69,742 research outputs found
Fronts with a Growth Cutoff but Speed Higher than
Fronts, propagating into an unstable state , whose asymptotic speed
is equal to the linear spreading speed of infinitesimal
perturbations about that state (so-called pulled fronts) are very sensitive to
changes in the growth rate for . It was recently found
that with a small cutoff, for ,
converges to very slowly from below, as . Here we show
that with such a cutoff {\em and} a small enhancement of the growth rate for
small behind it, one can have , {\em even} in the
limit . The effect is confirmed in a stochastic lattice model
simulation where the growth rules for a few particles per site are accordingly
modified.Comment: 4 pages, 4 figures, to appear in Rapid Comm., Phys. Rev.
Theory of Orbital Kondo Effect with Assisted Hopping in Strongly Correlated Electron Systems: Parquet Equations, Superconductivity and Mass Enhancement
Orbital Kondo effect is treated in a model, where additional to the
conduction band there are localized orbitals close to the Fermi energy. If the
hopping between the conduction band and the localized heavy orbitals depends on
the occupation of the atomic orbitals in the conduction band then orbital Kondo
correlation occurs. The noncommutative nature of the coupling required for the
Kondo effect is formally due to the form factors associated with the assisted
hopping which in the momentum representation depends on the momenta of the
conduction electrons involved. The leading logarithmic vertex corrections are
due to the local Coulomb interaction between the electrons on the heavy orbital
and in the conduction band. The renormalized vertex functions are obtained as a
solution of a closed set of differential equations and they show power
behavior. The amplitude of large renormalization is determined by an infrared
cutoff due to finite energy and dispersion of the heavy particles. The enhanced
assisted hopping rate results in mass enhancement and attractive interaction in
the conduction band. The superconductivity transition temperature calculated is
largest for intermediate mass enhancement, . For larger mass
enhancement the small one particle weight () in the Green's function reduces
the transition temperature which may be characteristic for otherComment: 32 pages, RevTeX 3.0, figures on reques
Performance Analysis and Enhancement of Multiband OFDM for UWB Communications
In this paper, we analyze the frequency-hopping orthogonal frequency-division
multiplexing (OFDM) system known as Multiband OFDM for high-rate wireless
personal area networks (WPANs) based on ultra-wideband (UWB) transmission.
Besides considering the standard, we also propose and study system performance
enhancements through the application of Turbo and Repeat-Accumulate (RA) codes,
as well as OFDM bit-loading. Our methodology consists of (a) a study of the
channel model developed under IEEE 802.15 for UWB from a frequency-domain
perspective suited for OFDM transmission, (b) development and quantification of
appropriate information-theoretic performance measures, (c) comparison of these
measures with simulation results for the Multiband OFDM standard proposal as
well as our proposed extensions, and (d) the consideration of the influence of
practical, imperfect channel estimation on the performance. We find that the
current Multiband OFDM standard sufficiently exploits the frequency selectivity
of the UWB channel, and that the system performs in the vicinity of the channel
cutoff rate. Turbo codes and a reduced-complexity clustered bit-loading
algorithm improve the system power efficiency by over 6 dB at a data rate of
480 Mbps.Comment: 32 pages, 10 figures, 1 table. Submitted to the IEEE Transactions on
Wireless Communications (Sep. 28, 2005). Minor revisions based on reviewers'
comments (June 23, 2006
Cooperative behavior of quantum dipole emitters coupled to a zero-index nanoscale waveguide
We study cooperative behavior of quantum dipole emitters coupled to a
rectangular waveguide with dielectric core and silver cladding. We investigate
cooperative emission and inter-emitter entanglement generation phenomena for
emitters whose resonant frequencies are near the frequency cutoff of the
waveguide, where the waveguide effectively behaves as zero-index metamaterial.
We show that coupling emitters to a zero-index waveguide allows one to relax
the constraint on precision positioning of emitters for observing inter-emitter
entanglement generation and extend the spatial scale at which the superradiance
can be observed
Stochastic Acceleration of He and He in Solar Flares by Parallel Propagating Plasma Waves: General Results
We study the acceleration in solar flares of He and He from a thermal
background by parallel propagating plasma waves with a general broken power-law
spectrum that takes into account the turbulence generation processes at large
scales and the thermal damping effects at small scales. The exact dispersion
relation for a cold plasma is used to describe the relevant wave modes. Because
low-energy -particles only interact with small scale waves in the
He-cyclotron branch, where the wave frequencies are below the
-particle gyro-frequency, their pitch angle averaged acceleration time
is at least one order of magnitude longer than that of He ions, which
mostly resonate with relatively higher frequency waves in the proton-cyclotron
(PC) branch. The -particle acceleration rate starts to approach that of
He beyond a few tens of keV nucleon, where -particles can
also interact with long wavelength waves in the PC branch. However, the He
acceleration rate is always smaller than that of He. Consequently, the
acceleration of He is suppressed significantly at low energies, and the
spectrum of the accelerated -particles is always softer than that of
He. The model gives reasonable account of the observed low-energy He
and He fluxes and spectra in the impulsive solar energetic particle events
observed with the {\it Advanced Composition Explorer}. We explore the model
parameter space to show how observations may be used to constrain the model.Comment: 29 pages, 11 Figures, Submitted to Ap
Alignment dependent enhancement of the photo-electron cutoff for multi-photon ionization of molecules
The multiphoton ionization rate of molecules depends on the alignment of the
molecular axis with respect to the ionizing laser polarization. By studying
molecular frame photo-electron angular distributions from N, O and
benzene, we illustrate how the angle-dependent ionization rate affects the
photo-electron cutoff energy. We find alignment can enhance the high energy
cutoff of the photo-electron spectrum when probing along a nodal plane or when
ionization is otherwise suppressed. This is supported by calculations using a
tunneling model with a single ion state.Comment: 4 pages, 4 figure
Thermal decoupling and the smallest subhalo mass in dark matter models with Sommerfeld-enhanced annihilation rates
We consider dark matter consisting of weakly interacting massive particles
(WIMPs) and revisit in detail its thermal evolution in the early universe, with
a particular focus on models where the annihilation rate is enhanced by the
Sommerfeld effect. After chemical decoupling, or freeze-out, dark matter no
longer annihilates but is still kept in local thermal equilibrium due to
scattering events with the much more abundant standard model particles. During
kinetic decoupling, even these processes stop to be effective, which eventually
sets the scale for a small-scale cutoff in the matter density fluctuations.
Afterwards, the WIMP temperature decreases more quickly than the heat bath
temperature, which causes dark matter to reenter an era of annihilation if the
cross-section is enhanced by the Sommerfeld effect. Here, we give a detailed
and self-consistent description of these effects. As an application, we
consider the phenomenology of simple leptophilic models that have been
discussed in the literature and find that the relic abundance can be affected
by as much two orders of magnitude or more. We also compute the mass of the
smallest dark matter subhalos in these models and find it to be in the range of
about 10^{-10} to 10 solar masses; even much larger cutoff values are possible
if the WIMPs couple to force carriers lighter than about 100 MeV. We point out
that a precise determination of the cutoff mass allows to infer new limits on
the model parameters, in particular from gamma-ray observations of galaxy
clusters, that are highly complementary to existing constraints from g-2 or
beam dump experiments.Comment: minor changes to match published versio
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