10,612 research outputs found
Thermally induced coherence in a Mott insulator of bosonic atoms
Conventional wisdom is that increasing temperature causes quantum coherence
to decrease. Using finite temperature perturbation theory and exact
calculations for the strongly correlated bosonic Mott insulating state we show
a practical counter-example that can be explored in optical lattice
experiments: the short-range coherence of the Mott insulating phase can
increase substantially with increasing temperature. We demonstrate that this
phenomenon originates from thermally produced defects that can tunnel with
ease. Since the near zero temperature coherence properties have been measured
with high precision we expect these results to be verifiable in current
experiments.Comment: 5 pages, 3 figure
Orientational transition in nematic liquid crystals under oscillatory Poiseuille flow
We investigate the orientational behaviour of a homeotropically aligned
nematic liquid crystal subjected to an oscillatory plane Poiseuille flow
produced by an alternating pressure gradient. For small pressure amplitudes the
director oscillates within the flow plane around the initial homeotropic
position, whereas for higher amplitudes a spatially homogeneous transition to
out-of-plane director motion was observed for the first time. The orientational
transition was found to be supercritical and the measured frequency dependence
of the critical pressure amplitude in the range between 2 and 20 Hz was in
quantitative agreement with a recent theory.Comment: 11 pages, 4 figures, submitted to Europhys. Let
Development of impact resistant boron/aluminum composites for turbojet engine fan blades
Composite fabrication was performed by vacuum press diffusion bonding by both the foil-filament array and preconsolidated monotape methods. The effect of matrix material, fiber diameter, matrix enhancement, fiber volume reinforcement, test temperature, angle-plying, notch, impact orientation, processing variables and fabrication methods on tensile strength and Charpy impact resistance are evaluated. Root attachment concepts, were evaluated by room and elevated temperature tensile testing, as well as by pendulum-Izod and ballistic impact testing. Composite resistance to foreign object damage was also evaluated by ballistic impacting of panels using projectiles of gelatin, RTV rubber and steel at various velocities, and impingement angles. A significant improvement in the pendulum impact resistance of B-Al composites was achieved
Ezetimibe therapy: mechanism of action and clinical update.
The lowering of low-density lipoprotein cholesterol (LDL-C) is the primary target of therapy in the primary and secondary prevention of cardiovascular events. Although statin therapy is the mainstay for LDL-C lowering, a significant percentage of patients prescribed these agents either do not achieve targets with statin therapy alone or have partial or complete intolerance to them. For such patients, the use of adjuvant therapy capable of providing incremental LDL-C reduction is advised. One such agent is ezetimibe, a cholesterol absorption inhibitor that targets uptake at the jejunal enterocyte brush border. Its primary target of action is the cholesterol transport protein Nieman Pick C1 like 1 protein. Ezetimibe is an effective LDL-C lowering agent and is safe and well tolerated. In response to significant controversy surrounding the use and therapeutic effectiveness of this drug, we provide an update on the biochemical mechanism of action for ezetimibe, its safety and efficacy, as well as the results of recent randomized studies that support its use in a variety of clinical scenarios
Entanglement loss in molecular quantum-dot qubits due to interaction with the environment
We study quantum entanglement loss due to environmental interaction in a
condensed matter system with a complex geometry relevant to recent proposals
for computing with single electrons at the nanoscale. We consider a system
consisting of two qubits, each realized by an electron in a double quantum dot,
which are initially in an entangled Bell state. The qubits are widely separated
and each interacts with its own environment. The environment for each is
modeled by surrounding double quantum dots placed at random positions with
random orientations. We calculate the unitary evolution of the joint system and
environment. The global state remains pure throughout. We examine the time
dependence of the expectation value of the bipartite Clauser-Horne-Shimony-Holt
(CHSH) and Brukner-Paunkovi\'c-Rudolph-Vedral (BPRV) Bell operators and explore
the emergence of correlations consistent with local realism. Though the details
of this transition depend on the specific environmental geometry, we show how
the results can be mapped on to a universal behavior with appropriate scaling.
