406 research outputs found
Normal and Anomalous Averages for Systems with Bose-Einstein Condensate
The comparative behaviour of normal and anomalous averages as functions of
momentum or energy, at different temperatures, is analysed for systems with
Bose-Einstein condensate. Three qualitatively distinct temperature regions are
revealed: The critical region, where the absolute value of the anomalous
average, for the main energy range, is much smaller than the normal average.
The region of intermediate temperatures, where the absolute values of the
anomalous and normal averages are of the same order. And the region of low
temperatures, where the absolute value of the anomalous average, for
practically all energies, becomes much larger than the normal average. This
shows the importance of the anomalous averages for the intermediate and,
especially, for low temperatures, where these anomalous averages cannot be
neglected.Comment: Latex file, 17 pages, 6 figure
Bose-Einstein-condensed gases in arbitrarily strong random potentials
Bose-Einstein-condensed gases in external spatially random potentials are
considered in the frame of a stochastic self-consistent mean-field approach.
This method permits the treatment of the system properties for the whole range
of the interaction strength, from zero to infinity, as well as for arbitrarily
strong disorder. Besides a condensate and superfluid density, a glassy number
density due to a spatially inhomogeneous component of the condensate occurs.
For very weak interactions and sufficiently strong disorder, the superfluid
fraction can become smaller than the condensate fraction, while at relatively
strong interactions, the superfluid fraction is larger than the condensate
fraction for any strength of disorder. The condensate and superfluid fractions,
and the glassy fraction always coexist, being together either nonzero or zero.
In the presence of disorder, the condensate fraction becomes a nonmonotonic
function of the interaction strength, displaying an antidepletion effect caused
by the competition between the stabilizing role of the atomic interaction and
the destabilizing role of the disorder. With increasing disorder, the
condensate and superfluid fractions jump to zero at a critical value of the
disorder parameter by a first-order phase transition
Condensate and superfluid fractions for varying interactions and temperature
A system with Bose-Einstein condensate is considered in the frame of the
self-consistent mean-field approximation, which is conserving, gapless, and
applicable for arbitrary interaction strengths and temperatures. The main
attention is paid to the thorough analysis of the condensate and superfluid
fractions in a wide region of interaction strengths and for all temperatures
between zero and the critical point T_c. The normal and anomalous averages are
shown to be of the same order for almost all interactions and temperatures,
except the close vicinity of T_c. But even in the vicinity of the critical
temperature, the anomalous average cannot be neglected, since only in the
presence of the latter the phase transition at T_c becomes of second order, as
it should be. Increasing temperature influences the condensate and superfluid
fractions in a similar way, by diminishing them. But their behavior with
respect to the interaction strength is very different. For all temperatures,
the superfluid fraction is larger than the condensate fraction. These coincide
only at T_c or under zero interactions. For asymptotically strong interactions,
the condensate is almost completely depleted, even at low temperatures, while
the superfluid fraction can be close to one.Comment: Latex file, 22 pages, 5 figure
Optimal trap shape for a Bose gas with attractive interactions
Dilute Bose gas with attractive interactions is considered at zero
temperature, when practically all atoms are in Bose-Einstein condensate. The
problem is addressed aiming at answering the question: What is the optimal trap
shape allowing for the condensation of the maximal number of atoms with
negative scattering lengths? Simple and accurate analytical formulas are
derived allowing for an easy analysis of the optimal trap shapes. These
analytical formulas are the main result of the paper.Comment: Latex file, 21 page
Bose-Einstein-condensed systems in random potentials
The properties of systems with Bose-Einstein condensate in external
time-independent random potentials are investigated in the frame of a
self-consistent stochastic mean-field approximation. General considerations are
presented, which are valid for finite temperatures, arbitrary strengths of the
interaction potential, and for arbitrarily strong disorder potentials. The
special case of a spatially uncorrelated random field is then treated in more
detail. It is shown that the system consists of three components, condensed
particles, uncondensed particles and a glassy density fraction, but that the
pure Bose glass phase with only a glassy density does not appear. The theory
predicts a first-order phase transition for increasing disorder parameter,
where the condensate fraction and the superfluid fraction simultaneously jump
to zero. The influence of disorder on the ground-state energy, the stability
conditions, the compressibility, the structure factor, and the sound velocity
are analyzed. The uniform ideal condensed gas is shown to be always
stochastically unstable, in the sense that an infinitesimally weak disorder
destroys the Bose-Einstein condensate, returning the system to the normal
state. But the uniform Bose-condensed system with finite repulsive interactions
becomes stochastically stable and exists in a finite interval of the disorder
parameter.Comment: Latex file, final published varian
From Light Nuclei to Nuclear Matter. The Role of Relativity?
