9,674 research outputs found
Sound propagation in elongated superfluid fermion clouds
We use hydrodynamic equations to study sound propagation in a superfluid
Fermi gas inside a strongly elongated cigar-shaped trap, with main attention to
the transition from the BCS to the unitary regime. We treat first the role of
the radial density profile in the quasi-onedimensional limit and then evaluate
numerically the effect of the axial confinement in a configuration in which a
hole is present in the gas density at the center of the trap. We find that in a
strongly elongated trap the speed of sound in both the BCS and the unitary
regime differs by a factor sqrt{3/5} from that in a homogeneous
three-dimensional superfluid. The predictions of the theory could be tested by
measurements of sound-wave propagation in a set-up such as that exploited by
M.R. Andrews et al. [Phys. Rev. Lett. 79, 553 (1997)] for an atomic
Bose-Einstein condensate
Boson-fermion mixtures inside an elongated cigar-shaped trap
We present mean-field calculations of the equilibrium state in a gaseous
mixture of bosonic and spin-polarized fermionic atoms with repulsive or
attractive interspecies interactions, confined inside a cigar-shaped trap under
conditions such that the radial thickness of the two atomic clouds is
approaching the magnitude of the s-wave scattering lengths. In this regime the
kinetic pressure of the fermionic component is dominant. Full demixing under
repulsive boson-fermion interactions can occur only when the number of fermions
in the trap is below a threshold, and collapse under attractive interactions is
suppressed within the range of validity of the mean-field model. Specific
numerical illustrations are given for values of system parameters obtaining in
7Li-6Li clouds.Comment: 12 pages, 6 figure
Transition to hydrodynamics in colliding fermion clouds
We study the transition from the collisionless to the hydrodynamic regime in
a two-component spin-polarized mixture of 40K atoms by exciting its dipolar
oscillation modes inside harmonic traps. The time evolution of the mixture is
described by the Vlasov-Landau equations and numerically solved with a fully
three-dimensional concurrent code. We observe a master/slave behaviour of the
oscillation frequencies depending on the dipolar mode that is excited.
Regardless of the initial conditions, the transition to hydrodynamics is found
to shift to lower values of the collision rate as temperature decreases.Comment: 11 pages, iop style. submitted to the proceedings of the Levico 2003
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Comment on ``Sound velocity and multibranch Bogoliubov spectrum of an elongated Fermi superfluid in the BEC-BCS crossover"
The work by T. K. Ghosh and K. Machida [cond-mat/0510160 and Phys. Rev. A 73,
013613 (2006)] on the sound velocity in a cylindrically confined Fermi
superfluid obeying a power-law equation of state is shown to make use of an
improper projection of the sound wave equation. This inaccuracy fully accounts
for the difference between their results and those previously reported by
Capuzzi et al. [cond-mat/0509323 and Phys. Rev. A 73, 021603(R) (2006)]. In
this Comment we show that both approaches lead exactly to the same result when
the correct weight function is used in the projection. Plots of the correct
behavior of the phonon and monopole-mode spectra in the BCS, unitary, and BEC
limits are also shown.Comment: Comment on cond-mat/051016
Effects of collisions against thermal impurities in the dynamics of a trapped fermion gas
We present a theoretical study of the dynamical behavior of a gas made of
ultracold fermionic atoms, which during their motions can collide with a much
smaller number of thermal bosonic impurities. The atoms are confined inside
harmonic traps and the interactions between the two species are treated as due
to s-wave scattering with a negative scattering length modeling the 40K-87Rb
fermion-boson system. We set the fermions into motion by giving a small shift
to their trap center and examine two alternative types of initial conditions,
referring to (i) a close-to-equilibrium situation in which the two species are
at the same temperature (well below the Fermi temperature and well above the
Bose-Einstein condensation temperature); and (ii) a far-from-equilibrium case
in which the impurities are given a Boltzmann distribution of momenta while the
fermions are at very low temperatures. The dynamics of the gas is evaluated by
the numerical solution of the Vlasov-Landau equations for the one-body
distribution functions, supported by some analytical results on the collisional
properties of a fermion gas. We find that the trapped gaseous mixture is close
to the collisionless regime for values of the parameters corresponding to
current experiments, but can be driven towards a collisional regime even
without increasing the strength of the interactions, either by going over to
heavier impurity masses or by matching the width of the momentum distribution
of the impurities to the Fermi momentum of the fermion gas.Comment: 7 pages, 4 figures, RevTeX 4, accepted in PR
Excised acoustic black holes: the scattering problem in the time domain
The scattering process of a dynamic perturbation impinging on a draining-tub
model of an acoustic black hole is numerically solved in the time domain.
Analogies with real black holes of General Relativity are explored by using
recently developed mathematical tools involving finite elements methods,
excision techniques, and constrained evolution schemes for strongly hyperbolic
systems. In particular it is shown that superradiant scattering of a
quasi-monochromatic wavepacket can produce strong amplification of the signal,
offering the possibility of a significant extraction of rotational energy at
suitable values of the angular frequency of the vortex and of the central
frequency of the wavepacket. The results show that theoretical tools recently
developed for gravitational waves can be brought to fruition in the study of
other problems in which strong anisotropies are present.Comment: 8 pages, 9 figure
Daily variability of Ceres' Albedo detected by means of radial velocities changes of the reflected sunlight
Bright features have been recently discovered by Dawn on Ceres, which extend
previous photometric and Space Telescope observations. These features should
produce distortions of the line profiles of the reflected solar spectrum and
therefore an apparent radial velocity variation modulated by the rotation of
the dwarf planet. Here we report on two sequences of observations of Ceres
performed in the nights of 31 July, 26-27 August 2015 by means of the
high-precision HARPS spectrograph at the 3.6-m La Silla ESO telescope. The
observations revealed a quite complex behaviour which likely combines a radial
velocity modulation due to the rotation with an amplitude of approx +/- 6 m/s
and an unexpected diurnal effect. The latter changes imply changes in the
albedo of Occator's bright features due to the blaze produced by the exposure
to solar radiation. The short-term variability of Ceres' albedo is on
timescales ranging from hours to months and can both be confirmed and followed
by means of dedicated radial velocity observations.Comment: 5 pag, 1fig, two tables, MNRAS Letters 201
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