1,224 research outputs found
On a slender dry patch in a liquid film draining under gravity down an inclined plane
In this paper two similarity solutions describing a steady, slender, symmetric dry patch in an infinitely wide liquid film draining under gravity down an inclined plane are obtained. The first solution, which predicts that the dry patch has a parabolic shape and that the transverse profile of the free surface always has a monotonically increasing shape, is appropriate for weak surface-tension effects and far from the apex of the dry patch. The second solution, which predicts that the dry patch has a quartic shape and that the transverse profile of the free surface has a capillary ridge near the contact line and decays in an oscillatory manner far from it, is appropriate for strong surface-tension effects (in particular, when the plane is nearly vertical) and near (but not too close) to the apex of the dry patch. With the average volume flux per unit width (or equivalently with the uniform height of the layer far from the dry patch) prescribed, both solutions contain a free parameter. For each value of this parameter there is a unique solution in the first case and either no solution or a one-parameter family of solutions in the second case. The solutions capture some of the qualitative features observed in experiments
Coupled Bose-Einstein condensate: Collapse for attractive interaction
We study the collapse in a coupled Bose-Einstein condensate of two types of
bosons 1 and 2 under the action of a trap using the time-dependent
Gross-Pitaevskii equation. The system may undergo collapse when one, two or
three of the scattering lengths for scattering of boson with ,
, are negative representing an attractive interaction. Depending
on the parameters of the problem a single or both components of the condensate
may experience collapse.Comment: 5 pages and 9 figures, small changes mad
Symbolic integration of a product of two spherical bessel functions with an additional exponential and polynomial factor
We present a mathematica package that performs the symbolic calculation of
integrals of the form \int^{\infty}_0 e^{-x/u} x^n j_{\nu} (x) j_{\mu} (x) dx
where and denote spherical Bessel functions of
integer orders, with and . With the real parameter
and the integer , convergence of the integral requires that . The package provides analytical result for the integral in its most
simplified form. The novel symbolic method employed enables the calculation of
a large number of integrals of the above form in a fraction of the time
required for conventional numerical and Mathematica based brute-force methods.
We test the accuracy of such analytical expressions by comparing the results
with their numerical counterparts.Comment: 17 pages; updated references for the introductio
Bose-Einstein condensate collapse: a comparison between theory and experiment
We solve the Gross-Pitaevskii equation numerically for the collapse induced
by a switch from positive to negative scattering lengths. We compare our
results with experiments performed at JILA with Bose-Einstein condensates of
Rb-85, in which the scattering length was controlled using a Feshbach
resonance. Building on previous theoretical work we identify quantitative
differences between the predictions of mean-field theory and the results of the
experiments. Besides the previously reported difference between the predicted
and observed critical atom number for collapse, we also find that the predicted
collapse times systematically exceed those observed experimentally. Quantum
field effects, such as fragmentation, that might account for these
discrepancies are discussed.Comment: 4 pages, 2 figure
Mean-field description of collapsing and exploding Bose-Einstein condensates
We perform numerical simulation based on the time-dependent mean-field
Gross-Pitaevskii equation to understand some aspects of a recent experiment by
Donley et al. on the dynamics of collapsing and exploding Bose-Einstein
condensates of Rb atoms. They manipulated the atomic interaction by an
external magnetic field via a Feshbach resonance, thus changing the repulsive
condensate into an attractive one and vice versa. In the actual experiment they
changed suddenly the scattering length of atomic interaction from positive to a
large negative value on a pre-formed condensate in an axially symmetric trap.