The success of non-relativistic quantum dynamics in accounting for the
binding energies and spectra of light nuclei with masses up to A=10 raises the
question whether the same dynamics applied to infinite nuclear matter agrees
with the empirical saturation properties of large nuclei.The simple unambiguous
relation between few-nucleon and many-nucleon Hamiltonians is directly related
to the Galilean covariance of nonrelativistic dynamics. Relations between the
irreducible unitary representations of the Galilei and Poincare groups indicate
thatthe ``nonrelativistic'' nuclear Hamiltonians may provide sufficiently
accurate approximations to Poincare invariant mass operators. In relativistic
nuclear dynamics based on suitable Lagrangeans the intrinsic nucleon parity is
an explicit, dynamically relevant, degree of freedom and the emphasis is on
properties of nuclear matter. The success of this approach suggests the
question how it might account for the spectral properties of light nuclei.Comment: conference proceedings "The 11th International Conference on Recent
Progress in Many-Body Theories" to be published by World Scientifi
Space Symmetries and Quantum Behavior of Finite Energy Configurations in SU(2)-Gauge Theory
The quantum properties of localized finite energy solutions to classical
Euler-Lagrange equations are investigated using the method of collective
coordinates. The perturbation theory in terms of inverse powers of the coupling
constant is constructed, taking into account the conservation laws of
momentum and angular momentum (invariance of the action with respect to the
group of motion M(3) of 3-dimensional Euclidean space) rigorously in every
order of perturbation theory.Comment: LaTex, 17 pages typos correcte
Topological Coherent Modes for Nonlinear Schr\"odinger Equation
Nonlinear Schr\"odinger equation, complemented by a confining potential,
possesses a discrete set of stationary solutions. These are called coherent
modes, since the nonlinear Schr\"odinger equation describes coherent states.
Such modes are also named topological because the solutions corresponding to
different spectral levels have principally different spatial dependences. The
theory of resonant excitation of these topological coherent modes is presented.
The method of multiscale averaging is employed in deriving the evolution
equations for resonant guiding centers. A rigorous qualitative analysis for
these nonlinear differential equations is given. Temporal behaviour of
fractional populations is illustrated by numerical solutions.Comment: 14 pages, Latex, no figure
Fast magnetization reversal of nanoclusters in resonator
An effective method for ultrafast magnetization reversal of nanoclusters is
suggested. The method is based on coupling a nanocluster to a resonant electric
circuit. This coupling causes the appearance of a magnetic feedback field
acting on the cluster, which drastically shortens the magnetization reversal
time. The influence of the resonator properties, nanocluster parameters, and
external fields on the magnetization dynamics and reversal time is analyzed.
The magnetization reversal time can be made many orders shorter than the
natural relaxation time. The reversal is studied for both the cases of a single
nanocluster as well as for the system of many nanoclusters interacting through
dipole forces.Comment: latex file, 21 pages, 7 figure
The structure of the quantum mechanical state space and induced superselection rules
The role of superselection rules for the derivation of classical probability
within quantum mechanics is investigated and examples of superselection rules
induced by the environment are discussed.Comment: 11 pages, Standard Latex 2.0
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