401 research outputs found
Linear density response in the random phase approximation for confined Bose vapours at finite temperature
A linear response framework is set up for the evaluation of collective
excitations in a confined vapour of interacting Bose atoms at finite
temperature. Focusing on the currently relevant case of contact interactions
between the atoms, the theory is developed within a random phase approximation
with exchange. This approach is naturally introduced in a two-fluid description
by expressing the density response of both the condensate and the
non-condensate in terms of the response of a Hartree-Fock reference gas to the
selfconsistent Hartree-Fock potentials. Such an approximate account of
correlations (i) preserves an interplay between the condensate and the
non-condensate through off-diagonal components of the response, which instead
vanish in the Hartree-Fock-Bogolubov approximation; and (ii) yields a common
resonant structure for the four partial response functions. The theory reduces
to the temperature-dependent Hartree-Fock-Bogolubov-Popov approximation for the
fluctuations of the condensate when its coupling with the density fluctuations
of the non-condensate is neglected. Analytic results are presented which are
amenable to numerical calculations and to inclusion of damping rates.Comment: 14 pages. To appear on J. Phys. : Condens. Matte
Bosonization and entanglement spectrum for one-dimensional polar bosons on disordered lattices
The extended Bose-Hubbard model subjected to a disordered potential is
predicted to display a rich phase diagram. In the case of uniform random
disorder one finds two insulating quantum phases -- the Mott-insulator and the
Haldane insulator -- in addition to a superfluid and a Bose glass phase. In the
case of a quasiperiodic potential further phases are found, eg the
incommensurate density wave, adiabatically connected to the Haldane insulator.
For the case of weak random disorder we determine the phase boundaries using a
perturbative bosonization approach. We then calculate the entanglement spectrum
for both types of disorder, showing that it provides a good indication of the
various phases.Comment: Submitted to NJ
Temperature-dependent density profiles of trapped boson-fermion mixtures
We present a semiclassical three-fluid model for a Bose-condensed mixture of
interacting Bose and Fermi gases confined in harmonic traps at finite
temperature. The model is used to characterize the experimentally relevant
behaviour of the equilibrium density profile of the fermions with varying
composition and temperature across the onset of degeneracy, for coupling
strengths relevant to a mixture of
K and K atoms.Comment: 9 pages, 2 postscript figures, accepted for publication in Eur. Phys.
Jour.
Kinetic energy of a trapped Fermi gas interacting with a Bose-Einstein condensate
We study a confined mixture of bosons and fermions in the regime of quantal
degeneracy, with particular attention to the effects of the interactions on the
kinetic energy of the fermionic component. We are able to explore a wide region
of system parameters by identifying two scaling variables which completely
determine its state at low temperature. These are the ratio of the
boson-fermion and boson-boson interaction strengths and the ratio of the radii
of the two clouds. We find that the effect of the interactions can be sizeable
for reasonable choices of the parameters and that its experimental study can be
used to infer the sign of the boson-fermion scattering length. The interplay
between interactions and thermal effects in the fermionic kinetic energy is
also discussed.Comment: REVTEX, 8 pages, 6 figures included. Small corrections to text and
figures, accepted for publication in EPJ
Anderson localization of a Bose-Einstein condensate in a 3D random potential
We study the effect of Anderson localization on the expansion of a
Bose-Einstein condensate, released from a harmonic trap, in a 3D random
potential. We use scaling arguments and the self-consistent theory of
localization to show that the long-time behavior of the condensate density is
controlled by a single parameter equal to the ratio of the mobility edge and
the chemical potential of the condensate. We find that the two critical
exponents of the localization transition determine the evolution of the
condensate density in time and space.Comment: 4 pages, 2 figure
One-dimensional non-interacting fermions in harmonic confinement: equilibrium and dynamical properties
We consider a system of one-dimensional non-interacting fermions in external
harmonic confinement. Using an efficient Green's function method we evaluate
the exact profiles and the pair correlation function, showing a direct
signature of the Fermi statistics and of the single quantum-level occupancy. We
also study the dynamical properties of the gas, obtaining the spectrum both in
the collisionless and in the collisional regime. Our results apply as well to
