357 research outputs found
Variance Reduction Result for a Projected Adaptive Biasing Force Method
This paper is committed to investigate an extension of the classical adaptive
biasing force method, which is used to compute the free energy related to the
Boltzmann-Gibbs measure and a reaction coordinate function. The issue of this
technique is that the approximated gradient of the free energy, called biasing
force, is not a gradient. The commitment to this field is to project the
estimated biasing force on a gradient using the Helmholtz decomposition. The
variance of the biasing force is reduced using this technique, which makes the
algorithm more efficient than the standard ABF method. We prove exponential
convergence to equilibrium of the estimated free energy, with a precise rate of
convergence in function of Logarithmic Sobolev inequality constants
A multiple replica approach to simulate reactive trajectories
A method to generate reactive trajectories, namely equilibrium trajectories
leaving a metastable state and ending in another one is proposed. The algorithm
is based on simulating in parallel many copies of the system, and selecting the
replicas which have reached the highest values along a chosen one-dimensional
reaction coordinate. This reaction coordinate does not need to precisely
describe all the metastabilities of the system for the method to give reliable
results. An extension of the algorithm to compute transition times from one
metastable state to another one is also presented. We demonstrate the interest
of the method on two simple cases: a one-dimensional two-well potential and a
two-dimensional potential exhibiting two channels to pass from one metastable
state to another one
Première découverte d'un Arthrodire (Placodermi, Vertebrata) dans le Dévonien d'Amérique du Sud
Des plaques dermiques d'un grand Arthrodire eubrachyhtoracide (Placodermi, Vertebrata), provisoirement attribuées à un Dunkleosteidae, ont été découvertes dans les faciès détritique de la Formation de Colpacucho (Famennien), sur la Péninsule de Cumana (Lac Titicaca, Bolivie). Il s'agit de la première découverte de restes d'Arthrodires en Amérique du Sud. Ces plaques d'Arthrodire sont associées à une épine de Chondrichthyen évoquant certaines espèces de "Ctenacanthus" du Famennien et du Carbonifère inférieur (Résumé d'auteur
Optical properties of Au colloids self-organized into rings via copolymer templates
The investigation of the Localized Surface Plasmon Resonance for plasmonic
nanoparticles has opened new perspectives for optical nanosensors. Today, an
issue in plasmonics is the development of large scale and low cost devices. We
focus here on the Langmuir-Blodgett technique to self-organize gold
nanoparticles (~ 7 nm) into rings (~ 60 nm) via
polystyrene-b-polymethylmethacrylate templates. In particular, we investigated
the optical properties of organized gold nanoparticle rings over large areas
and report experimental evidence for plasmon resonances of both individual
nanoparticles and collective modes. This paves the way for designing devices
with multiple resonances in the visible-Infra-red spectrum and developing
optical sensors
Magnetization jump in the XXZ chain with next-nearest-neighbor exchange
We study the dependence of the magnetization M with magnetic field B at zero
temperature in the spin-1/2 XXZ chain with nearest-neighbor (NN) J1 and next-NN
J2 exchange interactions, with anisotropies Delta1 and Delta2 respectively. The
region of parameters for which a jump in M(B) exists is studied using numerical
diagonalization, and analytical results for two magnons on a ferromagnetic
background in the thermodynamic limit. We find a line in the parameter space
(J2/J1, Delta1/J1, Delta2/J2) (determined by two simple equations) at which the
ground state is highly degenerate. M(B) has a jump near this line, and at or
near the isotropic case with ferromagnetic J1 and antiferromagnetic J2, with
|J2/J1| near 1/4. These results are relevant for some systems containing CuO
chains with edge-sharing CuO4 units.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
On the sign of the linear magnetoelectric coefficient in CrO
We establish the sign of the linear magnetoelectric (ME) coefficient,
, in chromia, CrO. CrO is the prototypical linear ME
material, in which an electric (magnetic) field induces a linearly proportional
magnetization (polarization), and a single magnetic domain can be selected by
annealing in combined magnetic (H) and electric (E) fields. Opposite
antiferromagnetic domains have opposite ME responses, and which
antiferromagnetic domain corresponds to which sign of response has previously
been unclear. We use density functional theory (DFT) to calculate the magnetic
response of a single antiferromagnetic domain of CrO to an applied
in-plane electric field at 0 K. We find that the domain with nearest neighbor
magnetic moments oriented away from (towards) each other has a negative
(positive) in-plane ME coefficient, , at 0 K. We show that this
sign is consistent with all other DFT calculations in the literature that
specified the domain orientation, independent of the choice of DFT code or
functional, the method used to apply the field, and whether the direct
(magnetic field) or inverse (electric field) ME response was calculated. Next,
we reanalyze our previously published spherical neutron polarimetry data to
determine the antiferromagnetic domain produced by annealing in combined E and
H fields oriented along the crystallographic symmetry axis at room temperature.
We find that the antiferromagnetic domain with nearest-neighbor magnetic
moments oriented away from (towards) each other is produced by annealing in
(anti-)parallel E and H fields, corresponding to a positive (negative) axial ME
coefficient, , at room temperature. Since
at 0 K and at room temperature are known to be of opposite
sign, our computational and experimental results are consistent.Comment: 11 pages, 5 figure
HST optical spectral index map of the jet of 3C 273
We present HST images at 622 nm and 300 nm of the jet in 3C273 and determine
the run of the optical spectral index at 0.2" along the jet. The smoothness of
spectral index changes shows that the physical conditions are varying smoothly
across the jet. There is no correlation between the optical flux and spectral
index, as would be expected for relativistic electrons suffering strong cooling
due to synchrotron emission. We find no evidence for localized acceleration or
loss sites. This suggests that the spectral shape is not changing much
throughout the jet. We show that relativistic beaming and/or sub-equipartition
magnetic fields cannot remove the discrepancy between light-travel time along
the jet and the lifetime of electrons emitting optical synchrotron radiation.
We consider this further evidence in favour of a distributed electron
acceleration process.Comment: Accepted for publication by Astronomy and Astrophysics (13 pages, 8
figures
Metamagnetism in the XXZ model with next-to-nearest-neighbor coupling
We investigate groundstate energies and magnetization curves in the one
dimensional XXZ-model with next to nearest neighbour coupling and
anisotropy () at T=0. In between the familiar
ferro- and antiferromagnetic phase we find a transition region -- called
metamagnetic phase -- where the magnetization curve is discontinuous at a
critical field .Comment: LaTeX file (text) + 5 PS files (5 figures
Ergodic properties of quasi-Markovian generalized Langevin equations with configuration dependent noise and non-conservative force
We discuss the ergodic properties of quasi-Markovian stochastic differential
equations, providing general conditions that ensure existence and uniqueness of
a smooth invariant distribution and exponential convergence of the evolution
operator in suitably weighted spaces, which implies the validity
of central limit theorem for the respective solution processes. The main new
result is an ergodicity condition for the generalized Langevin equation with
configuration-dependent noise and (non-)conservative force
- …