1,939 research outputs found
Breakdown of time-dependent mean-field theory for a one-dimensional condensate of impenetrable bosons
We show that the time-dependent nonlinear Schrodinger equation of mean-field
theory has limited utility for a one-dimensional condensate of impenetrable
bosons. Mean-field theory with its associated order parameter predicts
interference between split condensates that are recombined, whereas an exact
many-body treatment shows minimal interference.Comment: 4 pages, 2 EPS figure
Realization of Bose-Einstein condensates in lower dimensions
Bose-Einstein condensates of sodium atoms have been prepared in optical and
magnetic traps in which the energy-level spacing in one or two dimensions
exceeds the interaction energy between atoms, realizing condensates of lower
dimensionality. The cross-over into two-dimensional and one-dimensional
condensates was observed by a change in aspect ratio and saturation of the
release energy when the number of trapped atoms was reduced
Extracellular ATP released by osteoblasts is a key local inhibitor of bone mineralisation
Previous studies have shown that exogenous ATP (>1µM) prevents bone formation in vitro by blocking mineralisation of the collagenous matrix. This effect is thought to be mediated via both P2 receptor-dependent pathways and a receptor-independent mechanism (hydrolysis of ATP to produce the mineralisation inhibitor pyrophosphate, PPi). Osteoblasts are also known to release ATP constitutively. To determine whether this endogenous ATP might exert significant biological effects, bone-forming primary rat osteoblasts were cultured with 0.5-2.5U/ml apyrase (which sequentially hydrolyses ATP to ADP to AMP + 2Pi). Addition of 0.5U/ml apyrase to osteoblast culture medium degraded extracellular ATP to <1% of control levels within 2 minutes; continuous exposure to apyrase maintained this inhibition for up to 14 days. Apyrase treatment for the first 72 hours of culture caused small decreases (≤25%) in osteoblast number, suggesting a role for endogenous ATP in stimulating cell proliferation. Continuous apyrase treatment for 14 days (≥0.5U/ml) increased mineralisation of bone nodules by up to 3-fold. Increases in bone mineralisation were also seen when osteoblasts were cultured with the ATP release inhibitors, NEM and brefeldin A, as well as with P2X1 and P2X7 receptor antagonists. Apyrase decreased alkaline phosphatase (TNAP) activity by up to 60%, whilst increasing the activity of the PPi-generating ecto-nucleotide pyrophosphatase/phosphodiesterases (NPPs) up to 2.7-fold. Both collagen production and adipocyte formation were unaffected. These data suggest that nucleotides released by osteoblasts in bone could act locally, via multiple mechanisms, to limit mineralisation
Zettawatt-Exawatt Lasers and Their Applications in Ultrastrong-Field Physics: High Energy Front
Since its birth, the laser has been extraordinarily effective in the study
and applications of laser-matter interaction at the atomic and molecular level
and in the nonlinear optics of the bound electron. In its early life, the laser
was associated with the physics of electron volts and of the chemical bond.
Over the past fifteen years, however, we have seen a surge in our ability to
produce high intensities, five to six orders of magnitude higher than was
possible before. At these intensities, particles, electrons and protons,
acquire kinetic energy in the mega-electron-volt range through interaction with
intense laser fields. This opens a new age for the laser, the age of nonlinear
relativistic optics coupling even with nuclear physics. We suggest a path to
reach an extremely high-intensity level W/cm in the coming
decade, much beyond the current and near future intensity regime W/cm, taking advantage of the megajoule laser facilities. Such a laser at
extreme high intensity could accelerate particles to frontiers of high energy,
tera-electron-volt and peta-electron-volt, and would become a tool of
fundamental physics encompassing particle physics, gravitational physics,
nonlinear field theory, ultrahigh-pressure physics, astrophysics, and
cosmology. We focus our attention on high-energy applications in particular and
the possibility of merged reinforcement of high-energy physics and ultraintense
laser.Comment: 25 pages. 1 figur
Stability of stationary states in the cubic nonlinear Schroedinger equation: applications to the Bose-Einstein condensate
The stability properties and perturbation-induced dynamics of the full set of
stationary states of the nonlinear Schroedinger equation are investigated
numerically in two physical contexts: periodic solutions on a ring and
confinement by a harmonic potential. Our comprehensive studies emphasize
physical interpretations useful to experimentalists. Perturbation by stochastic
white noise, phase engineering, and higher order nonlinearity are considered.
We treat both attractive and repulsive nonlinearity and illustrate the
soliton-train nature of the stationary states.Comment: 9 pages, 11 figure
“White Flight” or positive contact? Local diversity and attitudes to immigration in Britain
Does the local presence of immigrant groups increase White hostility to immigration? Most research finds that diverse neighborhoods reduce White opposition to minorities and immigration. However, most studies at higher geographies find the reverse effect. We confirm this pattern for England and Wales for 2009-2012. Yet, contextual studies are open to selection bias, which is where this article makes its main contribution. Is White tolerance in diverse neighborhoods the result of a positive effect of inter-ethnic contact, or does it arise from White flight, with anti-immigrant Whites exiting diverse areas but remaining within wider geographies as radicalized opponents of immigration? We provide the first attempt we are aware of to track the opinions of in- and out-migrants, as well as stayers, from local areas over an extended period. We use 20 years of large-scale geocoded British longitudinal data and find only limited evidence of selection effects associated with White flight
Optical dipole traps and atomic waveguides based on Bessel light beams
We theoretically investigate the use of Bessel light beams generated using
axicons for creating optical dipole traps for cold atoms and atomic
waveguiding. Zeroth-order Bessel beams can be used to produce highly elongated
dipole traps allowing for the study of one-dimensional trapped gases and
realization of a Tonks gas of impentrable bosons. First-order Bessel beams are
shown to be able to produce tight confined atomic waveguides over centimeter
distances.Comment: 20 pages, 5 figures, to appear in Phys. Rev.
- …