30,472 research outputs found
Response of Bose gases in time-dependent optical superlattices
The dynamic response of ultracold Bose gases in one-dimensional optical
lattices and superlattices is investigated based on exact numerical time
evolutions in the framework of the Bose-Hubbard model. The system is excited by
a temporal amplitude modulation of the lattice potential, as it was done in
recent experiments. For regular lattice potentials, the dynamic signatures of
the superfluid to Mott-insulator transition are studied and the position and
the fine-structure of the resonances is explained by a linear response
analysis. Using direct simulations and the perturbative analysis it is shown
that in the presence of a two-colour superlattice the excitation spectrum
changes significantly when going from the homogeneous Mott-insulator the quasi
Bose-glass phase. A characteristic and experimentally accessible signature for
the quasi Bose-glass is the appearance of low-lying resonances and a
suppression of the dominant resonance of the Mott-insulator phase.Comment: 20 pages, 9 figures; added references and corrected typo
Depletion potentials near geometrically structured substrates
Using the recently developed so-called White Bear version of Rosenfeld's
Fundamental Measure Theory we calculate the depletion potentials between a
hard-sphere colloidal particle in a solvent of small hard spheres and simple
models of geometrically structured substrates: a right-angled wedge or edge. In
the wedge geometry, there is a strong attraction beyond the corresponding one
near a planar wall that significantly influences the structure of colloidal
suspensions in wedges. In accordance with an experimental study, for the edge
geometry we find a free energy barrier of the order of several which
repels a big colloidal particle from the edge.Comment: 7 pages, 7 figure
Density functional theory for hard-sphere mixtures: the White-Bear version Mark II
In the spirit of the White-Bear version of fundamental measure theory we
derive a new density functional for hard-sphere mixtures which is based on a
recent mixture extension of the Carnahan-Starling equation of state. In
addition to the capability to predict inhomogeneous density distributions very
accurately, like the original White-Bear version, the new functional improves
upon consistency with an exact scaled-particle theory relation in the case of
the pure fluid. We examine consistency in detail within the context of
morphological thermodynamics. Interestingly, for the pure fluid the degree of
consistency of the new version is not only higher than for the original
White-Bear version but also higher than for Rosenfeld's original fundamental
measure theory.Comment: 16 pages, 3 figures; minor changes; J. Phys.: Condens. Matter,
accepte
Solvent mediated interactions close to fluid-fluid phase separation: microscopic treatment of bridging in a soft core fluid
Using density functional theory we calculate the density profiles of a binary
solvent adsorbed around a pair of big solute particles. All species interact
via repulsive Gaussian potentials. The solvent exhibits fluid-fluid phase
separation and for thermodynamic states near to coexistence the big particles
can be surrounded by a thick adsorbed `wetting' film of the coexisting solvent
phase. On reducing the separation between the two big particles we find there
can be a `bridging' transition as the wetting films join to form a fluid
bridge. The potential between the two big particles becomes long ranged and
strongly attractive in the bridged configuration. Within our mean-field
treatment the bridging transition results in a discontinuity in the solvent
mediated force. We demonstrate that accounting for the phenomenon of bridging
requires the presence of a non-zero bridge function in the correlations between
the solute particles when our model fluid is described within a full mixture
theory based upon the Ornstein-Zernike equations.Comment: 28 pages, 8 figure
Mode expansion for the density profile of crystal-fluid interfaces: Hard spheres as a test case
We present a technique for analyzing the full three-dimensional density
profiles of a planar crystal-fluid interface in terms of density modes. These
density modes can also be related to crystallinity order parameter profiles
which are used in coarse-grained, phase field type models of the statics and
dynamics of crystal-fluid interfaces and are an alternative to crystallinity
order parameters extracted from simulations using local crystallinity criteria.
We illustrate our results for the hard sphere system using finely-resolved,
three-dimensional density profiles from density functional theory of
fundamental measure type.Comment: submitted for the special issue of the CODEF III conferenc
Quantum phases of atomic boson-fermion mixtures in optical lattices
The zero-temperature phase diagram of a binary mixture of bosonic and
fermionic atoms in an one-dimensional optical lattice is studied in the
framework of the Bose-Fermi-Hubbard model. By exact numerical solution of the
associated eigenvalue problems, ground state observables and the response to an
external phase twist are evaluated. The stiffnesses under phase variations
provide measures for the boson superfluid fraction and the fermionic Drude
weight. Several distinct quantum phases are identified as function of the
strength of the repulsive boson-boson and the boson-fermion interaction.
Besides the bosonic Mott-insulator phase, two other insulating phases are
found, where both the bosonic superfluid fraction and the fermionic Drude
weight vanish simultaneously. One of these double-insulator phases exhibits a
crystalline diagonal long-range order, while the other is characterized by
spatial separation of the two species.Comment: 4 pages, 3 figures, using REVTEX
Hadronic multiparticle production in extensive air showers and accelerator experiments
Using CORSIKA for simulating extensive air showers, we study the relation
between the shower characteristics and features of hadronic multiparticle
production at low energies. We report about investigations of typical energies
and phase space regions of secondary particles which are important for muon
production in extensive air showers. Possibilities to measure relevant
quantities of hadron production in existing and planned accelerator experiments
are discussed.Comment: To be published in Proceedings of ICRC 2005, 29th International
Cosmic Ray Conferenc
Di-boson Production beyond NLO QCD and Anomalous Couplings
In these proceedings, we review results for several di-boson production
processes beyond NLO QCD at high transverse momenta using the VBFNLO
Monte-Carlo program together with the LOOPSIM method. Additionally, we show for
the WZ production process how higher order QCD corrections can resemble
anomalous coupling effects.Comment: Conference Proceedings:C15-05-25.
Development of a relatchable cover mechanism for a cryogenic IR-sensor
A cover mechanism for use on the Infrared Background Signature Survey (IBSS) cryostat was developed. The IBSS IR-instrument is scheduled for STS launch in early 1991 as a payload of the Shuttle Payload Satellite (SPS) 2. The cover is hinged, with a motorized rope drive. During ground processing, launch, entry, and landing, the cryostat, which houses the IR-instrument, is required to be a sealed vacuum tight container for cooling purposes and contamination prevention. When on orbit, the cover is opened to provide an unobstructed field of view for the IR-instrument. A positive seal is accomplished through the use of latch mechanism. The cover and the latch are driven by a common redundant actuator consisting of dc motors, spur gears, and a differential gear. Hall probe limit switches and position sensors (rotary variable transformer) are used to determine the position of the cover and the latch. The cover mechanism was successfully qualified for thermal vacuum (-25 to 35 C), acoustic noise, vibration (6 Gs sine, 9.7 G RMS) and life cycles. Constricting requirements, mechanical and electronic control design, specific design details, test results of functional performance, and environmental and life tests are described
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