Consequently, the condensate collapses and ejects atoms via explosion. We find
that the present mean-field analysis can explain some aspects of the dynamics
of the collapsing and exploding Bose-Einstein condensates.Comment: 9 Latex pages, 10 ps and eps files, version accepted in Physical
Review A, minor changes mad
Scaling limit of virtual states of triatomic systems
For a system with three identical atoms, the dependence of the wave
virtual state energy on the weakly bound dimer and trimer binding energies is
calculated in a form of a universal scaling function. The scaling function is
obtained from a renormalizable three-body model with a pairwise Dirac-delta
interaction. It was also discussed the threshold condition for the appearance
of the trimer virtual state.Comment: 9 pages, 3 figure
Low-Prandtl-number B\'enard-Marangoni convection in a vertical magnetic field
The effect of a homogeneous magnetic field on surface-tension-driven
B\'{e}nard convection is studied by means of direct numerical simulations. The
flow is computed in a rectangular domain with periodic horizontal boundary
conditions and the free-slip condition on the bottom wall using a
pseudospectral Fourier-Chebyshev discretization. Deformations of the free
surface are neglected. Two- and three-dimensional flows are computed for either
vanishing or small Prandtl number, which are typical of liquid metals. The main
focus of the paper is on a qualitative comparison of the flow states with the
non-magnetic case, and on the effects associated with the possible
near-cancellation of the nonlinear and pressure terms in the momentum equations
for two-dimensional rolls. In the three-dimensional case, the transition from a
stationary hexagonal pattern at the onset of convection to three-dimensional
time-dependent convection is explored by a series of simulations at zero
Prandtl number.Comment: 26 pages, 9 figure
Scaling predictions for radii of weakly bound triatomic molecules
The mean-square radii of the molecules He, HeLi,
HeLi and HeNa are calculated using a three-body model
with contact interactions. They are obtained from a universal scaling function
calculated within a renormalized scheme for three particles interacting through
pairwise Dirac-delta interaction. The root-mean-square distance between two
atoms of mass in a triatomic molecule are estimated to be of de order of
, where is the dimer and the
trimer binding energies, and is a constant (varying from
to ) that depends on the ratio between and . Considering
previous estimates for the trimer energies, we also predict the sizes of
Rubidium and Sodium trimers in atomic traps.Comment: 7 pages, 2 figure
Telomere-based proliferative lifespan barriers in Werner-syndrome fibroblasts involve both p53-dependent and p53-independent mechanisms
Werner-syndrome fibroblasts have a reduced in vitro life span before entering replicative senescence. Although this has been thought to be causal in the accelerated ageing of this disease, controversy remains as to whether Werner syndrome is showing the acceleration of a normal cellular ageing mechanism or the occurrence of a novel Werner-syndrome-specific process. Here, we analyse the signalling pathways responsible for senescence in Werner-syndrome fibroblasts. Cultured Werner-syndrome (AG05229) fibroblasts senesced after approximately 20 population doublings with most of the cells having a 2N content of DNA. This was associated with hypophosphorylated pRb and high levels of p16(Ink4a) and p21(Waf1). Senescent AG05229 cells re-entered the cell cycle following microinjection of a p53-neutralizing antibody. Similarly, production of the human papilloma virus 16 E6 oncoprotein in presenescent AG05229 cells resulted in senescence being bypassed and extended cellular life span. Werner-syndrome fibroblasts expressing E6 did not proliferate indefinitely but reached a second proliferative lifespan barrier, termed M(int), that could be bypassed by forced production of telomerase in post-M1 E6-producing cells. The conclusions from these studies are that: (1) replicative senescence in Werner-syndrome fibroblasts is a telomere-induced p53-dependent event; and (2) the intermediate lifespan barrier M(int) is also a telomere-induced event, although it appears to be independent of p53. Werner-syndrome fibroblasts resemble normal human fibroblasts for both these proliferative lifespan barriers, with the strong similarity between the signalling pathway linking telomeres to cell-cycle arrest in Werner-syndrome and normal fibroblasts providing further support for the defect in Werner syndrome causing the acceleration of a normal ageing mechanism
Infrared optical properties of the spin-1/2 quantum magnet
We report results on the electrodynamic response of , a
low-dimensional spin-1/2 quantum magnet that shows a spin gap formation for
T= 67 . The Fano-like shape of a few selected infrared active
phonons suggests an interaction between lattice vibrations and a continuum of
low frequency (spin) excitations. The temperature dependence of the phonon mode
parameters extends over a broad temperature range well above ,
indicating the presence of an extended fluctuation regime. In the temperature
interval between 200 and there is a progressive dimensionality
crossover (from two to one), as well as a spectral weight shift from low
towards high frequencies. This allows us to identify a characteristic energy
scale of about 430 , ascribed to a pseudo spin-gap
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