257 research outputs found
Dynamics of highly unbalanced Bose-Bose mixtures: miscible vs immiscible gases
We study the collective modes of the minority component of a highly
unbalanced Bose-Bose mixtures. In the miscible case the minority component
feels an effective external potential and we derive an analytical expression
for the mode frequencies. The latter is independent of the minority component
interaction strength. In the immiscible case we find that the ground state can
be a two-domain walls soliton. Although the mode frequencies are continuous at
the transition, their behaviour is very different with respect to the miscible
case. The dynamical behaviour of the solitonic structure and the frequency
dependence on the inter- and intra-species interaction is numerically studied
using coupled Gross-Pitaevskii equations.Comment: 6 pages, 10 figure
Casimir forces and quantum friction from Ginzburg radiation in atomic BECs
We theoretically propose an experimentally viable scheme to use an impurity
atom in an atomic Bose-Einstein condensate, in order to realize
condensed-matter analogs of quantum vacuum effects. In a suitable atomic level
configuration, the collisional interaction between the impurity atom and the
density fluctuations in the condensate can be tailored to closely reproduce the
electric-dipole coupling of quantum electrodynamics. By virtue of this analogy,
we recover and extend the paradigm of electromagnetic vacuum forces to the
domain of cold atoms, showing in particular the emergence, at supersonic atomic
speeds, of a novel power-law scaling of the Casimir force felt by the atomic
impurity, as well as the occurrence of a quantum frictional force, accompanied
by the Ginzburg emis- sion of Bogoliubov quanta. Observable consequences of
these quantum vacuum effects in realistic spectroscopic experiments are
discussed.Comment: 5 pages, 2 figures. Revised version accepted in PR
Magnetic phase transition in coherently coupled Bose gases in optical lattices
We describe the ground state of a gas of bosonic atoms with two coherently
coupled internal levels in a deep optical lattice in a one dimensional
geometry. In the single-band approximation this system is described by a
Bose-Hubbard Hamiltonian. The system has a superfluid and a Mott insulating
phase which can be either paramagnetic or ferromagnetic. We characterize the
quantum phase transitions at unit filling by means of a density matrix
renormalization group technique and compare it with a mean-field approach. The
presence of the ferromagnetic Ising-like transition modifies the Mott lobes. In
the Mott insulating region the system maps to the ferromagnetic spin-1/2 XXZ
model in a transverse field and the numerical results compare very well with
the analytical results obtained from the spin model. In the superfluid regime
quantum fluctuations strongly modify the phase transition with respect to the
well established mean-field three dimensional classical bifurcation.Comment: 6 pages, 3 figure
Bogoliubov Theory of acoustic Hawking radiation in Bose-Einstein Condensates
We apply the microscopic Bogoliubov theory of dilute Bose-Einstein
condensates to analyze quantum and thermal fluctuations in a flowing atomic
condensate in the presence of a sonic horizon. For the simplest case of a
step-like horizon, closed-form analytical expressions are found for the
spectral distribution of the analog Hawking radiation and for the density
correlation function. The peculiar long-distance density correlations that
appear as a consequence of the Hawking emission features turns out to be
reinforced by a finite initial temperature of the condensate. The analytical
results are in good quantitative agreement with first principle numerical
calculations.Comment: 11 pages, 7 figure
Spin oscillations of the normal polarized Fermi gas at Unitarity
Using density functional theory in a time dependent approach we determine the
frequencies of the compressional modes of the normal phase of a Fermi gas at
unitarity as a function of its polarization. Our energy functional accounts for
the typical elastic deformations exhibited by Landau theory of Fermi liquids.
The comparison with the available experiments is biased by important
collisional effects affecting both the {\it in phase} and the {\it out of
phase} oscillations even at the lowest temperatures. New experiments in the
collisionless regime would provide a crucial test of the applicability of
Landau theory to the dynamics of these strongly interacting normal Fermi gases.Comment: 5 pages, 1 figur
Quadrupole oscillation in a dipolar Fermi gas: hydrodynamic vs collisionless regime
The surface quadrupole mode of an harmonically trapped dipolar Fermi gas is
studied in both the hydrodynamic and collisionless regimes. The anisotropy and
long range effects of the dipolar force as well as the role of the trapping
geometry are explicitly investigated. In the hydrodynamic regime the frequency
is always slightly smaller than the value holding for
gases interacting with contact interactions. In the collisionless regime the
frequency can be either pretty smaller or larger than the non-interacting value
, depending on the cloud aspect ratio. Our results suggest that
the frequency of the surface quadrupole oscillation can provide a useful test
for studying, at very low temperatures, the transition between the normal and
the superfluid phase and, in the normal phase at higher temperatures, the
crossover between the collisional and collisionless regimes. The consequences
of the anisotropy of the dipolar force on the virial theorem are also
discussed.Comment: 8 pages, 4 figure
Dipolar Drag in Bilayer Harmonically Trapped Gases
We consider two separated pancake-shaped trapped gases interacting with a
dipolar (either magnetic or electric) force. We study how the center of mass
motion propagates from one cloud to the other as a consequence of the
long-range nature of the interaction. The corresponding dynamics is fixed by
the frequency difference between the in-phase and the out-of-phase center of
mass modes of the two clouds, whose dependence on the dipolar interaction
strength and the cloud separation is explicitly investigated. We discuss Fermi
gases in the degenerate as well as in the classical limit and comment on the
case of Bose-Einsten condensed gases.Comment: Submitted to EPJD, EuroQUAM special issue "Cold Quantum Matter -
Achievements and Prospects
Andreev-Bashkin effect in superfluid cold gases mixture
We study a mixture of two superfluids with density-density and
current-current (Andreev-Bashkin) interspecies interactions. The
Andreev-Bashkin coupling gives rise to a dissipationless drag (or entrainment)
between the two superfluids. Within the quantum hydrodynamics approximation, we
study the relations between speeds of sound, susceptibilities and static
structure factors, in a generic model in which the density and spin dynamics
decouple. Due to translational invariance, the density channel does not feel
the drag. The spin channel, instead, does not satisfy the usual Bijl-Feynman
relation, since the f-sum rule is not exhausted by the spin phonons. The very
same effect on one dimensional Bose mixtures and their Luttinger liquid
description is analysed within perturbation theory. Using diffusion quantum
Monte Carlo simulations of a system of dipolar gases in a double layer
configuration, we confirm the general results. Given the recent advances in
measuring the counterflow instability, we also study the effect of the
entrainment on the dynamical stability of a superfluid mixture with non-zero
relative velocity.Comment: 12 pages, 4 figure
- …