We determine the relevant disentanglement times based on realistic physical
parameters for molecular double-dots.Comment: 14 pages, 3 figure
Stellar Winds on the Main-Sequence I: Wind Model
Aims: We develop a method for estimating the properties of stellar winds for
low-mass main-sequence stars between masses of 0.4 and 1.1 solar masses at a
range of distances from the star.
Methods: We use 1D thermal pressure driven hydrodynamic wind models run using
the Versatile Advection Code. Using in situ measurements of the solar wind, we
produce models for the slow and fast components of the solar wind. We consider
two radically different methods for scaling the base temperature of the wind to
other stars: in Model A, we assume that wind temperatures are fundamentally
linked to coronal temperatures, and in Model B, we assume that the sound speed
at the base of the wind is a fixed fraction of the escape velocity. In Paper II
of this series, we use observationally constrained rotational evolution models
to derive wind mass loss rates.
Results: Our model for the solar wind provides an excellent description of
the real solar wind far from the solar surface, but is unrealistic within the
solar corona. We run a grid of 1200 wind models to derive relations for the
wind properties as a function of stellar mass, radius, and wind temperature.
Using these results, we explore how wind properties depend on stellar mass and
rotation.
Conclusions: Based on our two assumptions about the scaling of the wind
temperature, we argue that there is still significant uncertainty in how these
properties should be determined. Resolution of this uncertainty will probably
require both the application of solar wind physics to other stars and detailed
observational constraints on the properties of stellar winds. In the final
section of this paper, we give step by step instructions for how to apply our
results to calculate the stellar wind conditions far from the stellar surface.Comment: 24 pages, 13 figures, 2 tables, Accepted for publication in A&
Theory of correlations between ultra-cold bosons released from an optical lattice
In this paper we develop a theoretical description of the correlations
between ultra-cold bosons after free expansion from confinement in an optical
lattice. We consider the system evolution during expansion and give criteria
for a far field regime. We develop expressions for first and second order
two-point correlations based on a variety of commonly used approximations to
the many-body state of the system including Bogoliubov, meanfield decoupling,
and particle-hole perturbative solution about the perfect Mott-insulator state.
Using these approaches we examine the effects of quantum depletion and pairing
on the system correlations. Comparison with the directly calculated correlation
functions is used to justify a Gaussian form of our theory from which we
develop a general three-dimensional formalism for inhomogeneous lattice systems
suitable for numerical calculations of realistic experimental regimes.Comment: 18 pages, 11 figures. To appear in Phys. Rev. A. (few minor changes
made and typos fixed
Stellar winds, dead zones, and coronal mass ejections
Axisymmetric stellar wind solutions are presented, obtained by numerically
solving the ideal magnetohydrodynamic (MHD) equations. Stationary solutions are
critically analysed using the knowledge of the flux functions. These flux
functions enter in the general variational principle governing all axisymmetric
stationary ideal MHD equilibria. The magnetized wind solutions for
(differentially) rotating stars contain both a `wind' and a `dead' zone. We
illustrate the influence of the magnetic field topology on the wind
acceleration pattern, by varying the coronal field strength and the extent of
the dead zone. This is evident from the resulting variations in the location
and appearance of the critical curves where the wind speed equals the slow,
Alfven, and fast speed. Larger dead zones cause effective, fairly isotropic
acceleration to super-Alfvenic velocities as the polar, open field lines are
forced to fan out rapidly with radial distance. A higher field strength moves
the Alfven transition outwards. In the ecliptic, the wind outflow is clearly
modulated by the extent of the dead zone. The combined effect of a fast stellar
rotation and an equatorial `dead' zone in a bipolar field configuration can
lead to efficient thermo-centrifugal equatorial winds. Such winds show both a
strong poleward collimation and some equatorward streamline bending due to
significant toroidal field pressure at mid-latitudes. We discuss how coronal
mass ejections are then simulated on top of the transonic outflows.Comment: scheduled for Astrophys. J. 530 #2, Febr.20 2000 issue. 9 figures (as
6 jpeg and 8 eps files
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