832 research outputs found
Thermodynamics of Bose-Condensed Atomic Hydrogen
We study the thermodynamics of the Bose-condensed atomic hydrogen confined in
the Ioffe-Pritchard potential. Such a trapping potential, that models the
magnetic trap used in recent experiments with hydrogen, is anharmonic and
strongly anisotropic. We calculate the ground-state properties, the condensed
and non-condensed fraction and the Bose-Einstein transition temperature. The
thermodynamics of the system is strongly affected by the anharmonicity of this
external trap. Finally, we consider the possibility to detect Josephson-like
currents by creating a double-well barrier with a laser beam.Comment: 11 pages, 4 figures, to be published in European Physical Journal
Thermodynamics of a trapped Bose condensate with negative scattering length
We study the Bose-Einstein condensation (BEC) for a system of atoms,
which have negative scattering length (attractive interaction), confined in a
harmonic potential. Within the Bogoliubov and Popov approximations, we
numerically calculate the density profile for both condensate and
non-condensate fractions and the spectrum of elementary excitations. In
particular, we analyze the temperature and number-of-boson dependence of these
quantities and evaluate the BEC transition temperature . We calculate
the loss rate for inelastic two- and three-body collisions. We find that the
total loss rate is strongly dependent on the density profile of the condensate,
but this density profile does not appreciably change by increasing the thermal
fraction. Moreover, we study, using the quasi-classical Popov approximation,
the temperature dependence of the critical number of condensed bosons,
for which there is the collapse of the condensate. There are different regimes
as a function of the total number of atoms. For the condensate is
always metastable but for the condensate is metastable only for
temperatures that exceed a critical value .Comment: RevTex, 7 postscript figures, to be published in Journal of Low
Temperature Phsyic
Exact Renormalization Group : A New Method for Blocking the Action
We consider the exact renormalization group for a non-canonical scalar field
theory in which the field is coupled to the external source in a special non
linear way. The Wilsonian action and the average effective action are then
simply related by a Legendre transformation up to a trivial quadratic form. An
exact mapping between canonical and non-canonical theories is obtained as well
as the relations between their flows. An application to the theory of liquids
is sketched
Modulational Instability and Complex Dynamics of Confined Matter-Wave Solitons
We study the formation of bright solitons in a Bose-Einstein condensate of
Li atoms induced by a sudden change in the sign of the scattering length
from positive to negative, as reported in a recent experiment (Nature {\bf
417}, 150 (2002)). The numerical simulations are performed by using the 3D
Gross-Pitaevskii equation (GPE) with a dissipative three-body term. We show
that a number of bright solitons is produced and this can be interpreted in
terms of the modulational instability of the time-dependent macroscopic wave
function of the Bose condensate. In particular, we derive a simple formula for
the number of solitons that is in good agreement with the numerical results of
3D GPE. By investigating the long time evolution of the soliton train solving
the 1D GPE with three-body dissipation we find that adjacent solitons repel
each other due to their phase difference. In addition, we find that during the
motion of the soliton train in an axial harmonic potential the number of
solitonic peaks changes in time and the density of individual peaks shows an
intermittent behavior. Such a complex dynamics explains the ``missing
solitons'' frequently found in the experiment.Comment: to be published in Phys. Rev. Let
Properties of derivative expansion approximations to the renormalization group
Approximation only by derivative (or more generally momentum) expansions,
combined with reparametrization invariance, turns the continuous
renormalization group for quantum field theory into a set of partial
differential equations which at fixed points become non-linear eigenvalue
equations for the anomalous scaling dimension . We review how these
equations provide a powerful and robust means of discovering and approximating
non-perturbative continuum limits. Gauge fields are briefly discussed.
