633 research outputs found
Convergent sequences of perturbative approximations for the anharmonic oscillator II. Compact time approach
We present an alternative pathway in the application of the variation
improvement of ordinary perturbation theory exposed in [1] which can preserve
the internal symmetries of a model by means of a time compactification.Comment: 21 pages, 4 Postscript figures available through anonymous ftp at
ftp://algol.lpm.univ-montp2.fr ; replaces version which could not be
postscripted presumably for lack of figures.uu fil
Convergent sequences of perturbative approximations for the anharmonic oscillator I. Harmonic approach
We present numerical evidence that a simple variational improvement of the
ordinary perturbation theory of the quantum anharmonic oscillator can give a
convergent sequence of approximations even in the extreme strong coupling
limit, the purely anharmonic case. Some of the new techniques of this paper can
be extended to renormalizable field theories.Comment: 29 pages, 12 Postscript figures available through anonymous ftp at
ftp://algol.lpm.univ-montp2.fr ; replaces earlier version which could not be
postscripted presumably due to lack of figures.uu fil
Coarse-graining schemes for stochastic lattice systems with short and long-range interactions
We develop coarse-graining schemes for stochastic many-particle microscopic
models with competing short- and long-range interactions on a d-dimensional
lattice. We focus on the coarse-graining of equilibrium Gibbs states and using
cluster expansions we analyze the corresponding renormalization group map. We
quantify the approximation properties of the coarse-grained terms arising from
different types of interactions and present a hierarchy of correction terms. We
derive semi-analytical numerical schemes that are accompanied with a posteriori
error estimates for coarse-grained lattice systems with short and long-range
interactions.Comment: 31 pages, 2 figure
Convergent sequences of perturbative approximations for the anharmonic oscillator; 1, harmonic approach
We present numerical evidence that a simple variational improvement of the ordinary perturbation theory of the quantum anharmonic oscillator can give a convergent sequence of approximations even in the extreme strong coupling limit, the purely anharmonic case. Some of the new techniques of this paper can be extended to renormalizable field theories
High In-content InGaN layers synthesized by plasma-assisted molecular-beam epitaxy: growth conditions, strain relaxation and In incorporation kinetics
We report the interplay between In incorporation and strain relaxation
kinetics in high-In-content InxGa1-xN (x = 0.3) layers grown by plasma-assisted
molecular-beam epitaxy. For In mole fractions x = 0.13-0.48, best structural
and morphological quality is obtained under In excess conditions, at In
accumulation limit, and at a growth temperature where InGaN decomposition is
active. Under such conditions, in situ and ex situ analysis of the evolution of
the crystalline structure with the growth thickness points to an onset of
misfit relaxation after the growth of 40 nm, and a gradual relaxation during
more than 200 nm which results in an inhomogeneous strain distribution along
the growth axis. This process is associated with a compositional pulling
effect, i.e. indium incorporation is partially inhibited in presence of
compressive strain, resulting in a compositional gradient with increasing In
mole fraction towards the surface
Ferromagnetic (Ga,Mn)N epilayers versus antiferromagnetic GaMnN clusters
Mn-doped wurtzite GaN epilayers have been grown by nitrogen plasma-assisted
molecular beam epitaxy. Correlated SIMS, structural and magnetic measurements
show that the incorporation of Mn strongly depends on the conditions of the
growth. Hysteresis loops which persist at high temperature do not appear to be
correlated to the presence of Mn. Samples with up to 2% Mn are purely
substitutional GaMnN epilayers, and exhibit paramagnetic
properties. At higher Mn contents, precipitates are formed which are identified
as GaMnN clusters by x-ray diffraction and absorption: this induces a
decrease of the paramagnetic magnetisation. Samples co-doped with enough Mg
exhibit a new feature: a ferromagnetic component is observed up to
K, which cannot be related to superparamagnetism of unresolved magnetic
precipitates.Comment: Revised versio
Transport Properties of a Chain of Anharmonic Oscillators with random flip of velocities
We consider the stationary states of a chain of anharmonic coupled
oscillators, whose deterministic hamiltonian dynamics is perturbed by random
independent sign change of the velocities (a random mechanism that conserve
energy). The extremities are coupled to thermostats at different temperature
and and subject to constant forces and . If
the forces differ the center of mass of the system will
move of a speed inducing a tension gradient inside the system. Our aim is
to see the influence of the tension gradient on the thermal conductivity. We
investigate the entropy production properties of the stationary states, and we
prove the existence of the Onsager matrix defined by Green-kubo formulas
(linear response). We also prove some explicit bounds on the thermal
conductivity, depending on the temperature.Comment: Published version: J Stat Phys (2011) 145:1224-1255 DOI
10.1007/s10955-011-0385-
Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns
We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films
grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures
(Tg) ranging from 80deg C to 200deg C, with average Mn contents between 1 % and
11 %. Their crystalline structure, morphology and composition have been
investigated by transmission electron microscopy (TEM), electron energy loss
spectroscopy and x-ray diffraction. In the whole range of growth temperatures
and Mn concentrations, we observed the formation of manganese rich
nanostructures embedded in a nearly pure germanium matrix. Growth temperature
mostly determines the structural properties of Mn-rich nanostructures. For low
growth temperatures (below 120deg C), we evidenced a two-dimensional spinodal
decomposition resulting in the formation of vertical one-dimensional
nanostructures (nanocolumns). Moreover we show in this paper the influence of
growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns
size and density. For temperatures higher than 180deg C, we observed the
formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns
and nanoclusters coexist. Combining high resolution TEM and superconducting
quantum interference device magnetometry, we could evidence at least four
different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn
atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc
nanocolumns (120 K 400 K) and
(iv) Ge3Mn5 clusters.Comment: 10 pages 2 colonnes revTex formatte
Large deviations of lattice Hamiltonian dynamics coupled to stochastic thermostats
We discuss the Donsker-Varadhan theory of large deviations in the framework
of Hamiltonian systems thermostated by a Gaussian stochastic coupling. We
derive a general formula for the Donsker-Varadhan large deviation functional
for dynamics which satisfy natural properties under time reversal. Next, we
discuss the characterization of the stationary state as the solution of a
variational principle and its relation to the minimum entropy production
principle. Finally, we compute the large deviation functional of the current in
the case of a harmonic chain thermostated by a Gaussian stochastic coupling.Comment: Revised version, published in Journal of Statistical Physic
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
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