describe a one-dimensional Bose gas with point-like hard-core interactions.Comment: 11 pages, 5 figure
The n-acetyl phenylalanine glucosamine derivative attenuates the inflammatory/catabolic environment in a chondrocyte-synoviocyte co-culture system
Osteoarthritis (OA), the most prevalent degenerative joint disease, still lacks a true disease-modifying therapy. The involvement of the NF-κB pathway and its upstream activating kinases in OA pathogenesis has been recognized for many years. The ability of the N-acetyl phenylalanine glucosamine derivative (NAPA) to increase anabolism and reduce catabolism via inhibition of IKKα kinase has been previously observed in vitro and in vivo. The present study aims to confirm the chondroprotective effects of NAPA in an in vitro model of joint OA established with primary cells, respecting both the crosstalk between chondrocytes and synoviocytes and their phenotypes. This model satisfactorily reproduces some features of the previously investigated DMM model, such as the prominent induction of ADAMTS-5 upon inflammatory stimulation. Both gene and protein expression analysis indicated the ability of NAPA to counteract key cartilage catabolic enzymes (ADAMTS-5) and effectors (MCP-1). Molecular analysis showed the ability of NAPA to reduce IKKα nuclear translocation and H3Ser10 phosphorylation, thus inhibiting IKKα transactivation of NF-κB signalling, a pivotal step in the NF-κB-dependent gene expression of some of its targets. In conclusion, our data confirm that NAPA could truly act as a disease-modifying drug in OA
Collective excitations of a trapped boson-fermion mixture across demixing
We calculate the spectrum of low-lying collective excitations in a mesoscopic
cloud formed by a Bose-Einstein condensate and a spin-polarized Fermi gas as a
function of the boson-fermion repulsions. The cloud is under isotropic harmonic
confinement and its dynamics is treated in the collisional regime by using the
equations of generalized hydrodynamics with inclusion of surface effects. For
large numbers of bosons we find that, as the cloud moves towards spatial
separation (demixing) with increasing boson-fermion coupling, the frequencies
of a set of collective modes show a softening followed by a sharp upturn. This
behavior permits a clear identification of the quantum phase transition. We
propose a physical interpretation for the dynamical transition point in a
confined mixture, leading to a simple analytical expression for its location.Comment: revtex4, 9 pages, 8 postscript file
Exact coherent states of a harmonically confined Tonks-Girardeau gas
Using a scaling transformation we exactly determine the dynamics of an
harmonically confined Tonks-Girardeau gas under arbitrary time variations of
the trap frequency. We show how during a one-dimensional expansion a
``dynamical fermionization'' occurs as the momentum distribution rapidly
approaches an ideal Fermi gas distribution, and that under a sudden change of
the trap frequency the gas undergoes undamped breathing oscillations displaying
alternating bosonic and fermionic character in momentum space. The absence of
damping in the oscillations is a peculiarity of the truly Tonks regime.Comment: 4 pages, 2 figures, published versio
Demixing in mesoscopic boson-fermion clouds inside cylindrical harmonic traps: quantum phase diagram and role of temperature
We use a semiclassical three-fluid thermodynamic model to evaluate the
phenomena of spatial demixing in mesoscopic clouds of fermionic and bosonic
atoms at high dilution under harmonic confinement, assuming repulsive
boson-boson and boson-fermion interactions and including account of a bosonic
thermal cloud at finite temperature T. The finite system size allows three
different regimes for the equilibrium density profiles at T=0: a fully mixed
state, a partially mixed state in which the overlap between the boson and
fermion clouds is decreasing, and a fully demixed state where the two clouds
have zero overlap. We propose simple analytical rules for the two cross-overs
between the three regimes as functions of the physical system parameters and
support these rules by extensive numerical calculations. A universal ``phase
diagram'' expressed in terms of simple scaling parameters is shown to be valid
for the transition to the regime of full demixing, inside which we identify
several exotic configurations for the two phase-separated clouds in addition to
simple ones consisting of a core of bosons enveloped by fermions and "vice
versa". With increasing temperature the main role of the growing thermal cloud
of bosons is to transform some exotic configurations into more symmetric ones,
until demixing is ultimately lost. For very high values of boson-fermion
repulsive coupling we also report demixing between the fermions and the
thermally excited bosons.Comment: 11 pages, 8 figure
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