Particular emphasis is placed on the r\^ole of reparametrization invariance,
and the convergence of the derivative expansion is addressed.Comment: (Minor touch ups of the lingo.) Invited talk at RG96, Dubna, Russia;
14 pages including 2 eps figures; uses LaTeX, epsf and sprocl.st
Thermodynamics of Multi-Component Fermi Vapors
We study the thermodynamical properties of Fermi vapors confined in a
harmonic external potential. In the case of the ideal Fermi gas, we compare
exact density profiles with their semiclassical approximation in the conditions
of recent experiments. Then, we consider the phase-separation of a
multi-component Fermi vapor. In particular, we analyze the phase-separation as
a function of temperature, number of particles and scattering length. Finally,
we discuss the effect of rotation on the stability and thermodynamics of the
trapped vapors.Comment: 15 pages, 5 figures, to be published in J. Phys. B (Atom. Mol.) as a
Special Issue Articl
Quantum Monte Carlo diagonalization for many-fermion systems
In this study we present an optimization method based on the quantum Monte
Carlo diagonalization for many-fermion systems. Using the Hubbard-Stratonovich
transformation, employed to decompose the interactions in terms of auxiliary
fields, we expand the true ground-state wave function. The ground-state wave
function is written as a linear combination of the basis wave functions. The
Hamiltonian is diagonalized to obtain the lowest energy state, using the
variational principle within the selected subspace of the basis functions. This
method is free from the difficulty known as the negative sign problem. We can
optimize a wave function using two procedures. The first procedure is to
increase the number of basis functions. The second improves each basis function
through the operators, , using the Hubbard-Stratonovich
decomposition. We present an algorithm for the Quantum Monte Carlo
diagonalization method using a genetic algorithm and the renormalization
method. We compute the ground-state energy and correlation functions of small
clusters to compare with available data
Role of backflow correlations for the non-magnetic phase of the t-t' Hubbard model
We introduce an efficient way to improve the accuracy of projected wave
functions, widely used to study the two-dimensional Hubbard model. Taking the
clue from the backflow contribution, whose relevance has been emphasized for
various interacting systems on the continuum, we consider many-body
correlations to construct a suitable approximation for the ground state at
intermediate and strong couplings. In particular, we study the phase diagram of
the frustrated Hubbard model on the square lattice and show
that, thanks to backflow correlations, an insulating and non-magnetic phase can
be stabilized at strong coupling and sufficiently large frustrating ratio
.Comment: 5 pages, 4 figure
Timing accuracy of the Swift X-Ray Telescope in WT mode
The X-Ray Telescope (XRT) on board Swift was mainly designed to provide
detailed position, timing and spectroscopic information on Gamma-Ray Burst
(GRB) afterglows. During the mission lifetime the fraction of observing time
allocated to other types of source has been steadily increased. In this paper,
we report on the results of the in-flight calibration of the timing
capabilities of the XRT in Windowed Timing read-out mode. We use observations
of the Crab pulsar to evaluate the accuracy of the pulse period determination
by comparing the values obtained by the XRT timing analysis with the values
derived from radio monitoring. We also check the absolute time reconstruction
measuring the phase position of the main peak in the Crab profile and comparing
it both with the value reported in literature and with the result that we
obtain from a simultaneous Rossi X-Ray Timing Explorer (RXTE) observation. We
find that the accuracy in period determination for the Crab pulsar is of the
order of a few picoseconds for the observation with the largest data time span.
The absolute time reconstruction, measured using the position of the Crab main
peak, shows that the main peak anticipates the phase of the position reported
in literature for RXTE by ~270 microseconds on average (~150 microseconds when
data are reduced with the attitude file corrected with the UVOT data). The
analysis of the simultaneous Swift-XRT and RXTE Proportional Counter Array
(PCA) observations confirms that the XRT Crab profile leads the PCA profile by
~200 microseconds. The analysis of XRT Photodiode mode data and BAT event data
shows a main peak position in good agreement with the RXTE, suggesting the
discrepancy observed in XRT data in Windowed Timing mode is likely due to a
systematic offset in the time assignment for this XRT read out mode.Comment: 6 pages, 4 figures. Accepted for publication on
Astronomy&Astrophysic
Effective chiral-spin Hamiltonian for odd-numbered coupled Heisenberg chains
An system of odd number of coupled Heisenberg spin chains
is studied using a degenerate perturbation theory, where is the number of
coupled chains. An effective chain Hamiltonian is derived explicitly in terms
of two spin half degrees of freedom of a closed chain of sites, valid in
the regime the inter-chain coupling is stronger than the intra-chain coupling.
The spin gap has been calculated numerically using the effective Hamiltonian
for for a finite chain up to ten sites. It is suggested that the
ground state of the effective Hamiltonian is correlated, by examining
variational states for the effective chiral-spin chain Hamiltonian.Comment: 9 Pages, Latex, report ICTP-94-28
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