2,488 research outputs found
Mesoscopic and microscopic dipole clusters: Structure and phase transitions
Two dimensional (2D) classical system of dipole particles confined by a
quadratic potential is studied. For clusters of N < 81 particles ground state
configurations and appropriate eigenfrequencies and eigenvectors for the normal
modes are found. Monte Carlo and molecular dynamic methods are used to study in
detail the order - disorder transition (the "melting" of clusters). In
mesoscopic clusters (N < 37) there is a hierarchy of transitions: at lower
temperatures an intershell orientational disordering of pairs of shells takes
place; at higher temperatures the intershell diffusion sets in and the shell
structure disappears. In "macroscopic" clusters (N > 37) an orientational
"melting" of only the outer shell is possible. The most stable clusters (having
both maximal lowest nonzero eigenfrequencies and maximal temperatures of total
melting) are that of completed crystal shells which are concentric groups of
nodes of 2D hexagonal lattice with a number of nodes placed in the center of
them. The study of different quantities shows that the melting temperature is a
nonmonotonic function of the number of particles in the system. The dynamical
equilibrium between "solidlike" and "orientationally disordered" forms of
clusters is considered.Comment: 12 pages, 16 Postscript figures. Submitted to Phys. Rev.
Volterra-series approach to stochastic nonlinear dynamics: linear response of the Van der Pol oscillator driven by white noise
The Van der Pol equation is a paradigmatic model of relaxation oscillations.
This remarkable nonlinear phenomenon of self-sustained oscillatory motion
underlies important rhythmic processes in nature and electrical engineering.
Relaxation oscillations in a real system are usually coupled to environmental
noise, which further enriches their dynamics, but makes theoretical analysis of
such systems and determination of the equation's parameter values a difficult
task. In a companion paper we have proposed an analytic approach to a similar
problem for another classical nonlinear model, the bistable Duffing oscillator.
Here we extend our techniques to the case of the Van der Pol equation driven by
white noise. We analyze the statistics of solutions and propose a method to
estimate parameter values from the oscillator's time series. We use
experimental data of active oscillations in a biological system to demonstrate
how our method applies to real observations and how it can be generalized for
more complex models.Comment: 12 pages, 6 figures, 1 tabl
Time evolution of a pair of distinguishable interacting spins subjected to controllable and noisy magnetic fields
The quantum dynamics of a
-conserving
Hamiltonian model describing two coupled spins and
under controllable and fluctuating time-dependent magnetic
fields is investigated. Each eigenspace of is dynamically
invariant and the Hamiltonian of the total system restricted to any one of such
eigenspaces, possesses the SU(2) structure of the
Hamiltonian of a single fictitious spin acted upon by the total magnetic field.
We show that such a reducibility holds regardless of the time dependence of the
externally applied field as well as of the statistical properties of the noise,
here represented as a classical fluctuating magnetic field. The time evolution
of the joint transition probabilities of the two spins and
between two prefixed factorized states is examined,
bringing to light peculiar dynamical properties of the system under scrutiny.
When the noise-induced non-unitary dynamics of the two coupled spins is
properly taken into account, analytical expressions for the joint Landau-Zener
transition probabilities are reported. The possibility of extending the
applicability of our results to other time-dependent spin models is pointed
out.Comment: 11 pages, 5 figure
Bouncing off the walls : the influence of gas-kinetic and van der Waals effects in drop impact
A model is developed for liquid drop impact on a solid surface that captures the thin film gas flow beneath the drop, even when the film’s thickness is below the mean free path in the gas so that gas kinetic effects (GKE) are important. Simulation results agree with experiments, with the impact speed threshold between bouncing and wetting reproduced to within 5 least 50 mapped and provides experimentally verifiable predictions. There are two principal modes of contact leading to wetting and both are associated with a van der Waals driven instability of the film
Development of Methods and Algorithms for Spectral Data Analysis for Vibroacoustic Diagnostics of Diesel-Generator Sets at NPPs
In this article, the main methods and algorithms for spectral data analysis for vibroacoustic diagnostics of diesel-generator sets at nuclear power plants are considered. To collect the diagnostic data, an experimental setup was developed, thanks to which the sound signals of the diesel generator were obtained under various operating conditions. The recording and processing of signals was carried out using the application package and MATLAB programming language. The article describes the application of correlation and spectral analysis for data processing and analysis. Also, the authors apply regression analysis to find the dependence of the speed of the diesel engine on the frequency of acoustic oscillations. The prediction of the number of revolutions from the frequency of sound vibrations makes it possible in the future to build a more accurate mathematical model of engine operation, and also to find diagnostic features for detecting malfunctions and anomalies in the operation of a diesel generator
Phase diagram of 2D array of mesoscopic granules
A lattice boson model is used to study ordering phenomena in regular 2D array
of superconductive mesoscopic granules, Josephson junctions or pores filled
with a superfluid helium. Phase diagram of the system, when quantum
fluctuations of both the phase and local superfluid density are essential, is
analyzed both analytically and by quantum Monte Carlo technique. For the system
of strongly interacting bosons it is found that as the boson density is
increased the boundary of ordered superconducting state shifts to {\it lower
temperatures} and at approaches its limiting position corresponding
to negligible relative fluctuations of moduli of the order parameter (as in an
array of "macroscopic" granules). In the region of weak quantum fluctuations of
phases mesoscopic phenomena manifest themselves up to . The mean
field theory and functional integral - expansion results are shown to
agree with that of quantum Monte Carlo calculations of the boson Hubbard model
and its quasiclassical limit, the quantum XY model.Comment: 7 pages, 5 Postscript figure
New model for system of mesoscopic Josephson contacts
Quantum fluctuations of the phases of the order parameter in 2D arrays of
mesoscopic Josephson junctions and their effect on the destruction of
superconductivity in the system are investigated by means of a quantum-cosine
model that is free of the incorrect application of the phase operator. The
proposed model employs trigonometric phase operators and makes it possible to
study arrays of small superconducting granules, pores filled with superfluid
helium, or Josephson junctions in which the average number of particles
(effective bosons, He atoms, and so on) is small, and the standard approach
employing the phase operator and the particle number operator as conjugate ones
is inapplicable. There is a large difference in the phase diagrams between
arrays of macroscopic and mesoscopic objects for and ( is
the characteristic interaction energy of the particle per granule and is
the Josephson coupling constant). Reentrant superconductivity phenomena are
discussed.Comment: 4 pages, 3 Postscript figure
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