14,267 research outputs found
Inversion of the Diffraction Pattern from an Inhomogeneously Strained Crystal using an Iterative Algorithm
The displacement field in highly non uniformly strained crystals is obtained
by addition of constraints to an iterative phase retrieval algorithm. These
constraints include direct space density uniformity and also constraints to the
sign and derivatives of the different components of the displacement field.
This algorithm is applied to an experimental reciprocal space map measured
using high resolution X-ray diffraction from an array of silicon lines and the
obtained component of the displacement field is in very good agreement with the
one calculated using a finite element model.Comment: 5 pages, 4 figure
Probing a non-biaxial behavior of infinitely thin hard platelets
We give a criterion to test a non-biaxial behavior of infinitely thin hard
platelets of symmetry based upon the components of three order
parameter tensors. We investigated the nematic behavior of monodisperse
infinitely thin rectangular hard platelet systems by using the criterion.
Starting with a square platelet system, and we compared it with rectangular
platelet systems of various aspect ratios. For each system, we performed
equilibration runs by using isobaric Monte Carlo simulations. Each system did
not show a biaxial nematic behavior but a uniaxial nematic one, despite of the
shape anisotropy of those platelets. The relationship between effective
diameters by simulations and theoretical effective diameters of the above
systems was also determined.Comment: Submitted to JPS
A unified evaluation of iterative projection algorithms for phase retrieval
Iterative projection algorithms are successfully being used as a substitute
of lenses to recombine, numerically rather than optically, light scattered by
illuminated objects. Images obtained computationally allow aberration-free
diffraction-limited imaging and the possibility of using radiation for which no
lenses exist. The challenge of this imaging technique is transfered from the
lenses to the algorithms. We evaluate these new computational ``instruments''
developed for the phase retrieval problem, and discuss acceleration strategies.Comment: 12 pages, 9 figures, revte
High-resolution imaging at the SOAR telescope
Bright single and binary stars were observed at the 4.1-m telescope with a
fast electron-multiplication camera in the regime of partial turbulence
correction by the visible-light adaptive optics system. We compare the angular
resolution achieved by simple averaging of AO-corrected images (long-exposure),
selection and re-centering (shift-and-add or "lucky" imaging) and speckle
interferometry. The effect of partial AO correction, vibrations, and image
post-processing on the attained resolution is shown. Potential usefulness of
these techniques is evaluated for reaching the diffraction limit in
ground-based optical imaging. Measurements of 75 binary stars obtained during
these tests are given and objects of special interest are discussed. We report
tentative resolution of the astrometric companion to Zeta Aqr B. A concept of
advanced high-resolution camera is outlined.Comment: Accepted for publication in PASP. 14 pages, 9 figures, 2 tabl
Directed motion emerging from two coupled random processes: Translocation of a chain through a membrane nanopore driven by binding proteins
We investigate the translocation of a stiff polymer consisting of M monomers
through a nanopore in a membrane, in the presence of binding particles
(chaperones) that bind onto the polymer, and partially prevent backsliding of
the polymer through the pore. The process is characterized by the rates: k for
the polymer to make a diffusive jump through the pore, q for unbinding of a
chaperone, and the rate q kappa for binding (with a binding strength kappa);
except for the case of no binding kappa=0 the presence of the chaperones give
rise to an effective force that drives the translocation process. Based on a
(2+1) variate master equation, we study in detail the coupled dynamics of
diffusive translocation and (partial) rectification by the binding proteins. In
particular, we calculate the mean translocation time as a function of the
various physical parameters.Comment: 22 pages, 5 figures, IOP styl
Recombination of H3+ Ions in the Afterglow of a He-Ar-H2 Plasma
Recombination of H3+ with electrons was studied in a low temperature plasma
in helium. The plasma recombination rate is driven by two body, H3+ + e, and
three-body, H3+ + e + He, processes with the rate coefficients 7.5x10^{-8}cm3/s
and 2.8x10^{-25}cm6/s correspondingly at 260K. The two-body rate coefficient is
in excellent agreement with results from storage ring experiments and
theoretical calculations. We suggest that the three-body recombination involves
formation of highly excited Rydberg neutral H3 followed by an l- or m- changing
collision with He. Plasma electron spectroscopy indicates the presence of H3.Comment: 4 figure
Spectroscopy of the Potential Profile in a Ballistic Quantum Constriction
We present a theory for the nonlinear current-voltage characteristics of a
ballistic quantum constriction. Nonlinear features first develop because of
above-barrier reflection from the potential profile, created by impurities in
the vicinity of the constriction. The nonlinearity appears on a small voltage
scale and makes it possible to determine distances between impurities as well
as the magnitude of the impurity potentials.Comment: 3 pages, 4 figures (availiable upon request), REVTEX, Applied Physics
Report 93-5
Dispersive stabilization of the inverse cascade for the Kolmogorov flow
It is shown by perturbation techniques and numerical simulations that the
inverse cascade of kink-antikink annihilations, characteristic of the
Kolmogorov flow in the slightly supercritical Reynolds number regime, is halted
by the dispersive action of Rossby waves in the beta-plane approximation. For
beta tending to zero, the largest excited scale is proportional to the
logarithm of one over beta and differs strongly from what is predicted by
standard dimensional phenomenology which ignores depletion of nonlinearity.Comment: 4 pages, LATEX, 3 figures. v3: revised version with minor correction
Finite to infinite steady state solutions, bifurcations of an integro-differential equation
We consider a bistable integral equation which governs the stationary
solutions of a convolution model of solid--solid phase transitions on a circle.
We study the bifurcations of the set of the stationary solutions as the
diffusion coefficient is varied to examine the transition from an infinite
number of steady states to three for the continuum limit of the
semi--discretised system. We show how the symmetry of the problem is
responsible for the generation and stabilisation of equilibria and comment on
the puzzling connection between continuity and stability that exists in this
problem
Geology and geochronology of the Tana Basin, Ethiopia: LIP volcanism, super eruptions and Eocene–Oligocene environmental change
New geological and geochronological data define four episodes of volcanism for the Lake Tana region in the northern Ethiopian portion of the Afro–Arabian Large Igneous Province (LIP): pre-31 Ma flood basalt that yielded a single 40Ar/39Ar age of 34.05±0.54/0.56 Ma; thick and extensive felsic ignimbrites and rhyolites (minimum volume of 2–3×103 km3) erupted between 31.108±0.020/0.041 Ma and 30.844±0.027/0.046 Ma (U–Pb CA-ID-TIMS zircon ages); mafic volcanism bracketed by 40Ar/39Ar ages of 28.90±0.12/0.14 Ma and 23.75±0.02/0.04 Ma; and localised scoraceous basalt with an 40Ar/39Ar age of 0.033±0.005/0.005 Ma. The felsic volcanism was the product of super eruptions that created a 60–80 km diameter caldera marked by km-scale caldera-collapse fault blocks and a steep-sided basin filled with a minimum of 180 m of sediment and the present-day Lake Tana. These new data enable mapping, with a finer resolution than previously possible, Afro–Arabian LIP volcanism onto the timeline of the Eocene–Oligocene transition and show that neither the mafic nor silicic volcanism coincides directly with perturbations in the geochemical records that span that transition. Our results reinforce the view that it is not the development of a LIP alone but its rate of effusion that contributes to inducing global-scale environmental change
- …