1,806 research outputs found
Electron energy loss spectroscopy with parallel readout of energy and momentum
We introduce a high energy resolution electron source that matches the
requirements for parallel readout of energy and momentum of modern
hemispherical electron energy analyzers. The system is designed as an add-on
device to typical photoemission chambers. Due to the multiplex gain, a complete
phonon dispersion of a Cu(111) surface was measured in seven minutes with 4 meV
energy resolution
Searching for activated transitions in complex magnetic systems
The process of finding activated transitions in localized spin systems with
continuous degrees of freedom is developed based on a magnetic variant of the
Activation-Relaxation Technique (mART). In addition to the description of the
method and the relevant local properties of the magnetic energy landscape, a
criterion to efficiently recognize failed attempts and an expression for the
step magnitude to control the convergence are proposed irrespective of the
physical system under study. The present implementation is validated on two
translational symmetric systems with isotropic exchange interactions. Then, in
one example, diffusion processes of a skyrmion vacancy and a skyrmion
interstitial are revealed for a skyrmion system on a square spin lattice. In
another example, the set of activation events about a metastable state of a 2D
dipolar spin glass is investigated and the corresponding energy barrier
distribution is found. Detailed inspection of the transition states reveals the
participation of nearest neighbour pairs affording a simplified analytical
understanding
Microscopic Derivation of Non-Markovian Thermalization of a Brownian Particle
In this paper, the first microscopic approach to the Brownian motion is
developed in the case where the mass density of the suspending bath is of the
same order of magnitude as that of the Brownian (B) particle. Starting from an
extended Boltzmann equation, which describes correctly the interaction with the
fluid, we derive systematicaly via the multiple time-scale analysis a reduced
equation controlling the thermalization of the B particle, i.e. the relaxation
towards the Maxwell distribution in velocity space. In contradistinction to the
Fokker-Planck equation, the derived new evolution equation is non-local both in
time and in velocity space, owing to correlated recollision events between the
fluid and particle B. In the long-time limit, it describes a non-markovian
generalized Ornstein-Uhlenbeck process. However, in spite of this complex
dynamical behaviour, the Stokes-Einstein law relating the friction and
diffusion coefficients is shown to remain valid. A microscopic expression for
the friction coefficient is derived, which acquires the form of the Stokes law
in the limit where the mean-free in the gas is small compared to the radius of
particle B.Comment: 28 pages, no figure, submitted to Journal of Statistical Physic
Thermally Activated Dynamics of the Capillary Condensation
This paper is devoted to the thermally activated dynamics of the capillary
condensation. We present a simple model which enables us to identify the
critical nucleus involved in the transition mechanism. This simple model is
then applied to calculate the nucleation barrier from which we can obtain
informations on the nucleation time. We present a simple estimation of the
nucleation barrier in slab geometry both in the two dimensional case and in the
three dimensional case. We extend the model in the case of rough surfaces which
is closer to the experimental case and allows comparison with experimental
datas.Comment: 6 pages, 3 figures, Submitted to J. Phys. : Condens. Matter,
Proceedings of the IV Liquid Matter Conference - Grenada(Spain) july 199
Les « pyramides écologiques » dans l’étude systématique des processus de radiocontamination
Bocquet Henri. Les Pyramides écologiques dans l’étude systématique des processus de radiocontamination. In: Bulletin de l'Académie Vétérinaire de France tome 112 n°10, 1959. pp. 707-720
Les « Pyramides écologiques » dans l’étude systématique des processus de radiocontamination (suite)
Bocquet Henri. Les «Pyramides écologiques» dans l’étude systématique des processus de radiocontamination (suite). In: Bulletin de l'Académie Vétérinaire de France tome 114 n°2, 1961. pp. 99-110
On the Maximum Mass of Differentially Rotating Neutron Stars
We construct relativistic equilibrium models of differentially rotating
neutron stars and show that they can support significantly more mass than their
nonrotating or uniformly rotating counterparts. We dynamically evolve such
``hypermassive'' models in full general relativity and show that there do exist
configurations which are dynamically stable against radial collapse and bar
formation. Our results suggest that the remnant of binary neutron star
coalescence may be temporarily stabilized by differential rotation, leading to
delayed collapse and a delayed gravitational wave burst.Comment: 4 pages, 2 figures, uses emulateapj.sty; to appear in ApJ Letter
A kinetic theory of plastic flow in soft glassy materials
A kinetic model for the elasto-plastic dynamics of a flowing jammed material
is proposed, which takes the form of a non-local -- Boltzmann-like -- kinetic
equation for the stress distribution function. Coarse-graining this equation
yields a non-local constitutive law for the flow, introducing as a key dynamic
quantity the local rate of plastic events. This quantity, interpreted as a
local fluidity, is spatially correlated, with a correlation length diverging in
the quasi-static limit, i.e. close to yielding. We predict finite size effects
in the flow behavior, as well as the absence of an intrinsic local flow curves.
These features are supported by recent experimental and numerical observations.Comment: 5 pages, 2 figure
Relativistic stars with purely toroidal magnetic fields
We investigate the effects of the purely toroidal magnetic field on the
equilibrium structures of the relativistic stars. The master equations for
obtaining equilibrium solutions of relativistic rotating stars containing
purely toroidal magnetic fields are derived for the first time. To solve these
master equations numerically, we extend the Cook-Shapiro-Teukolsky scheme for
calculating relativistic rotating stars containing no magnetic field to
incorporate the effects of the purely toroidal magnetic fields. By using the
numerical scheme, we then calculate a large number of the equilibrium
configurations for a particular distribution of the magnetic field in order to
explore the equilibrium properties. We also construct the equilibrium sequences
of the constant baryon mass and/or the constant magnetic flux, which model the
evolution of an isolated neutron star as it loses angular momentum via the
gravitational waves. Important properties of the equilibrium configurations of
the magnetized stars obtained in this study are summarized as follows ; (1) For
the non-rotating stars, the matter distribution of the stars is prolately
distorted due to the toroidal magnetic fields. (2) For the rapidly rotating
stars, the shape of the stellar surface becomes oblate because of the
centrifugal force. But, the matter distribution deep inside the star is
sufficiently prolate for the mean matter distribution of the star to be
prolate. (3) The stronger toroidal magnetic fields lead to the mass-shedding of
the stars at the lower angular velocity. (4) For some equilibrium sequences of
the constant baryon mass and magnetic flux, the stars can spin up as they lose
angular momentum.Comment: 13 figures, 7 tables, submitted to PR
Optimal eavesdropping on QKD without quantum memory
We consider the security of the BB84, six-state and SARG04 quantum key
distribution protocols when the eavesdropper doesn't have access to a quantum
memory. In this case, Eve's most general strategy is to measure her ancilla
with an appropriate POVM designed to take advantage of the post-measurement
information that will be released during the sifting phase of the protocol.
After an optimization on all the parameters accessible to Eve, our method
provides us with new bounds for the security of six-state and SARG04 against a
memoryless adversary. In particular, for the six-state protocol we show that
the maximum QBER for which a secure key can be extracted is increased from
12.6% (for collective attacks) to 20.4% with the memoryless assumption.Comment: 7 pages, 3 figures. Analysis of six-state and SARG04 QKD protocols
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