45 research outputs found
Dipolar Bose-Einstein condensates at Finite temperature
We study a Bose-Einstein condensate (BEC) of a dilute gas with dipolar
interactions, at finite temperature, using the Hartree-Fock-Bogoliubov (HFB)
theory within the Popov approximation. An additional approximation involving
the dipolar exchange interaction is made to facilitate the computation. We
calculate the temperature dependence of the condensate fraction of a condensate
confined in a cylindrically symmetric harmonic trap. We show that the
bi-concave shaped condensates found in Ref. \cite{Ronen07} in certain pancake
traps at zero temperature, are also stable at finite temperature. Surprisingly,
the dip in the central density of these structured condensates is actually
enhanced at low finite temperatures. We explain this effect.Comment: 9 figure
Radial and angular rotons in trapped dipolar gases
We study Bose-Einstein condensates with purely dipolar interactions in oblate
(pancake) traps. We find that the condensate always becomes unstable to
collapse when the number of particles is sufficiently large. We analyze the
instability, and find that it is the trapped-gas analogue of the
``roton-maxon'' instability previously reported for a gas that is unconfined in
two dimensions. In addition, we find that under certain circumstances, the
condensate wave function attains a biconcave shape, with its maximum density
away from the center of the gas. These biconcave condensates become unstable
due to azimuthl excitation - an angular roton.Comment: 4 pages, 3 figure
Dynamical pattern formation during growth of a dual-species Bose-Einstein condensate
We simulate the growth of a dual species Bose-Einstein condensate using a
Gross-Pitaevskii equation with an additional gain term giving rise to the
growth. Such growth occurs during simultaneous evaporative cooling of a mixture
of two gases. The ground state of a dual condensate is normally either a
miscible mixture, or an immiscible phase with two spatially separated
components. In a cigar trap the ground state typically consists of one
component in the center, and the other component flanking it. Our simulations
show that when the condensates are formed in a cigar trap and the mixture is
phase separated, then the final state upon the end of the growth is generally
far from the true ground state of the system. Instead it consists of multiple,
interleaved bubbles of the two species. Such a pattern was observed recently in
an experiment by Wieman's group at JILA, and our simulations are in good
qualitative agreement with the experiment. We explain the pattern formation as
due to the onset of modulation instability during growth, and study the
dependence of the final state pattern on various parameters of the system
Critical superfluid velocity in a trapped dipolar gas
We investigate the superfluid properties of a dipolar Bose-Einstein
condensate (BEC) in a fully three-dimensional trap. Specifically, we calculate
a superfluid critical velocity for this system by applying the Landau criterion
to its discrete quasiparticle spectrum. We test this critical velocity by
direct numerical simulation of condensate depletion as a blue-detuned laser
moves through the condensate. In both cases, the presence of the roton in the
spectrum serves to lower the critical velocity beyond a critical particle
number. Since the shape of the dispersion, and hence the roton minimum, is
tunable as a function of particle number, we thereby propose an experiment that
can simultaneously measure the Landau critical velocity of a dipolar BEC and
demonstrate the presence of the roton in this system.Comment: 5 pages, 4 figures, version accepted to PR
Dipolar Bose-Einstein condensates with dipole-dependent scattering length
We consider a Bose-Einstein condensate of polar molecules in a harmonic trap,
where the effective dipole may be tuned by an external field. We demonstrate
that taking into account the dependence of the scattering length on the dipole
moment is essential to reproducing the correct energies and for predicting the
stability of the condensate. We do this by comparing Gross-Pitaevskii
calculations with diffusion Monte Carlo calculations. We find very good
agreement between the results obtained by these two approaches once the dipole
dependence of the scattering length is taken into account. We also examine the
behavior of the condensate in non-isotropic traps
Manifestations of the Roton Mode in Dipolar Bose-Einstein Condensates
We investigate the structure of trapped Bose-Einstein condensates (BECs) with
long-range anisotropic dipolar interactions. We find that a small perturbation
in the trapping potential can lead to dramatic changes in the condensate's
density profile for sufficiently large dipolar interaction strengths and trap
aspect ratios. By employing perturbation theory, we relate these oscillations
to a previously-identified "roton-like" mode in dipolar BECs. The same physics
is responsible for radial density oscillations in vortex states of dipolar BECs
that have been predicted previously.Comment: 4 pages, 3 figures, version accepted to PR
The 2dF Galaxy Redshift Survey: spectral types and luminosity functions
We describe the 2dF Galaxy Redshift Survey (2dFGRS) and the current status of the observations. In this exploratory paper, we apply a principal component analysis to a preliminary sample of 5869 galaxy spectra and use the two most significant components to split the sample into five spectral classes. These classes are defined by considering visual classifications of a subset of the 2dF spectra, and also by comparison with high-quality spectra of local galaxies. We calculate a luminosity function for each of the different classes and find that later-type galaxies have a fainter characteristic magnitude, and a steeper faint-end slope. For the whole sample we find M*=-19.7 (for Ω=1, H_0=100kms^-1Mpc^-1), α=-1.3, φ*=0.017. For class 1 (`early-type') we find M*=-19.6, α=-0.7, while for class 5 (`late-type') we find M*=-19.0, α=-1.7. The derived 2dF luminosity functions agree well with other recent luminosity function estimates
Bogoliubov modes of a dipolar condensate in a cylindrical trap
The calculation of properties of Bose-Einstein condensates with dipolar
interactions has proven a computationally intensive problem due to the long
range nature of the interactions, limiting the scope of applications. In
particular, the lowest lying Bogoliubov excitations in three dimensional
harmonic trap with cylindrical symmetry were so far computed in an indirect
way, by Fourier analysis of time dependent perturbations, or by approximate
variational methods. We have developed a very fast and accurate numerical
algorithm based on the Hankel transform for calculating properties of dipolar
Bose-Einstein condensates in cylindrically symmetric traps. As an application,
we are able to compute many excitation modes by directly solving the
Bogoliubov-De Gennes equations. We explore the behavior of the excited modes in
different trap geometries. We use these results to calculate the quantum
depletion of the condensate by a combination of a computation of the exact
modes and the use of a local density approximation
Principal Component Analysis Of Synthetic Galaxy Spectra
We analyse synthetic galaxy spectra from the evolutionary models of
Bruzual&Charlot and Fioc&Rocca-Volmerange using the method of Principal
Component Analysis (PCA). We explore synthetic spectra with different ages,
star formation histories and metalicities, and identify the Principal
Components (PCs) of variance in the spectra due to these different model
parameters. The PCA provides a more objective and informative alternative to
diagnostics by individual spectral lines. We discuss how the PCs can be used to
estimate the input model parameters and explore the impact of noise in this
inverse problem. We also discuss how changing the sampling of the ages and
other model parameters affects the resulting PCs. Our first two synthetic PCs
agree with a similar analysis on observed spectra obtained by Kennicutt and the
2dF redshift survey. We conclude that with a good enough signal-to-noise (S/N>>
10) it is possible to derive age, star formation history and metallicity from
observed galaxy spectra using PCA.Comment: 11 pages, 17 figures, submitted to MNRA