29,658 research outputs found
Generation of coherent two-color pulses at the two adjacent harmonics in a seeded free-electron laser
The growing requirements of pump-probe techniques and nonlinear optics
experiments greatly promote the studies of two-color free-electron lasers
(FELs). We propose a new method to generate coherent two-color pulses in a
high-gain harmonic generation (HGHG) FEL. In this scheme, an initial tilted
electron beam is sent though the modulator and dispersive section of an HGHG
FEL to generate the bunching at harmonics of the seed laser. Then a transverse
gradient undulator (TGU) is adopted as the radiator and in such radiator, only
two separated fractions of the tilted beam will resonate at two adjacent
harmonics of the seed laser and are enabled to emit the coherent two-color
pulses simultaneously. The time separation between the two pulses are on the
order of hundreds of femtoseconds, and can be precisely controlled by varying
the tilted amplitude of the electron beam and/or the transverse gradient of the
TGU radiator. Numerical simulations confirm the validity and feasibility of
this scheme in the EUV waveband
Heterogeneity in structurally arrested hard spheres
When cooled or compressed sufficiently rapidly, a liquid vitrifies into a glassy amorphous state. Vitrification in a dense liquid is associated with jamming of the particles. For hard spheres, the density and degree of order in the final structure depend on the compression rate: simple intuition suggests, and previous computer simulation demonstrates, that slower compression results in states that are both denser and more ordered. In this work, we use the Lubachevsky-Stillinger algorithm to generate a sequence of structurally arrested hard-sphere states by varying the compression rate. We find that while the degree of order, as measured by both bond-orientation and translation order parameters, increases monotonically with decreasing compression rate, the density of the arrested state first increases, then decreases, then increases again, as the compression rate decreases, showing a minimum at an intermediate compression rate. Examination of the distribution of the local order parameters and the distribution of the root-mean-square fluctuation of the particle positions, as well as direct visual inspection of the arrested structures, reveal that they are structurally heterogeneous, consisting of disordered, amorphous regions and locally ordered crystal-like domains. In particular, the low-density arrested states correspond with many interconnected small crystal clusters that form a polycrystalline network interspersed in an amorphous background, suggesting that jamming by the domains may be an important mechanism for these states
Dynamics and correlation length scales of a glass-forming liquid in quiescent and sheared conditions
We numerically study dynamics and correlation length scales of a colloidal
liquid in both quiescent and sheared conditions to further understand the
origin of slow dynamics and dynamic heterogeneity in glass-forming systems. The
simulation is performed in a weakly frustrated two-dimensional liquid, where
locally preferred order is allowed to develop with increasing density. The
four-point density correlations and bond-orientation correlations, which have
been frequently used to capture dynamic and static length scales in a
quiescent condition, can be readily extended to a system under steady shear in
this case. In the absence of shear, we confirmed the previous findings that the
dynamic slowing down accompanies the development of dynamic heterogeneity. The
dynamic and static length scales increase with -relaxation time
as power-law with . In the
presence of shear, both viscosity and have power-law dependence
on shear rate in the marked shear thinning regime. However, dependence of
correlation lengths cannot be described by power laws in the same regime.
Furthermore, the relation between length scales
and dynamics holds for not too strong shear where thermal fluctuations and
external forces are both important in determining the properties of dense
liquids. Thus, our results demonstrate a link between slow dynamics and
structure in glass-forming liquids even under nonequilibrium conditions.Comment: 9 pages, 17 figures. Accepted by J. Phys.: Condens. Matte
Threshold Resummation Effects in Neutral Higgs Boson Production by Bottom Quark Fusion at the CERN Large Hadron Collider
We investigate the QCD effects in the production of neutral Higgs bosons via
bottom quark fusion in both the standard model and the minimal supersymmetric
standard model at the CERN Large Hadron Collider. We include the
next-to-leading order (NLO) QCD corrections (including supersymmetric QCD) and
the threshold resummation effects. We use the soft-collinear effective theory
to resum the large logarithms near threshold from soft gluon emission. Our
results show that the resummation effects can enhance the total cross sections
by about 5% compared with the NLO results.Comment: 29pages, 14 figures, version to appear in Physical Review
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