7,414 research outputs found
Nonlinear envelope equation for broadband optical pulses in quadratic media
We derive a nonlinear envelope equation to describe the propagation of
broadband optical pulses in second order nonlinear materials. The equation is
first order in the propagation coordinate and is valid for arbitrarily wide
pulse bandwidth. Our approach goes beyond the usual coupled wave description of
phenomena and provides an accurate modelling of the evolution of
ultra-broadband pulses also when the separation into different coupled
frequency components is not possible or not profitable
A Heuristic Strategy to Compute Ensemble of Trajectories for 3D Low Cost Earth-Moon Transfers
The problem of finding optimal trajectories is essential for modern space mission design. When considering multibody
gravitational dynamics and exploiting both low-thrust and high-thrust and alternative forms of propulsion such
as solar sailing, sets of good initial guesses are fundamental for the convergence to local or global optimal solutions,
using both direct or indirect methods available to solve the optimal control problem. This paper deals with obtaining
preliminary trajectories that are designed to be good initial guesses as input to search optimal low-energy short-time
Earth-Moon transfers with ballistic capture. A more realistic modelling is introduced, in which the restricted four-body
system Sun-Earth-Moon-Spacecraft is decoupled in two patched planar Circular Restricted Three-Body Problems,
taking into account the inclination of the orbital plane of the Moon with respect to the ecliptic. We present a heuristic
strategy based on the hyperbolic invariant manifolds of the Lyapunov orbits around the Lagrangian points of the Earth-
Moon system to obtain ballistic capture orbits around the Moon that fulfill specific mission requirements. Moreover,
quasi-periodic orbits of the Sun-Earth system are exploited using a genetic algorithm to find optimal solutions with
respect to total Dv, time of flight and altitude at departure. Finally, the procedure is illustrated and the full transfer
trajectories assessed in view of relevant properties. The proposed methodology provides sets of low-cost and shorttime
initial guesses to serve as inputs to compute fully optimized three-dimensional solutions considering different
propulsion technologies, such as low, high, and hybrid thrust, and/or using more realistic models
Future weak lensing constraints in a dark coupled universe
Coupled cosmologies can predict values for the cosmological parameters at low
redshifts which may differ substantially from the parameters values within
non-interacting cosmologies. Therefore, low redshift probes, as the growth of
structure and the dark matter distribution via galaxy and weak lensing surveys
constitute a unique tool to constrain interacting dark sector models. We focus
here on weak lensing forecasts from future Euclid and LSST-like surveys
combined with the ongoing Planck cosmic microwave background experiment. We
find that these future data could constrain the dimensionless coupling to be
smaller than a few . The coupling parameter is strongly
degenerate with the cold dark matter energy density and the
Hubble constant .These degeneracies may cause important biases in the
cosmological parameter values if in the universe there exists an interaction
among the dark matter and dark energy sectors.Comment: 8 pages, 6 figure
Constraining Modified Gravity with Euclid
Future proposed satellite missions as Euclid can offer the opportunity to
test general relativity on cosmic scales through mapping of the galaxy weak
lensing signal. In this paper we forecast the ability of these experiments to
constrain modified gravity scenarios as those predicted by scalar-tensor and
theories. We found that Euclid will improve constraints expected from
the PLANCK satellite on these modified gravity models by two orders of
magnitude. We discuss parameter degeneracies and the possible biases introduced
by modified gravity
Irreversible thermodynamics of open chemical networks I: Emergent cycles and broken conservation laws
In this and a companion paper we outline a general framework for the
thermodynamic description of open chemical reaction networks, with special
regard to metabolic networks regulating cellular physiology and biochemical
functions. We first introduce closed networks "in a box", whose thermodynamics
is subjected to strict physical constraints: the mass-action law, elementarity
of processes, and detailed balance. We further digress on the role of solvents
and on the seemingly unacknowledged property of network independence of free
energy landscapes. We then open the system by assuming that the concentrations
of certain substrate species (the chemostats) are fixed, whether because
promptly regulated by the environment via contact with reservoirs, or because
nearly constant in a time window. As a result, the system is driven out of
