164 research outputs found
Molecular production at a wide Feshbach resonance in Fermi-gas of cooled atoms
The problem of molecular production from degenerate gas of fermions at a wide
Feshbach resonance, in a single-mode approximation, is reduced to the linear
Landau-Zener problem for operators. The strong interaction leads to significant
renormalization of the gap between adiabatic levels. In contrast to static
problem the close vicinity of exact resonance does not play substantial role.
Two main physical results of our theory is the high sensitivity of molecular
production to the initial value of magnetic field and generation of a large BCS
condensate distributed over a broad range of momenta in inverse process of the
molecule dissociation.Comment: 4 pages, no figure
Classical fields approximation for cold weakly interacting bosons without free parameters
Classical fields approximation to cold weakly interacting bosons allows for a
unified treatment of condensed and uncondensed parts of the system. Until now,
however, the quantitative predictions were limited by a dependence of the
results on a grid chosen for numerical implementation of the method. In this
paper we propose replacing this unphysical ambiguity by an additional
postulate: the temperature of the gas at thermal equilibrium should be the same
as that of an ideal Bose gas with the same fraction of condensed atoms. As it
turns-out, with this additional assumption, nearly all atoms are within the
classical fields, thus the method applies to the whole system
Genuine phase diffusion of a Bose-Einstein condensate in the microcanonical ensemble: A classical field study
Within the classical field model, we find that the phase of a Bose-Einstein
condensate undergoes a true diffusive motion in the microcanonical ensemble,
the variance of the condensate phase change between time zero and time
growing linearly in . The phase diffusion coefficient obeys a simple scaling
law in the double thermodynamic and Bogoliubov limit. We construct an
approximate calculation of the diffusion coefficient, in fair agreement with
the numerical results over the considered temperature range, and we extend this
approximate calculation to the quantum field.Comment: 9 pages, 6 figure
Anisotropic pseudo-potential for polarized dilute quantum gases
Anisotropic pseudopotential relevant to collisions of two particles polarized
by external field is rigorously derived and its properties are investigated.
Such low-energy pseudopotential may be useful in describing collective
properties of dilute quantum gases, such as molecules polarized by electric
field or metastable atoms polarized by magnetic field. The
pseudopotential is expressed in terms of reactance (K--) matrix and derivatives
of Dirac delta-function. In most applications it may be represented as a sum of
traditional spherically-symmetric contact term and anisotropic part. The former
contribution may be parameterized by a generalized scattering length. The
anisotropic part of pseudopotential may be characterized by off-diagonal
scattering length for dipolar interactions and off-diagonal scattering volume
for quadrupolar interactions. Two-body matrix element of the pseudopotential in
a basis of plane waves is also derived.Comment: 1 Fig, submitted to Phys. Rev.
Two point correlations of a trapped interacting Bose gas at finite temperature
We develop a computationally tractable method for calculating correlation
functions of the finite temperature trapped Bose gas that includes the effects
of s-wave interactions. Our approach uses a classical field method to model the
low energy modes and treats the high energy modes using a Hartree-Fock
description. We present results of first and second order correlation
functions, in position and momentum space, for an experimentally realistic
system in the temperature range of to . We also characterize
the spatial coherence length of the system. Our theory should be applicable in
the critical region where experiments are now able to measure first and second
order correlations.Comment: 9 pages, 4 figure
Splitting and merging an elongated Bose-Einstein condensate at finite temperature
We analyze coherence effects during the splitting of a quasi one-dimensional
condensate into two spatially separated ones and their subsequent merging into
a single condensate. Our analysis takes into account finite-temperature
effects, where phase fluctuations play an important role. We show that, at
zero-temperature, the two split condensates can be merged into a single one
with a negligible phase difference. By increasing temperature to a finite value
below the critical point for condensation (), i.e., , a
considerable enhancement of phase and density fluctuations appears during the
process of splitting and merging. Our results show that if the process of
splitting and merging is sufficiently adiabatic, the whole process is quite
insensitive to phase fluctuations and even at high temperatures, a single
condensate can be produced.Comment: 8 pages, 6 figure
Full counting statistics of heteronuclear molecules from Feshbach-assisted photo association
We study the effects of quantum statistics on the counting statistics of
ultracold heteronuclear molecules formed by Feshbach-assisted photoassociation
[Phys. Rev. Lett. {\bf 93}, 140405 (2004)]. Exploiting the formal similarities
with sum frequency generation and using quantum optics methods we consider the
cases where the molecules are formed from atoms out of two Bose-Einstein
condensates, out of a Bose-Einstein condensate and a gas of degenerate
fermions, and out of two degenerate Fermi gases with and without superfluidity.
Bosons are treated in a single mode approximation and fermions in a degenerate
model. In these approximations we can numerically solve the master equations
describing the system's dynamics and thus we find the full counting statistics
of the molecular modes. The full quantum dynamics calculations are complemented
by mean field calculations and short time perturbative expansions. While the
molecule production rates are very similar in all three cases at this level of
approximation, differences show up in the counting statistics of the molecular
fields. The intermediate field of closed-channel molecules is for short times
second-order coherent if the molecules are formed from two Bose-Einstein
condensates or a Bose-Fermi mixture. They show counting statistics similar to a
thermal field if formed from two normal Fermi gases. The coherence properties
of molecule formation in two superfluid Fermi gases are intermediate between
the two previous cases. In all cases the final field of deeply-bound molecules
is found to be twice as noisy as that of the intermediate state. This is a
consequence of its coupling to the lossy optical cavity in our model, which
acts as an input port for quantum noise, much like the situation in an optical
beam splitter.Comment: replacement of earlier manuscript cond-mat/0508080
''Feshbach-assisted photoassociation of ultracold heteronuclear molecules''
with minor revision
Classical field techniques for condensates in one-dimensional rings at finite temperatures
For a condensate in a one-dimensional ring geometry, we compare the
thermodynamic properties of three conceptually different classical field
techniques: stochastic dynamics, microcanonical molecular dynamics, and the
classical field method. Starting from non-equilibrium initial conditions, all
three methods approach steady states whose distribution and correlation
functions are in excellent agreement with an exact evaluation of the partition
function in the high-temperature limit. Our study helps to establish these
various classical field techniques as powerful non-perturbative tools for
systems at finite temperatures.Comment: 7 pages, 7 figures; minor changes, one reference adde
Dipolar Relaxation in an ultra-cold Gas of magnetically trapped chromium atoms
We have investigated both theoretically and experimentally dipolar relaxation
in a gas of magnetically trapped chromium atoms. We have found that the large
magnetic moment of 6 results in an event rate coefficient for dipolar
relaxation processes of up to cms at a magnetic
field of 44 G. We present a theoretical model based on pure dipolar coupling,
which predicts dipolar relaxation rates in agreement with our experimental
observations. This very general approach can be applied to a large variety of
dipolar gases.Comment: 9 pages, 9 figure
Structure and thickness of Y2O3 coatings deposited by plasma spray physical vapour deposition (PS-PvD) method on graphite
Graphite is one of materials used in metallurgical applications; however, it is characterized by low oxidation resistance. In the article, an yttrium oxide coating was deposited using Plasma Spray Physical Vapour deposition method (PS-PVD) on graphite. Next, the influence of selected process parameters (power current, powder feed rate, or plasma gasses composition) on coating thickness and structure were discussed. The obtained coatings were characterized by hybrid structure with partially formed columns. The linear relationship between power current and coating thickness was observed. There was no significant influence of other analyses’ process parameters on coating thickness or microstructure
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