carbon star exhibiting an extended molecular
circumstellar envelope. Its spectral properties are therefore the template for
an entire class of objects. The main goal is to systematically study the
1.3 cm spectral line characteristics of IRC +10216. We carried
out a spectral line survey with the Effelsberg-100 m telescope toward IRC
+10216. It covers the frequency range between 17.8 GHz and 26.3 GHz (K-band).
In the circumstellar shell of IRC +10216, we find 78 spectral lines, among
which 12 remain unidentified. The identified lines are assigned to 18 different
molecules and radicals. A total of 23 lines from species known to exist in this
envelope are detected for the first time outside the Solar System and there are
additional 20 lines first detected in IRC +10216. The potential orgin of "U"
lines is also discussed. Assuming local thermodynamic equilibrium (LTE), we
then determine rotational temperatures and column densities of 17 detected
molecules. Molecular abundances relative to H are also estimated. A
non-LTE analysis of NH shows that the bulk of its emission arises from
the inner envelope with a kinetic temperature of 7020 K. Evidence for
NH emitting gas with higher kinetic temperature is also obtained, and
potential abundance differences between various C-bearing isotopologues
of HCN are evaluated. Overall, the isotopic C/C ratio is
estimated to be 499. Finally, a comparison of detected molecules in the
1.3 cm range with the dark cloud TMC-1 indicates that
silicate-bearing molecules are more predominant in IRC +10216.Comment: 32 pages, 9 figures, Accepted by A&
A 1.3 cm Line Survey toward Orion KL
Orion KL has served as a benchmark for spectral line searches throughout the
(sub)millimeter regime. The main goal is to systematically study spectral
characteristics of Orion KL in the 1.3 cm band. We carried out a spectral line
survey (17.9 GHz to 26.2 GHz) with the Effelsberg-100 m telescope towards Orion
KL. We find 261 spectral lines, yielding an average line density of about 32
spectral features per GHz above 3. The identified lines include 164
radio recombination lines (RRLs) and 97 molecular lines. A total of 23
molecular transitions from species known to exist in Orion KL are detected for
the first time in the interstellar medium. Non-metastable 15NH3 transitions are
detected in Orion KL for the first time. Based on the velocity information of
detected lines and the ALMA images, the spatial origins of molecular emission
are constrained and discussed. A narrow feature is found in SO2
(), possibly suggesting the presence of a maser line. Column
densities and fractional abundances relative to H2 are estimated for 12
molecules with LTE methods. Rotational diagrams of non-metastable 14NH3
transitions with J=K+1 to J=K+4 yield different results; metastable 15NH3 is
found to have a higher excitation temperature than non-metastable 15NH3,
indicating that they may trace different regions. Elemental and isotopic
abundance ratios are estimated: 12C/13C=63+-17, 14N/15N=100+-51,
D/H=0.0083+-0.0045. The dispersion of the He/H ratios derived from
H/He pairs to H/He pairs is very small, which
is consistent with theoretical predictions that the departure coefficients bn
factors for hydrogen and helium are nearly identical. Based on a non-LTE code
neglecting excitation by the infrared radiation field and a likelihood
analysis, we find that the denser regions have lower kinetic temperature, which
favors an external heating of the Hot Core.Comment: 70 pages, 26 figures, 12 tables, accepted for publication in A&A.
Figs. 1, 2, 8, 9 have been downsize
Detailed Study of the Influence of InGaAs Matrix on the Strain Reduction in the InAs Dot-In-Well Structure
InAs/InGaAs dot-in-well (DWELL) structures have been investigated with the systematically varied InGaAs thickness. Both the strained buffer layer (SBL) below the dot layer and the strain-reducing layer (SRL) above the dot layer were found to be responsible for the redshift in photoluminescence (PL) emission of the InAs/InGaAs DWELL structure. A linear followed by a saturation behavior of the emission redshift was observed as a function of the SBL and SRL thickness, respectively. The PL intensity is greatly enhanced by applying both of the SRL and SBL. Finite element analysis simulation and transmission electron microscopy (TEM) measurement were carried out to analyze the strain distribution in the InAs QD and the InGaAs SBL. The results clearly indicate the strain reduction in the QD induced by the SBL, which are likely the main cause for the emission redshift
Photon Momentum Transfer in Single-Photon Double Ionization of Helium
We theoretically and experimentally investigate the photon momentum transfer in single-photon double ionization of helium at various large photon energies. We find that the forward shifts of the momenta along the light propagation of the two photoelectrons are roughly proportional to their fraction of the excess energy. The mean value of the forward momentum is about 8/5 of the electron energy divided by the speed of light. This holds for fast and slow electrons despite the fact that the energy sharing is highly asymmetric and the slow electron is known to be ejected by secondary processes of shake off and knockout rather than directly taking its energy from the photon. The biggest deviations from this rule are found for the region of equal energy sharing where the quasifree mechanism dominates double ionization
Frequency variations of gravity waves interacting with a time-varying tide
Using a nonlinear, 2-D time-dependent numerical model, we simulate the
propagation of gravity waves (GWs) in a time-varying tide. Our simulations
show that when a GW packet propagates in a time-varying tidal-wind
environment, not only its intrinsic frequency but also its ground-based
frequency would change significantly. The tidal horizontal-wind acceleration
dominates the GW frequency variation. Positive (negative) accelerations
induce frequency increases (decreases) with time. More interestingly,
tidal-wind acceleration near the critical layers always causes the GW
frequency to increase, which may partially explain the observations that
high-frequency GW components are more dominant in the middle and upper
atmosphere than in the lower atmosphere. The combination of the increased
ground-based frequency of propagating GWs in a time-varying tidal-wind field
and the transient nature of the critical layer induced by a time-varying
tidal zonal wind creates favorable conditions for GWs to penetrate their
originally expected critical layers. Consequently, GWs have an impact on the
background atmosphere at much higher altitudes than expected, which indicates
that the dynamical effects of tidal–GW interactions are more complicated
than usually taken into account by GW parameterizations in global models
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