879 research outputs found
Single-particle vs. pair superfluidity in a bilayer system of dipolar bosons
We consider the ground state of a bilayer system of dipolar bosons, where
dipoles are oriented by an external field in the direction perpendicular to the
parallel planes. Quantum Monte Carlo methods are used to calculate the
ground-state energy, the one-body and two-body density matrix, and the
superfluid response as a function of the separation between layers. We find
that by decreasing the interlayer distance for fixed value of the strength of
the dipolar interaction, the system undergoes a quantum phase transition from a
single-particle to a pair superfluid. The single-particle superfluid is
characterized by a finite value of both the atomic condensate and the
super-counterfluid density. The pair superfluid phase is found to be stable
against formation of many-body cluster states and features a gap in the
spectrum of elementary excitations.Comment: 4 figure
The critical temperature of a trapped, weakly interacting Bose gas
We report on measurements of the critical temperature of a harmonically
trapped, weakly interacting Bose gas as a function of atom number. Our results
exclude ideal-gas behavior by more than two standard deviations, and agree
quantitatively with mean-field theory. At our level of sensitivity, we find no
additional shift due to critical fluctuations. In the course of this
measurement, the onset of hydrodynamic expansion in the thermal component has
been observed. Our thermometry method takes this feature into account.Comment: version 2, 20 octobre 200
Superfluidity vs Bose-Einstein condensation in a Bose gas with disorder
We investigate the phenomenon of Bose-Einstein condensation and superfluidity
in a Bose gas at zero temperature with disorder. By using the Diffusion
Monte-Carlo method we calculate the superfluid and the condensate fraction of
the system as a function of density and strength of disorder. In the regime of
weak disorder we find agreement with the analytical results obtained within the
Bogoliubov model. For strong disorder the system enters an unusual regime where
the superfluid fraction is smaller than the condensate fraction.Comment: 4 pages, 4 Postscript figure
Dilute Bose gas with correlated disorder: A Path Integral Monte Carlo study
We investigate the thermodynamic properties of a dilute Bose gas in a
correlated random potential using exact path integral Monte Carlo methods. The
study is carried out in continuous space and disorder is produced in the
simulations by a 3D speckle pattern with tunable intensity and correlation
length. We calculate the shift of the superfluid transition temperature due to
disorder and we highlight the role of quantum localization by comparing the
critical chemical potential with the classical percolation threshold. The
equation of state of the gas is determined in the regime of strong disorder,
where superfluidity is suppressed and the normal phase exists down to very low
temperatures. We find a dependence of the energy in agreement with the
expected behavior in the Bose glass phase. We also discuss the major role
played by the disorder correlation length and we make contact with a
Hartree-Fock mean-field approach that holds valid if the correlation length is
very large. The density profiles are analyzed as a function of temperature and
interaction strength. Effects of localization and the depletion of the order
parameter are emphasized in the comparison between local condensate and total
density. At very low temperature we find that the energy and the particle
distribution of the gas are very well described by the T=0 Gross-Pitaevskii
theory even in the regime of very strong disorder.Comment: 27 pages, 20 figure
Energy and Structure of Hard-Sphere Bose Gases in three and two dimensions
The energy and structure of dilute gases of hard spheres in three dimensions
is discussed, together with some aspects of the corresponding 2D systems. A
variational approach in the framework of the Hypernetted Chain Equations (HNC)
is used starting from a Jastrow wavefunction that is optimized to produce the
best two--body correlation factor with the appropriate long range. Relevant
quantities describing static properties of the system are studied as a function
of the gas parameter where , and are the density,
--wave scattering length of the potential and dimensionality of the space,
respectively. The occurrence of a maximum in the radial distribution function
and in the momentum distribution is a natural effect of the correlations when
increases. Some aspects of the asymptotic behavior of the functions
characterizing the structure of the systems are also investigated.Comment: Proceedings of the QFS2004 conference in Trento. To appear in JLT
Collective and single particle excitations of a trapped Bose gas
The density of states of a Bose-condensed gas confined in a harmonic trap is
investigated. The predictions of Bogoliubov theory are compared with the ones
of Hartree-Fock theory and of the hydrodynamic model. We show that the
Hartree-Fock scheme provides an excellent description of the excitation
spectrum in a wide range of energy, revealing a major role played by
single-particle excitations in these confined systems. The crossover from the
hydrodynamic regime, holding at low energies, to the independent particle
regime is explicitly explored by studying the frequency of the surface mode as
a function of their angular momentum. The applicability of the semiclassical
approximation for the excited states is also discussed. We show that the
semiclassical approach provides simple and accurate formulae for the density of
states and the quantum depletion of the condensate.Comment: 16 pages, REVTeX, 6 figures; misprints corrected; some clarifying
remarks include
Ablation of kynurenine 3-monooxygenase rescues plasma inflammatory cytokine levels in the R6/2 mouse model of Huntington's disease
Kynurenine 3-monooxygenase (KMO) regulates the levels of neuroactive metabolites in the kynurenine pathway (KP), dysregulation of which is associated with Huntington’s disease (HD) pathogenesis. KMO inhibition leads to increased levels of neuroprotective relative to neurotoxic metabolites, and has been found to ameliorate disease-relevant phenotypes in several HD models. Here, we crossed KMO knockout mice to R6/2 HD mice to examine the effect of KMO depletion in the brain and periphery. KP genes were dysregulated in peripheral tissues from R6/2 mice and KMO ablation normalised levels of a subset of these. KP metabolites were also assessed, and KMO depletion led to increased levels of neuroprotective kynurenic acid in brain and periphery, and dramatically reduced neurotoxic 3-hydroxykunurenine levels in striatum and cortex. Notably, the increased levels of pro-inflammatory cytokines TNFa, IL1β, IL4 and IL6 found in R6/2 plasma were normalised upon KMO deletion. Despite these improvements in KP dysregulation and peripheral inflammation, KMO ablation had no effect upon several behavioural phenotypes. Therefore, although genetic inhibition of KMO in R6/2 mice modulates several metabolic and inflammatory parameters, these do not translate to improvements in primary disease indicators—observations which will likely be relevant for other interventions targeted at peripheral inflammation in HD
Optimization of Pyro-gasification of Carbon Fiber Reinforced Polymers (CFRPs)
This work focuses on the optimization of pyro-gasification process of carbon fiber reinforced polymers (CFRPs) with the aim of recovering carbon fibers (CFs) with properties suitable for the production of new more sustainable composites with high performances. In particular, the pyro-gasification process is carried out on cured CFRPs panels based on both epoxy (EC) and vinyl ester (VC) matrices, which are the two most used resins for CFRPs. The matrix degradation is evaluated via sample's weight loss measurement and the recovered CFs obtained after different time of treatment are analyzed to identify convenient pyro-gasification conditions to avoid damaging of the recovered CFs. The obtained results highlight the importance of the thickness of the composites to be treated for the identification of the more suitable pyro-gasification conditions
- …