equilibrium. A rich algebraic and topological structure ensues in the network
of internal species: Emergent irreversible cycles are associated to
nonvanishing affinities, whose symmetries are dictated by the breakage of
conservation laws. These central results are resumed in the relation between the number of fundamental affinities , that of broken
conservation laws and the number of chemostats . We decompose the
steady state entropy production rate in terms of fundamental fluxes and
affinities in the spirit of Schnakenberg's theory of network thermodynamics,
paving the way for the forthcoming treatment of the linear regime, of
efficiency and tight coupling, of free energy transduction and of thermodynamic
constraints for network reconstruction.Comment: 18 page
Liquid-vapor interface of a polydisperse fluid
We report a Grand Canonical Monte Carlo simulation study of the liquid-vapor
interface of a model fluid exhibiting polydispersity in terms of the particle
size . The bulk density distribution, , of the system
is controlled by the imposed chemical potential distribution . We
choose the latter such that assumes a Schulz form with
associated degree of polydispersity . By introducing a smooth
attractive wall, a planar liquid-vapor interface is formed for bulk state
points within the region of liquid-vapor coexistence. Owing to fractionation,
the pure liquid phase is enriched in large particles, with respect to the
coexisting vapor. We investigate how the spatial non-uniformity of the density
near the liquid-vapor interface affects the evolution of the local distribution
of particle sizes between the limiting pure phase forms. We find (as previously
predicted by density functional theory, Bellier-Castella {\em et al}, Phys.
Rev. {\bf E65}, 021503 (2002)) a segregation of smaller particles to the
interface. The magnitude of this effect is quantified for various via
measurements of the relative adsorption. Additionally, we consider the utility
of various estimators for the interfacial width and highlight the difficulties
of isolating the intrinsic contribution of polydispersity to this width.Comment: 9 pages, 10 Fig
Magnetism in nanometer-thick magnetite
The oldest known magnetic material, magnetite, is of current interest for use
in spintronics as a thin film. An open question is how thin can magnetite films
be and still retain the robust ferrimagnetism required for many applications.
We have grown one-nanometer-thick magnetite crystals and characterized them in
situ by electron and photoelectron microscopies including selected-area x-ray
circular dichroism. Well-defined magnetic patterns are observed in individual
nano-crystals up to at least 520 K, establishing the retention of
ferrimagnetism in magnetite two-unit-cells thick.Comment: 5 pages, 4 figure
Financial instability from local market measures
We study the emergence of instabilities in a stylized model of a financial
market, when different market actors calculate prices according to different
(local) market measures. We derive typical properties for ensembles of large
random markets using techniques borrowed from statistical mechanics of
disordered systems. We show that, depending on the number of financial
instruments available and on the heterogeneity of local measures, the market
moves from an arbitrage-free phase to an unstable one, where the complexity of
the market - as measured by the diversity of financial instruments - increases,
and arbitrage opportunities arise. A sharp transition separates the two phases.
Focusing on two different classes of local measures inspired by real markets
strategies, we are able to analytically compute the critical lines,
corroborating our findings with numerical simulations.Comment: 17 pages, 4 figure
Enhanced magnetic moment and conductive behavior in NiFe2O4 spinel ultrathin films
Bulk NiFe2O4 is an insulating ferrimagnet. Here, we report on the epitaxial
growth of spinel NiFe2O4 ultrathin films onto SrTiO3 single-crystals. We will
show that - under appropriate growth conditions - epitaxial stabilization leads
to the formation of a spinel phase with magnetic and electrical properties that
radically differ from those of the bulk material : an enhanced magnetic moment
(Ms) - about 250% larger - and a metallic character. A systematic study of the
thickness dependence of Ms allows to conclude that its enhanced value is due to
an anomalous distribution of the Fe and Ni cations among the A and B sites of
the spinel structure resulting from the off-equilibrium growth conditions and
to interface effects. The relevance of these findings for spinel- and, more
generally, oxide-based heterostructures is discussed. We will argue that this
novel material could be an alternative ferromagetic-metallic electrode in
magnetic tunnel junctions.Comment: accepted for publication in Phys. Rev.
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