875 research outputs found
Fluctuation indices for atomic systems with Bose-Einstein condensate
The notion of fluctuation indices, characterizing thermodynamic stability of
statistical systems, is advanced. These indices are especially useful for
investigating the stability of nonuniform and trapped atomic assemblies. The
fluctuation indices are calculated for several systems with Bose-Einstein
condensate. It is shown that: the ideal uniform Bose-condensed gas is
thermodynamically unstable; trapped ideal gases are stable for the confining
dimension larger than two; trapped gases, under the confining dimension two,
are weakly unstable; harmonically trapped gas is stable only for the spatial
dimension three; one-dimensional harmonically trapped gas is unstable;
two-dimensional gas in a harmonic trap represents a marginal case, being weakly
unstable; interacting nonuniform three-dimensional Bose-condensed gas is
stable. There are no thermodynamically anomalous particle fluctuations in
stable Bose-condensed systems.Comment: Latex file, 12 page
Particle fluctuations in nonuniform and trapped Bose gases
The problem of particle fluctuations in arbitrary nonuniform systems with
Bose-Einstein condensate is considered. This includes the case of trapped Bose
atoms. It is shown that the correct description of particle fluctuations for
any nonuniform system of interacting atoms always results in thermodynamically
normal fluctuations.Comment: Latex file, 16 page
Turbulent superfluid as continuous vortex mixture
A statistical model is advanced for describing quantum turbulence in a
superfluid system with Bose-Einstein condensate. Such a turbulent superfluid
can be realized for trapped Bose atoms subject to either an alternating
trapping potential or to an alternating magnetic field modulating the atomic
scattering length by means of Feshbach resonance. The turbulent system is
represented as a continuous mixture of states each of which is characterized by
its own vorticity corresponding to a particular vortex.Comment: Latex file, 22 pages, one figur
Method of self-similar factor approximants
The method of self-similar factor approximants is completed by defining the
approximants of odd orders, constructed from the power series with the largest
term of an odd power. It is shown that the method provides good approximations
for transcendental functions. In some cases, just a few terms in a power series
make it possible to reconstruct a transcendental function exactly. Numerical
convergence of the factor approximants is checked for several examples. A
special attention is paid to the possibility of extrapolating the behavior of
functions, with arguments tending to infinity, from the related asymptotic
series at small arguments. Applications of the method are thoroughly
illustrated by the examples of several functions, nonlinear differential
equations, and anharmonic models.Comment: Latex file, 21 pages, 4 tables, 4 figure
Coherent spin radiation by magnetic nanomolecules and nanoclusters
The peculiarities of coherent spin radiation by magnetic nanomolecules is
investigated by means of numerical simulation. The consideration is based on a
microscopic Hamiltonian taking into account realistic dipole interactions.
Superradiance can be realized only when the molecular sample is coupled to a
resonant electric circuit. The feedback mechanism allows for the achievement of
a fast spin reversal time and large radiation intensity. The influence on the
level of radiation, caused by sample shape and orientation, is analysed. The
most powerful coherent radiation is found to occur for an elongated sample
directed along the resonator magnetic field.Comment: Latex file, 11 figure
Temporal Dynamics in Perturbation Theory
Perturbation theory can be reformulated as dynamical theory. Then a sequence
of perturbative approximations is bijective to a trajectory of dynamical system
with discrete time, called the approximation cascade. Here we concentrate our
attention on the stability conditions permitting to control the convergence of
approximation sequences. We show that several types of mapping multipliers and
Lyapunov exponents can be introduced and, respectively, several types of
conditions controlling local stability can be formulated. The ideas are
illustrated by calculating the energy levels of an anharmonic oscillator.Comment: 1 file, 21 pages, RevTex, 2 table
Self-Similar Law of Energy Release before Materials Fracture
A general law of energy release is derived for stressed heterogeneous
materials, being valid from the starting moment of loading till the moment of
materials fracture. This law is obtained by employing the extrapolation
technique of the self-similar approximation theory. Experiments are
accomplished measuring the energy release for industrial composite samples. The
derived analytical law is confronted with these experimental data as well as
with the known experimental data for other materials.Comment: Latex, 15 pages, no figure
Statistical Mechanics of Structural Fluctuations
The theory of mesoscopic fluctuations is applied to inhomogeneous solids
consisting of chaotically distributed regions with different crystalline
structure. This approach makes it possible to describe statistical properties
of such mixture by constructing a renormalized Hamiltonian. The relative
volumes occupied by each of the coexisting structures define the corresponding
geometric probabilities. In the case of a frozen heterophase system these
probabilities should be given a priori. And in the case of a thermal
heterophase mixture the structural probabilities are to be defined
self-consistently by minimizing a thermodynamical potential. This permits to
find the temperature behavior of the probabilities which is especially
important near the points of structural phase transitions. The presense of
these structural fluctuations yields a softening of a crystal and a decrease of
the effective Debye temperature. These effects can be directly seen by nuclear
gamma resonance since the occurrence of structural fluctuations is accompanied
by a noticeable sagging of the M\"ossbauer factor at the point of structural
phase transition. The structural fluctuations also lead to the attenuation of
sound and increase of isothermic compressibility.Comment: 1 file, 18 pages, RevTex, no figure
Spin superradiance versus atomic superradiance
A comparative analysis is given of spin superradiance and atomic
superradiance. Their similarities and distinctions are emphasized. It is shown
that, despite a close analogy, these phenomena are fundamentally different. In
atomic systems, superradiance is a self-organized process, in which both the
initial cause, being spontaneous emission, as well as the collectivizing
mechanism of their interactions through the common radiation field, are of the
same physical nature. Contrary to this, in actual spin systems with dipole
interactions, the latter are the major reason for spin motion. Electromagnetic
spin interactions through radiation are negligible and can never produce
collective effects. The possibility of realizing superradiance in molecular
magnets by coupling them to a resonant circuit is discussed.Comment: Latex file, 12 pages, no figure
Self-similar factor approximants for evolution equations and boundary-value problems
The method of self-similar factor approximants is shown to be very convenient
for solving different evolution equations and boundary-value problems typical
of physical applications. The method is general and simple, being a
straightforward two-step procedure. First, the solution to an equation is
represented as an asymptotic series in powers of a variable. Second, the series
are summed by means of the self-similar factor approximants. The obtained
expressions provide highly accurate approximate solutions to the considered
equations. In some cases, it is even possible to reconstruct exact solutions
for the whole region of variables, starting from asymptotic series for small
variables. This can become possible even when the solution is a transcendental
function. The method is shown to be more simple and accurate than different
variants of perturbation theory with respect to small parameters, being
applicable even when these parameters are large. The generality and accuracy of
the method are illustrated by a number of evolution equations as well as
boundary value problems.Comment: Latex file, 27 pages, 2 figures, 5 table
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