5,852 research outputs found
Two-fluid dynamics for a Bose-Einstein condensate out of local equilibrium with the non-condensate
We extend our recent work on the two-fluid hydrodynamics of a Bose-condensed
gas by including collisions involving both condensate and non-condensate atoms.
These collisions are essential for establishing a state of local thermodynamic
equilibrium between the condensate and non-condensate. Our theory is more
general than the usual Landau two-fluid theory, to which it reduces in the
appropriate limit, in that it allows one to describe situations in which a
state of complete local equilibrium between the two components has not been
reached. The exchange of atoms between the condensate and non-condensate is
associated with a new relaxational mode of the gas.Comment: 4 pages, revtex, 1 postscript figure, Fig.1 has been correcte
Bose-Einstein Condensation of Helium and Hydrogen inside Bundles of Carbon Nanotubes
Helium atoms or hydrogen molecules are believed to be strongly bound within
the interstitial channels (between three carbon nanotubes) within a bundle of
many nanotubes. The effects on adsorption of a nonuniform distribution of tubes
are evaluated. The energy of a single particle state is the sum of a discrete
transverse energy Et (that depends on the radii of neighboring tubes) and a
quasicontinuous energy Ez of relatively free motion parallel to the axis of the
tubes. At low temperature, the particles occupy the lowest energy states, the
focus of this study. The transverse energy attains a global minimum value
(Et=Emin) for radii near Rmin=9.95 Ang. for H2 and 8.48 Ang.for He-4. The
density of states N(E) near the lowest energy is found to vary linearly above
this threshold value, i.e. N(E) is proportional to (E-Emin). As a result, there
occurs a Bose-Einstein condensation of the molecules into the channel with the
lowest transverse energy. The transition is characterized approximately as that
of a four dimensional gas, neglecting the interactions between the adsorbed
particles. The phenomenon is observable, in principle, from a singular heat
capacity. The existence of this transition depends on the sample having a
relatively broad distribution of radii values that include some near Rmin.Comment: 21 pages, 9 figure
Finite temperature excitations of a trapped Bose gas
We present a detailed study of the temperature dependence of the condensate
and noncondensate density profiles of a Bose-condensed gas in a parabolic trap.
These quantitites are calculated self-consistently using the
Hartree-Fock-Bogoliubov equations within the Popov approximation. Below the
Bose-Einstein transition the excitation frequencies have a realtively weak
temperature dependence even though the condensate is strongly depleted. As the
condensate density goes to zero through the transition, the excitation
frequencies are strongly affected and approach the frequencies of a
noninteracting gas in the high temperature limit.Comment: 4 pages, Latex, 4 postscript figures. Submitted to Physical Review
Letter
Theory of acoustic surface plasmons
Recently, a novel low-energy collective excitation has been predicted to
exist at metal surfaces where a quasi two-dimensional (2D) surface-state band
coexists with the underlying three-dimensional (3D) continuum. Here we present
a model in which the screening of a semiinfinite 3D metal is incorporated into
the description of electronic excitations in a 2D electron gas through the
introduction of an effective 2D dielectric function. Our self-consistent
calculations of the dynamical response of the 3D substrate indicate that an
acoustic surface plasmon exists for all possible locations of the 2D sheet
relative to the metal surface. This low-energy excitation, which exhibits
linear dispersion at low wave vectors, is dictated by the nonlocality of the 3D
dynamical response providing incomplete screening of the 2D electron-density
oscillations.Comment: 10 pages, 7 figures, to appear in Phys. Rev.
