151 research outputs found
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
Dynamics of Nonground-State Bose-Einstein Condensates
Dilute Bose gases, cooled down to low temperatures below the Bose-Einstein
condensation temperature, form coherent ensembles described by the
Gross-Pitaevskii equation. Stationary solutions to the latter are topological
coherent modes. The ground state, corresponding to the lowest energy level,
defines the standard Bose-Einstein condensate, while the states with higher
energy levels represent nonground-state condensates. The higher modes can be
generated by alternating fields, whose frequencies are in resonance with the
associated transition frequencies. The condensate with topological coherent
modes exhibits a variety of nontrivial effects. Here it is demonstrated that
the dynamical transition between the mode-locked and mode-unlocked regimes is
accompanied by noticeable changes in the evolutional entanglement production.Comment: latex file, 5 pages, 2 figures, Figs. 1,2 are not include
Absence of spin superradiance in resonatorless magnets
A spin system is considered with a Hamiltonian typical of molecular magnets,
having dipole-dipole interactions and a single-site magnetic anisotropy. In
addition, spin interactions through the common radiation field are included. A
fully quantum-mechanical derivation of the collective radiation rate is
presented. An effective narrowing of the dipole-dipole attenuation, due to high
spin polarization is taken into account. The influence of the radiation rate on
spin dynamics is carefully analysed. It is shown that this influence is
completely negligible. No noticeable collective effects, such as superradiance,
can appear in molecular magnets, being caused by electromagnetic spin
radiation. Spin superradiance can arise in molecular magnets only when these
are coupled to a resonant electric circuit, as has been suggested earlier by
one of the authors in Laser Phys. {\bf 12}, 1089 (2002).Comment: Latex file, 14 pages, 5 figure
From Coherent Modes to Turbulence and Granulation of Trapped Gases
The process of exciting the gas of trapped bosons from an equilibrium initial
state to strongly nonequilibrium states is described as a procedure of symmetry
restoration caused by external perturbations. Initially, the trapped gas is
cooled down to such low temperatures, when practically all atoms are in
Bose-Einstein condensed state, which implies the broken global gauge symmetry.
Excitations are realized either by imposing external alternating fields,
modulating the trapping potential and shaking the cloud of trapped atoms, or it
can be done by varying atomic interactions by means of Feshbach resonance
techniques. Gradually increasing the amount of energy pumped into the system,
which is realized either by strengthening the modulation amplitude or by
increasing the excitation time, produces a series of nonequilibrium states,
with the growing fraction of atoms for which the gauge symmetry is restored. In
this way, the initial equilibrium system, with the broken gauge symmetry and
all atoms condensed, can be excited to the state, where all atoms are in the
normal state, with completely restored gauge symmetry. In this process, the
system, starting from the regular superfluid state, passes through the states
of vortex superfluid, turbulent superfluid, heterophase granular fluid, to the
state of normal chaotic fluid in turbulent regime. Both theoretical and
experimental studies are presented.Comment: Latex file, 25 pages, 4 figure
Anomalous Transient Current in Nonuniform Semiconductors
Nonequilibrium processes in semiconductors are considered with highly
nonuniform initial densities of charge carriers. It is shown that there exist
such distributions of charge densities under which the electric current through
a sample displays quite abnormal behaviour flowing against the applied voltage.
The appearance of this negative electric current is a transient phenomenon
occurring at the initial stage of the process. After this anomalous negative
fluctuation, the electric current becomes normal, i.e. positive as soon as the
charge density becomes more uniform. Several possibilities for the practical
usage of this effect are suggested.Comment: 1 file, 11 pages, RevTex, no figure
Self-Similar Factor Approximants
The problem of reconstructing functions from their asymptotic expansions in
powers of a small variable is addressed by deriving a novel type of
approximants. The derivation is based on the self-similar approximation theory,
which presents the passage from one approximant to another as the motion
realized by a dynamical system with the property of group self-similarity. The
derived approximants, because of their form, are named the self-similar factor
approximants. These complement the obtained earlier self-similar exponential
approximants and self-similar root approximants. The specific feature of the
self-similar factor approximants is that their control functions, providing
convergence of the computational algorithm, are completely defined from the
accuracy-through-order conditions. These approximants contain the Pade
approximants as a particular case, and in some limit they can be reduced to the
self-similar exponential approximants previously introduced by two of us. It is
proved that the self-similar factor approximants are able to reproduce exactly
a wide class of functions which include a variety of transcendental functions.
For other functions, not pertaining to this exactly reproducible class, the
factor approximants provide very accurate approximations, whose accuracy
surpasses significantly that of the most accurate Pade approximants. This is
illustrated by a number of examples showing the generality and accuracy of the
factor approximants even when conventional techniques meet serious
difficulties.Comment: 22 pages + 11 ps figure
Self-similar Approximants of the Permeability in Heterogeneous Porous Media from Moment Equation Expansions
We use a mathematical technique, the self-similar functional renormalization,
to construct formulas for the average conductivity that apply for large
heterogeneity, based on perturbative expansions in powers of a small parameter,
usually the log-variance of the local conductivity. Using
perturbation expansions up to third order and fourth order in
obtained from the moment equation approach, we construct the general functional
dependence of the transport variables in the regime where is of
order 1 and larger than 1. Comparison with available numerical simulations give
encouraging results and show that the proposed method provides significant
improvements over available expansions.Comment: Latex, 14 pages + 5 ps figure
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
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
Nonequilibrium Bose systems and nonground-state Bose-Einstein condensates
The theory of resonant generation of nonground-state Bose-Einstein
condensates is extended to Bose-condensed systems at finite temperature. The
generalization is based on the notion of representative statistical ensembles
for Bose systems with broken global gauge symmetry. Self-consistent equations
are derived describing an arbitrary nonequilibrium nonuniform Bose system. The
notion of finite-temperature topological coherent modes, coexisting with a
cloud of noncondensed atoms, is introduced. It is shown that resonant
generation of these modes is feasible for a gas of trapped Bose atoms at finite
temperature.Comment: Latex file, 16 pages, no figure
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