794 research outputs found
Beyond Tonks-Girardeau: strongly correlated regime in quasi-one-dimensional Bose gases
We consider a homogeneous 1D Bose gas with contact interactions and large
attractive coupling constant. This system can be realized in tight waveguides
by exploiting a confinement induced resonance of the effective 1D scattering
amplitude. By using a variational {\it ansatz} for the many-body wavefunction,
we show that for small densities the gas-like state is stable and the
corresponding equation of state is well described by a gas of hard rods. By
calculating the compressibility of the system, we provide an estimate of the
critical density at which the gas-like state becomes unstable against cluster
formation. Within the hard-rod model we calculate the one-body density matrix
and the static structure factor of the gas. The results show that in this
regime the system is more strongly correlated than a Tonks-Girardeau gas. The
frequency of the lowest breathing mode for harmonically trapped systems is also
discussed as a function of the interaction strength.Comment: 4 pages, 4 figure
Critical temperature of Bose-Einstein condensation in trapped atomic Bose-Fermi mixtures
We calculate the shift in the critical temperature of Bose-Einstein
condensation for a dilute Bose-Fermi mixture confined by a harmonic potential
to lowest order in both the Bose-Bose and Bose-Fermi coupling constants. The
relative importance of the effect on the critical temperature of the
boson-boson and boson-fermion interactions is investigated as a function of the
parameters of the mixture. The possible relevance of the shift of the
transition temperature in current experiments on trapped Bose-Fermi mixtures is
discussed.Comment: 15 pages, 2 figures, submitted to J. Phys.
Analyzing a Bose polaron across resonant interactions
Recently, two independent experiments reported the observation of long-lived
polarons in a Bose-Einstein condensate, providing an excellent setting to study
the generic scenario of a mobile impurity interacting with a quantum reservoir.
Here, we expand the experimental analysis by disentangling the effects of trap
inhomogeneities and the many-body continuum in one of these experiments. This
makes it possible to extract the energy of the polaron at a well-defined
density as a function of the interaction strength. Comparisons with quantum
Monte-Carlo as well as diagrammatic calculations show good agreement, and
provide a more detailed picture of the polaron properties at stronger
interactions than previously possible. Moreover, we develop a semi-classical
theory for the motional dynamics and three-body loss of the polarons, which
partly explains a previously unresolved discrepancy between theory and
experimental observations for repulsive interactions. Finally, we utilize
quantum Monte-Carlo calculations to demonstrate that the findings reported in
the two experiments are consistent with each other
Picking battles: The impact of trust assumptions on the elaboration of security requirements
This position paper describes work on trust assumptions in the con-text of security requirements. We show how trust assumptions can affect the scope of the analysis, derivation of security requirements, and in some cases how functionality is realized. An example shows how trust assumptions are used by a requirements engineer to help define and limit the scope of analysis and to document the decisions made during the process
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
Survey Simulations of a New Near-Earth Asteroid Detection System
We have carried out simulations to predict the performance of a new
space-based telescopic survey operating at thermal infrared wavelengths that
seeks to discover and characterize a large fraction of the potentially
hazardous near-Earth asteroid (NEA) population. Two potential architectures for
the survey were considered: one located at the Earth-Sun L1 Lagrange point, and
one in a Venus-trailing orbit. A sample cadence was formulated and tested,
allowing for the self-follow-up necessary for objects discovered in the daytime
sky on Earth. Synthetic populations of NEAs with sizes >=140 m in effective
spherical diameter were simulated using recent determinations of their physical
and orbital properties. Estimates of the instrumental sensitivity, integration
times, and slew speeds were included for both architectures assuming the
properties of new large-format 10 um detector arrays capable of operating at
~35 K. Our simulation included the creation of a preliminary version of a
moving object processing pipeline suitable for operating on the trial cadence.
We tested this pipeline on a simulated sky populated with astrophysical sources
such as stars and galaxies extrapolated from Spitzer and WISE data, the catalog
of known minor planets (including Main Belt asteroids, comets, Jovian Trojans,
etc.), and the synthetic NEA model. Trial orbits were computed for simulated
position-time pairs extracted from the synthetic surveys to verify that the
tested cadence would result in orbits suitable for recovering objects at a
later time. Our results indicate that the Earth-Sun L1 and Venus-trailing
surveys achieve similar levels of integral completeness for potentially
hazardous asteroids larger than 140 m; placing the telescope in an interior
orbit does not yield an improvement in discovery rates. This work serves as a
necessary first step for the detailed planning of a next-generation NEA survey.Comment: AJ accepted; corrected typ
A Critical Evaluation of Mechanical and Fire Performance of Flax Fiber Epoxy Resin Composites
In the present work, the mechanical behavior of flax fiber reinforced polymers (FFRPs) intended for racing applications is evaluated when subjected to different environmental conditions. A significant drop of mechanical performance in the presence of water (both 100% relative humidity and water submersion) is observed, highlighting also the fact that panels themselves already contain a fraction of water, probably absorbed onto the flax fibers prior their impregnation with the resin, that, where removed, may influence the mechanical behavior. Moreover, the flame behavior of the FFRP composite is also assessed in comparison with the widely applied carbon fiber reinforced polymers (CFRPs) to highlight the effect of the different reinforcement. Both FRPs are produced with the same flame retarded resin to highlight the contribution of the different reinforcement. The evaluation of the flame behavior of the FFRP panels shows that it completely burns during the cone-calorimetric test, involving in the fire both the matrix and the reinforcement with a stronger and faster heat release than the corresponding CFRP based on the same resin. The above observations seem thus to discourage their use in critical conditions, where the decrease of mechanical performance and the event of fire incidental condition may dramatically and negatively affect the final application
Quasi-one-dimensional Bose gases with large scattering length
Bose gases confined in highly-elongated harmonic traps are investigated over
a wide range of interaction strengths using quantum Monte Carlo techniques. We
find that the properties of a Bose gas under tight transverse confinement are
well reproduced by a 1d model Hamiltonian with contact interactions. We point
out the existence of a unitary regime, where the properties of the quasi-1d
Bose gas become independent of the actual value of the 3d scattering length. In
this unitary regime, the energy of the system is well described by a hard rod
equation of state. We investigate the stability of quasi-1d Bose gases with
positive and negative 3d scattering length.Comment: 5 pages, 3 figure
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