Meeting the Expectations of Your Heritage Culture: Links between Attachment Style, Intragroup Marginalisation, and Psychological Adjustment
This article has been made available through the Brunel Open Access Publishing Fund.This article has been made available through the Brunel Open Access Publishing Fund.Do insecurely-attached individuals perceive greater rejection from their heritage culture? Few studies have examined the antecedents and outcomes of this perceived rejection – termed intragroup marginalisation – in spite of its implications for the adjustment of cultural migrants to the mainstream culture. The present study investigated whether anxious and avoidant attachment orientations among cultural migrants were associated with greater intragroup marginalisation and, in turn, with lower subjective well-being and flourishing, and higher acculturative stress. Anxious attachment was associated with heightened intragroup marginalisation from friends and, in turn, with increased acculturative stress; anxious attachment was also associated with increased intragroup marginalisation from family. Avoidant attachment was linked with increased intragroup marginalisation from family and, in turn, with decreased subjective well-being
Prediction of the Aerodynamic Environment and Heat Transfer for Rotor-Stator Configurations,’’
ABSTRACT A numerical study of the aerodynamic and thermal environment associated with axial turbine stages is presented. Computations were performed using a modification of the unsteady viscous code, ROTORI, and an improved version of the steady inviscid cascade system, MERIDL-TSONIC, coupled with boundary layer codes, BLAYER and STAN5. Two different turbine stages were analyzed: the first stage of the United Technologies Research Center Large Scale Rotating Rig (LSRR) and the first stage of the Space Shuttle Main Engine (SSME) high pressure fuel turbopump turbine. The time-averaged airfoil midspan pressure and heat transfer profiles were predicted for numerous thermal boundary conditions including adiabatic wall, prescribed surface temperature, and prescribed heat flux. Computed solutions are compared with each other and with experimental data in the case of the LSRR calculations. Modified ROTOR1 predictions of unsteady pressure envelopes and instantaneous contour plots are also presented. Relative merits of the two computational approaches are discussed
The Herschel-SPIRE instrument and its in-flight performance
The Spectral and Photometric Imaging REceiver (SPIRE), is the Herschel Space Observatory`s submillimetre camera and spectrometer. It contains a three-band imaging photometer operating at 250, 350 and 500 μm, and an imaging Fourier-transform spectrometer (FTS) which covers simultaneously its whole operating range of 194–671 μm (447–1550 GHz). The SPIRE detectors are arrays of feedhorn-coupled bolometers cooled to 0.3 K. The photometer has a field of view of 4´× 8´, observed simultaneously in the three spectral bands. Its main operating mode is scan-mapping, whereby the field of view is scanned across the sky to achieve full spatial sampling and to cover large areas if desired. The spectrometer has an approximately circular field of view with a diameter of 2.6´. The spectral resolution can be adjusted between 1.2 and 25 GHz by changing the stroke length of the FTS scan mirror. Its main operating mode involves a fixed telescope pointing with multiple scans of the FTS mirror to acquire spectral data. For extended source measurements, multiple position offsets are implemented by means of an internal beam steering mirror to achieve the desired spatial sampling and by rastering of the telescope pointing to map areas larger than the field of view. The SPIRE instrument consists of a cold focal plane unit located inside the Herschel cryostat and warm electronics units, located on the spacecraft Service Module, for instrument control and data handling. Science data are transmitted to Earth with no on-board data compression, and processed by automatic pipelines to produce calibrated science products. The in-flight performance of the instrument matches or exceeds predictions based on pre-launch testing and modelling: the photometer sensitivity is comparable to or slightly better than estimated pre-launch, and the spectrometer sensitivity is also better by a factor of 1.5–2
Casimir energy and variational methods in AdS spacetime
Following the subtraction procedure for manifolds with boundaries, we
calculate by variational methods, the Schwarzschild-Anti-de Sitter and the
Anti-de Sitter space energy difference. By computing the one loop approximation
for TT tensors we discover the existence of an unstable mode at zero
temperature, which can be stabilized by the boundary reduction method.
Implications on a foam-like space are discussed.Comment: Submitted to Classical and Quantum Gravit
Self-consistent perturbation expansion for Bose-Einstein condensates satisfying Goldstone's theorem and conservation laws
Quantum-field-theoretic descriptions of interacting condensed bosons have
suffered from the lack of self-consistent approximation schemes satisfying
Goldstone's theorem and dynamical conservation laws simultaneously. We present
a procedure to construct such approximations systematically by using either an
exact relation for the interaction energy or the Hugenholtz-Pines relation to
express the thermodynamic potential in a Luttinger-Ward form. Inspection of the
self-consistent perturbation expansion up to the third order with respect to
the interaction shows that the two relations yield a unique identical result at
each order, reproducing the conserving-gapless mean-field theory [T. Kita, J.
Phys. Soc. Jpn. 74, 1891 (2005)] as the lowest-order approximation. The
uniqueness implies that the series becomes exact when infinite terms are
retained. We also derive useful expressions for the entropy and superfluid
density in terms of Green's function and a set of real-time dynamical equations
to describe thermalization of the condensate.Comment: 15 pages, 6 figures, to appear in Phys. Rev.
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