926 research outputs found
Metamorphosis of helical magnetorotational instability in the presence of axial electric current
This paper presents numerical linear stability analysis of a cylindrical
Taylor-Couette flow of liquid metal carrying axial electric current in a
generally helical external magnetic field. Axially symmetric disturbances are
considered in the inductionless approximation corresponding to zero magnetic
Prandtl number. Axial symmetry allows us to reveal an entirely new
electromagnetic instability. First, we show that the electric current passing
through the liquid can extend the range of helical magnetorotational
instability (HMRI) indefinitely by transforming it into a purely
electromagnetic instability. Two different electromagnetic instability
mechanisms are identified. The first is an internal pinch-type instability,
which is due to the interaction of the electric current with its own magnetic
field. Axisymmetric mode of this instability requires a free-space component of
the azimuthal magnetic field. When the azimuthal component of the magnetic
field is purely rotational and the axial component is nonzero, a new kind of
electromagnetic instability emerges. The latter driven by the interaction of
electric current with a weak collinear magnetic field in a quiescent fluid
gives rise to a steady meridional circulation coupled with azimuthal rotation.Comment: 10 pages, 12 figures, final versio
Quantum graphs where back-scattering is prohibited
We describe a new class of scattering matrices for quantum graphs in which
back-scattering is prohibited. We discuss some properties of quantum graphs
with these scattering matrices and explain the advantages and interest in their
study. We also provide two methods to build the vertex scattering matrices
needed for their construction.Comment: 15 page
Stock assessment of sciaenid resources of India
B e catches of Sew fishes decrensed froin 1'379 to 1980 and later increased in sages frum 1961 to
1984,1985 to 87 and 1988~089 . Gujantand Mahamshtra togehrwntribured 52%of zdal sdaenid calches
along she wcsl coast. hi b.%h the states, the landings during tht: f~stfe w years showed an incmsing trend
whercss during the remaining period, the catches and permrage eontnbuticn varied. Along the east coast
dso i?r,e sciaenid landings showed annual fluctuations
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The spectrum of myelodysplastic syndromes post-solid organ transplantation: A single institutional experience
An increased incidence of acute myeloid leukemia (AML) has recently been documented in patients post-solid organ transplantation but the incidence and types of myelodysplastic syndromes (MDS) occurring in this patient population are not known. We identified 5 patients (3M, 2F, age 48–64 years) who developed MDS ranging from 1.8 to 25 years (median 4.2 years) post-solid organ transplantation, only 2 patients had received azathioprine. The cumulative incidence of MDS in heart and lung transplant recipients at 15 years was 0.5% and 1.8%, respectively, which is markedly higher compared to the general population. Low-risk types of MDS predominated, 3 of 5 patients are alive (median 3.9 years) since diagnosis. Deletions of chromosome 20q, which have not been previously reported in post-transplant MDS/AML, were identified in 3 cases. Our findings expand the morphologic and cytogenetic spectrum of MDS occurring post-solid organ transplantation and suggest that mechanisms beside azathioprine toxicity might be important in disease pathogenesis
Profile scaling in decay of nanostructures
The flattening of a crystal cone below its roughening transition is studied
by means of a step flow model. Numerical and analytical analyses show that the
height profile, h(r,t), obeys the scaling scenario dh/dr = F(r t^{-1/4}). The
scaling function is flat at radii r<R(t) \sim t^{1/4}. We find a one parameter
family of solutions for the scaling function, and propose a selection criterion
for the unique solution the system reaches.Comment: 4 pages, RevTex, 3 eps figure
Simulations of energetic beam deposition: from picoseconds to seconds
We present a new method for simulating crystal growth by energetic beam
deposition. The method combines a Kinetic Monte-Carlo simulation for the
thermal surface diffusion with a small scale molecular dynamics simulation of
every single deposition event. We have implemented the method using the
effective medium theory as a model potential for the atomic interactions, and
present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to
35 eV. The method is capable of following the growth of several monolayers at
realistic growth rates of 1 monolayer per second, correctly accounting for both
energy-induced atomic mobility and thermal surface diffusion. We find that the
energy influences island and step densities and can induce layer-by-layer
growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag),
which correlates with where the net impact-induced downward interlayer
transport is at a maximum. A high step density is needed for energy induced
layer-by-layer growth, hence the effect dies away at increased temperatures,
where thermal surface diffusion reduces the step density. As part of the
development of the method, we present molecular dynamics simulations of single
atom-surface collisions on flat parts of the surface and near straight steps,
we identify microscopic mechanisms by which the energy influences the growth,
and we discuss the nature of the energy-induced atomic mobility
Observation of coherent backscattering of light by cold atoms
Coherent backscattering (CBS) of light waves by a random medium is a
signature of interference effects in multiple scattering. This effect has been
studied in many systems ranging from white paint to biological tissues.
Recently, we have observed CBS from a sample of laser-cooled atoms, a
scattering medium with interesting new properties. In this paper we discuss
various effects, which have to be taken into account for a quantitative study
of coherent backscattering of light by cold atoms.Comment: 25 pages LaTex2e, 17 figures, submitted to J. Opt. B: Quant. Semicl.
Op
Amicable pairs and aliquot cycles for elliptic curves
An amicable pair for an elliptic curve E/Q is a pair of primes (p,q) of good
reduction for E satisfying #E(F_p) = q and #E(F_q) = p. In this paper we study
elliptic amicable pairs and analogously defined longer elliptic aliquot cycles.
We show that there exist elliptic curves with arbitrarily long aliqout cycles,
but that CM elliptic curves (with j not 0) have no aliqout cycles of length
greater than two. We give conjectural formulas for the frequency of amicable
pairs. For CM curves, the derivation of precise conjectural formulas involves a
detailed analysis of the values of the Grossencharacter evaluated at a prime
ideal P in End(E) having the property that #E(F_P) is prime. This is especially
intricate for the family of curves with j = 0.Comment: 53 page
The profile of a decaying crystalline cone
The decay of a crystalline cone below the roughening transition is studied.
We consider local mass transport through surface diffusion, focusing on the two
cases of diffusion limited and attachment-detachment limited step kinetics. In
both cases, we describe the decay kinetics in terms of step flow models.
Numerical simulations of the models indicate that in the attachment-detachment
limited case the system undergoes a step bunching instability if the repulsive
interactions between steps are weak. Such an instability does not occur in the
diffusion limited case. In stable cases the height profile, h(r,t), is flat at
radii r<R(t)\sim t^{1/4}. Outside this flat region the height profile obeys the
scaling scenario \partial h/\partial r = {\cal F}(r t^{-1/4}). A scaling ansatz
for the time-dependent profile of the cone yields analytical values for the
scaling exponents and a differential equation for the scaling function. In the
long time limit this equation provides an exact description of the discrete
step dynamics. It admits a family of solutions and the mechanism responsible
for the selection of a unique scaling function is discussed in detail. Finally
we generalize the model and consider permeable steps by allowing direct adatom
hops between neighboring terraces. We argue that step permeability does not
change the scaling behavior of the system, and its only effect is a
renormalization of some of the parameters.Comment: 25 pages, 18 postscript figure
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