8,908 research outputs found
Hermite Coherent States for Quadratic Refractive Index Optical Media
Producción CientíficaLadder and shift operators are determined for the set of Hermite–Gaussian modes associated with an optical medium with quadratic refractive index profile. These operators allow to establish irreducible representations of the su(1, 1) and su(2) algebras. Glauber coherent states, as well as su(1, 1) and su(2) generalized coherent states, were constructed as solutions of differential equations admitting separation of variables. The dynamics of these coherent states along the optical axis is also evaluated.MINECO grant MTM2014-57129-C2-1-P and Junta de Castilla y Leon grant VA057U16
Spin wave spectrum of the quantum ferromagnet on the pyrochlore lattice Lu2V2O7
Neutron inelastic scattering has been used to probe the spin dynamics of the
quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu2V2O7. Well-defined
spin waves are observed at all energies and wavevectors, allowing us to
determine the parameters of the Hamiltonian of the system. The data are found
to be in excellent overall agreement with a minimal model that includes a
nearest- neighbour Heisenberg exchange J = 8:22(2) meV and a
Dzyaloshinskii-Moriya interaction (DMI) D =1:5(1) meV. The large DMI term
revealed by our study is broadly consistent with the model developed by Onose
et al. to explain the magnon Hall effect they observed in Lu2V2O7 [1], although
our ratio of D=J = 0:18(1) is roughly half of their value and three times
larger than calculated by ab initio methods [2].Comment: 5 pages, 4 figure
Nature of the f_0(600) from its N_c dependence at two loops in unitarized Chiral Perturbation Theory
By using unitarized two-loop Chiral Perturbation Theory partial waves to
describe pion-pion scattering we find that the dominant component of the
lightest scalar meson does not follow the q-qbar dependence on the number of
colors that, in contrast, is obeyed by the lightest vectors. The method
suggests that a subdominant q-qbar component of the f_0(600) possibly
originates around 1 GeV.Comment: 4 pages, 1 Figure. To appear in Phys. Rev. Let
Model of the meniscus of an ionic liquid ion source.
A simple model of the transfer of charge and ion evaporation in the meniscus of an ionic-liquid ion source working in the purely ionic regime is proposed on the basis of order-of-magnitude estimates which show that, in this regime, _i_ the flow in the meniscus is dominated by the viscosity of the liquid and is affected very little by the mass flux accompanying ion evaporation, and _ii_ the effect of the space charge around the evaporating surface is negligible and the evaporation current is controlled by the finite electrical conductivity of the liquid. The model predicts that a stationary meniscus of a very polar liquid undergoing ion evaporation is nearly hydrostatic and can exist only below a certain value of the applied electric field, at which the meniscus attains its maximum elongation but stays smooth. The electric current vs applied electric field characteristic displays a frozen regime of negligible ion evaporation at low fields and a conduction-controlled regime at higher fields, with a sharp transition between the two regimes owing to the high sensitivity of the ion evaporation rate to the electric field. A simplified treatment of the flow in the capillary or liquid layer through which liquid is delivered to the meniscus shows that the size of the meniscus decreases and the maximum attainable current increases when the feeding pressure is decreased, and that appropriate combinations of feeding pressure and pressure drop may lead to high maximum currents
Self-Averaging in the Three Dimensional Site Diluted Heisenberg Model at the critical point
We study the self-averaging properties of the three dimensional site diluted
Heisenberg model. The Harris criterion \cite{critharris} states that disorder
is irrelevant since the specific heat critical exponent of the pure model is
negative. According with some analytical approaches \cite{harris}, this implies
that the susceptibility should be self-averaging at the critical temperature
(). We have checked this theoretical prediction for a large range of
dilution (including strong dilution) at critically and we have found that the
introduction of scaling corrections is crucial in order to obtain
self-averageness in this model. Finally we have computed critical exponents and
cumulants which compare very well with those of the pure model supporting the
Universality predicted by the Harris criterion.Comment: 11 pages, 11 figures, 14 tables. New analysis (scaling corrections in
the g2=0 scenario) and new numerical simulations. Title and conclusions
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The effects of intrinsic noise on the behaviour of bistable cell regulatory systems under quasi-steady state conditions
We analyse the effect of intrinsic fluctuations on the properties of bistable
stochastic systems with time scale separation operating under1 quasi-steady
state conditions. We first formulate a stochastic generalisation of the
quasi-steady state approximation based on the semi-classical approximation of
the partial differential equation for the generating function associated with
the Chemical Master Equation. Such approximation proceeds by optimising an
action functional whose associated set of Euler-Lagrange (Hamilton) equations
provide the most likely fluctuation path. We show that, under appropriate
conditions granting time scale separation, the Hamiltonian can be re-scaled so
that the set of Hamilton equations splits up into slow and fast variables,
whereby the quasi-steady state approximation can be applied. We analyse two
particular examples of systems whose mean-field limit has been shown to exhibit
bi-stability: an enzyme-catalysed system of two mutually-inhibitory proteins
and a gene regulatory circuit with self-activation. Our theory establishes that
the number of molecules of the conserved species are order parameters whose
variation regulates bistable behaviour in the associated systems beyond the
predictions of the mean-field theory. This prediction is fully confirmed by
direct numerical simulations using the stochastic simulation algorithm. This
result allows us to propose strategies whereby, by varying the number of
molecules of the three conserved chemical species, cell properties associated
to bistable behaviour (phenotype, cell-cycle status, etc.) can be controlled.Comment: 33 pages, 9 figures, accepted for publication in the Journal of
Chemical Physic
Ferromagnetism in the Two-Dimensional Periodic Anderson Model
Using the constrained-path Monte Carlo method, we studied the magnetic
properties of the two-dimensional periodic Anderson model for electron fillings
between 1/4 and 1/2. We also derived two effective low energy theories to
assist in interpreting the numerical results. For 1/4 filling we found that the
system can be a Mott or a charge transfer insulator, depending on the relative
values of the Coulomb interaction and the charge transfer gap between the two
non-interacting bands. The insulator may be a paramagnet or antiferromagnet. We
concentrated on the effect of electron doping on these insulating phases. Upon
doping we obtained a partially saturated ferromagnetic phase for low
concentrations of conduction electrons. If the system were a charge transfer
insulator, we would find that the ferromagnetism is induced by the well-known
RKKY interaction. However, we found a novel correlated hopping mechanism
inducing the ferromagnetism in the region where the non-doped system is a Mott
insulator. Our regions of ferromagnetism spanned a much smaller doping range
than suggested by recent slave boson and dynamical mean field theory
calculations, but they were consistent with that obtained by density matrix
renormalization group calculations of the one-dimensional periodic Anderson
model
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