1,403 research outputs found
Optical supercavitation in soft-matter
We investigate theoretically, numerically and experimentally nonlinear
optical waves in an absorbing out-of-equilibrium colloidal material at the
gelification transition. At sufficiently high optical intensity, absorption is
frustrated and light propagates into the medium. The process is mediated by the
formation of a matter-shock wave due to optically induced thermodiffusion, and
largely resembles the mechanism of hydrodynamical supercavitation, as it is
accompanied by a dynamic phase-transition region between the beam and the
absorbing material.Comment: 4 pages, 5 figures, revised version: corrected typos and reference
The frustrated Brownian motion of nonlocal solitary waves
We investigate the evolution of solitary waves in a nonlocal medium in the
presence of disorder. By using a perturbational approach, we show that an
increasing degree of nonlocality may largely hamper the Brownian motion of
self-trapped wave-packets. The result is valid for any kind of nonlocality and
in the presence of non-paraxial effects. Analytical predictions are compared
with numerical simulations based on stochastic partial differential equationComment: 4 pages, 3 figures
Kinetic equation for a dense soliton gas
We propose a general method to derive kinetic equations for dense soliton
gases in physical systems described by integrable nonlinear wave equations. The
kinetic equation describes evolution of the spectral distribution function of
solitons due to soliton-soliton collisions. Owing to complete integrability of
the soliton equations, only pairwise soliton interactions contribute to the
solution and the evolution reduces to a transport of the eigenvalues of the
associated spectral problem with the corresponding soliton velocities modified
by the collisions. The proposed general procedure of the derivation of the
kinetic equation is illustrated by the examples of the Korteweg -- de Vries
(KdV) and nonlinear Schr\"odinger (NLS) equations. As a simple physical example
we construct an explicit solution for the case of interaction of two cold NLS
soliton gases.Comment: 4 pages, 1 figure, final version published in Phys. Rev. Let
Higher-order splitting algorithms for solving the nonlinear Schr\"odinger equation and their instabilities
Since the kinetic and the potential energy term of the real time nonlinear
Schr\"odinger equation can each be solved exactly, the entire equation can be
solved to any order via splitting algorithms. We verified the fourth-order
convergence of some well known algorithms by solving the Gross-Pitaevskii
equation numerically. All such splitting algorithms suffer from a latent
numerical instability even when the total energy is very well conserved. A
detail error analysis reveals that the noise, or elementary excitations of the
nonlinear Schr\"odinger, obeys the Bogoliubov spectrum and the instability is
due to the exponential growth of high wave number noises caused by the
splitting process. For a continuum wave function, this instability is
unavoidable no matter how small the time step. For a discrete wave function,
the instability can be avoided only for \dt k_{max}^2{<\atop\sim}2 \pi, where
.Comment: 10 pages, 8 figures, submitted to Phys. Rev.
Nonresponse of native Cottonwood trees to water additions during summer drought
Studies have demonstrated that some riparian trees may switch their reliance on surface soil water (unsaturated or vadose zone) to groundwater (saturated zone) sources during the growing season in association with changes in moisture availability. A closely related question is: How do these trees respond to pulse increases in water availability in previously dry zones? We tested the whole-tree physiological response of 6 natural Populus genotypes to water additions during the peak of summer drought in northern Utah, USA. We found clear evidence that trees were insensitive to water additions to the surface soil that were twice the magnitude of whole-tree transpiration rates. Our results suggest that some cottonwoods may have little immediate transpiration of leaf conductance response to pulse soil moisture increases. This lack of response may be related to water-use strategy associated with regional climate patterns (i.e. genetic or environmental programming), cavitation recovery, or other physical determinants of water use such as depth to groundwater. Our data suggest that it is important to consider potential nonresponsiveness to changes in soil water availability when evaluating the impact of climate change on these important and productive ecosystems
Variational Approach to Gaussian Approximate Coherent States: Quantum Mechanics and Minisuperspace Field Theory
This paper has a dual purpose. One aim is to study the evolution of coherent
states in ordinary quantum mechanics. This is done by means of a Hamiltonian
approach to the evolution of the parameters that define the state. The
stability of the solutions is studied. The second aim is to apply these
techniques to the study of the stability of minisuperspace solutions in field
theory. For a theory we show, both by means of perturbation
theory and rigorously by means of theorems of the K.A.M. type, that the
homogeneous minisuperspace sector is indeed stable for positive values of the
parameters that define the field theory.Comment: 26 pages, Plain TeX, no figure
Within-species variation in foliar chemistry influences aquatic leaf litter decomposition
Leaf-litter inputs provide substrate and energy to stream systems. These contributions vary based on species-specific differences in litter quality, but little is known about how differences in litter quality within a species can affect ecosystem processes. Genetic variation within tree species, such as oaks and cottonwoods, affects ecosystem processes including decomposition and nutrient cycling in forest ecosystems and has the potential to do the same in streams. We collected litter from 5 genotypes of each of 4 different cottonwood cross types (Populus fremontii, Populus angustifolia, and natural F1 and backcross hybrids), grown in a common garden, and measured their decomposition rates using litter bags in the Weber River, Utah. The proportion of 35 species-specific P. fremontii restriction-fragment length polymorphism markers in the genotype explained 46% and genetically controlled phytochemical mechanisms (e.g., % soluble condensed tannin in litter) explained .72% of the variation in leaf-litter decomposition rate, respectively. Understanding how natural genetic variation in plants can affect ecosystem processes will provide baseline information with which to address the loss of genetic variation (through habitat fragmentation and global change) and altered genetic variation through hybridization with cultivars and transgenic manipulations in the wild
Scattering Parameter Measurements of the Long Wavelength Array Antenna and Front End Electronics
We present recent 2-port vector network analyzer (VNA) measurements of the
complete set of scattering parameters for the antenna used within the Long
Wavelength Array (LWA) and the associated front end electronics (FEEs). Full
scattering parameter measurements of the antenna yield not only the reflection
coefficient for each polarization, S11 and S22, but also the coupling between
polarizations, S12 and S21. These had been previously modeled using
simulations, but direct measurements had not been obtained until now. The
measurements are used to derive a frequency dependent impedance mismatch factor
(IMF) which represents the fraction of power that is passed through the
antenna-FEE interface and not reflected due to a mismatch between the impedance
of the antenna and the impedance of the FEE. We also present results from a two
antenna experiment where each antenna is hooked up to a separate port on the
VNA. This allows for cross-antenna coupling to be measured for all four
possible polarization combinations. Finally, we apply the newly measured IMF
and FEE forward gain corrections to LWA data to investigate how well they
remove instrumental effects.Comment: 16 pages, 12 figures, Accepted for publication in Publications of the
Astronomical Society of the Pacifi
Multiple hydrodynamical shocks induced by Raman effect in photonic crystal fibres
We theoretically predict the occurrence of multiple hydrodynamical-like shock
phenomena in the propagation of ultrashort intense pulses in a suitably
engineered photonic crystal fiber. The shocks are due to the Raman effect,
which acts as a nonlocal term favoring their generation in the focusing regime.
It is shown that the problem is mapped to shock formation in the presence of a
slope and a gravity-like potential. The signature of multiple shocks in XFROG
signals is unveiled
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