3,767 research outputs found
Monte Carlo simulation with time step quantification in terms of Langevin dynamics
For the description of thermally activated dynamics in systems of classical
magnetic moments numerical methods are desirable. We consider a simple model
for isolated magnetic particles in a uniform field with an oblique angle to the
easy axis of the particles. For this model, a comparison of the Monte Carlo
method with Langevin dynamics yields new insight in the interpretation of the
Monte Carlo process, leading to the implementation of a new algorithm where the
Monte Carlo step is time-quantified. The numeric results for the characteristic
time of the magnetisation reversal are in excellent agreement with asymptotic
solutions which itself are in agreement with the exact numerical results
obtained from the Fokker-Planck equation for the Neel-Brown model.Comment: 5 pages, Revtex, 4 Figures include
Development and testing of laser Doppler system components for wake vortex monitoring. Volume 1: Scanner development, laboratory and field testing and system modeling
A servo-controlled range/elevation scanner for the laser Doppler velocimeter (LDV) was developed and tested in the field to assess its performance in detecting and monitoring aircraft trailing vortices in an airport environment. The elevation scanner provides a capability to manually point the LDV telescope at operator chosen angles from 3.2 deg. to 89.6 deg within 0.2 deg, or to automatically scan the units between operator chosen limits at operator chosen rates of 0.1 Hz to 0.5 Hz. The range scanner provides a capability to manually adjust the focal point of the system from a range of 32 meters to a range of 896 meters under operator control, or to scan between operator chosen limits and at rates from 0.1 Hz to 6.9 Hz. The scanner controls are designed to allow simulataneous range and elevation scanning so as to provide finger scan patterns, arc scan patterns, and vertical line scan patterns. The development and testing of the unit is discussed, along with a fluid dynamic model of the wake vortex developed in a laser Doppler vortex sensor simulation program
The Quantum Mechanics of Hyperion
This paper is motivated by the suggestion [W. Zurek, Physica Scripta, T76,
186 (1998)] that the chaotic tumbling of the satellite Hyperion would become
non-classical within 20 years, but for the effects of environmental
decoherence. The dynamics of quantum and classical probability distributions
are compared for a satellite rotating perpendicular to its orbital plane,
driven by the gravitational gradient. The model is studied with and without
environmental decoherence. Without decoherence, the maximum quantum-classical
(QC) differences in its average angular momentum scale as hbar^{2/3} for
chaotic states, and as hbar^2 for non-chaotic states, leading to negligible QC
differences for a macroscopic object like Hyperion. The quantum probability
distributions do not approach their classical limit smoothly, having an
extremely fine oscillatory structure superimposed on the smooth classical
background. For a macroscopic object, this oscillatory structure is too fine to
be resolved by any realistic measurement. Either a small amount of smoothing
(due to the finite resolution of the apparatus) or a very small amount of
environmental decoherence is sufficient ensure the classical limit. Under
decoherence, the QC differences in the probability distributions scale as
(hbar^2/D)^{1/6}, where D is the momentum diffusion parameter. We conclude that
decoherence is not essential to explain the classical behavior of macroscopic
bodies.Comment: 17 pages, 24 figure
On the Theory of Relativistic Strong Plasma Waves
The influence of motion of ions and electron temperature on nonlinear
one-dimensional plasma waves with velocity close to the speed of light in
vacuum is investigated. It is shown that although the wavebreaking field weakly
depends on mass of ions, the nonlinear relativistic wavelength essentially
changes. The nonlinearity leads to the increase of the strong plasma
wavelength, while the motion of ions leads to the decrease of the wavelength.
Both hydrodynamic approach and kinetic one, based on Vlasov-Poisson equations,
are used to investigate the relativistic strong plasma waves in a warm plasma.
The existence of relativistic solitons in a thermal plasma is predicted.Comment: 13 pages, 8 figure
Accurate Results from Perturbation Theory for Strongly Frustrated Heisenberg Spin Clusters
We investigate the use of perturbation theory in finite sized frustrated spin
systems by calculating the effect of quantum fluctuations on coherent states
derived from the classical ground state. We first calculate the ground and
first excited state wavefunctions as a function of applied field for a 12-site
system and compare with the results of exact diagonalization. We then apply the
technique to a 20-site system with the same three fold site coordination as the
12-site system. Frustration results in asymptotically convergent series for
both systems which are summed with Pad\'e approximants.
We find that at zero magnetic field the different connectivity of the two
systems leads to a triplet first excited state in the 12-site system and a
singlet first excited state in the 20-site system, while the ground state is a
singlet for both. We also show how the analytic structure of the Pad\'e
approximants at evolves in the complex plane at
the values of the applied field where the ground state switches between spin
sectors and how this is connected with the non-trivial dependence of the
number on the strength of quantum fluctuations. We discuss the origin
of this difference in the energy spectra and in the analytic structures. We
also characterize the ground and first excited states according to the values
of the various spin correlation functions.Comment: Final version, accepted for publication in Physical review
Longitudinal relaxation and thermoactivation of quantum superparamagnets
The relaxation mechanisms of a quantum nanomagnet are discussed in the frame
of linear response theory. We use a spin Hamiltonian with a uniaxial potential
barrier plus a Zeeman term. The spin, having arbitrary , is coupled to a
bosonic environment. From the eigenstructure of the relaxation matrix, we
identify two main mechanisms, namely, thermal activation over the barrier, with
a time scale \eival_1^{-1}, and a faster dynamics inside the potential wells,
with characteristic time \eivalW^{-1}. This allows to introduce a simple
analytical formula for the response, which agrees well with the exact numerical
results, and cover experiments even under moderate to strong fields in the
superparamagnetic range. In passing, we generalize known classical results for
a number of quantities (e.g., integral relaxation times, initial decay time,
Kramers rate), results that are recovered in the limit .Comment: submitted to Phys. Rev.
Human physiology during exposure to the cave environment: A systematic review with implications for aerospace medicine
Electrodynamics of a Clean Vortex Lattice
We report on a microscopic evaluation of electrodynamic response for the
vortex lattice state of a model s-wave superconductor. Our calculation accounts
self-consistently for both quasiparticle and order parameter response and
establishes the collective nature of linear response in the clean limit. We
discuss the effects of homogeneous and inhomogeneous pinning on the optical
conductivity and the penetration depth, and comment on the relationship between
macroscopic and local penetration depths. We find unexpected relationships
between pinning arrangements and conductivity due to the strongly non-local
response.Comment: 4 pages, 2 figure
Depinning transition in type-II superconductors
The surface impedance Z(f) of conventional isotropic materials has been
carefully measured for frequencies f ranging from 1 kHz to 3 MHz, allowing a
detailed investigation of the depinning transition. Our results exhibit the
irrelevance of classical ideas to the dynamics of vortex pinning. We propose a
new picture, where the linear ac response is entirely governed by disordered
boundary conditions of a rough surface, whereas in the bulk vortices respond
freely. The universal law for Z(f) thus predicted is in remarkable agreement
with experiment, and tentatively applies to microwave data in YBaCuO films.Comment: 4 pages, 4 figures, 14 reference
Sharpenings of Li's criterion for the Riemann Hypothesis
Exact and asymptotic formulae are displayed for the coefficients
used in Li's criterion for the Riemann Hypothesis. For we obtain
that if (and only if) the Hypothesis is true,
(with and explicitly given, also for the case of more general zeta or
-functions); whereas in the opposite case, has a non-tempered
oscillatory form.Comment: 10 pages, Math. Phys. Anal. Geom (2006, at press). V2: minor text
corrections and updated reference
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