1,288 research outputs found
Optical imaging of the effect of in-plane fields on cholesteric liquid crystals
Sharon A. Jewell and J. Roy Sambles, Physical Review E, Vol. 78, article 012701 (2008). Copyright © 2008 by the American Physical Society.The effects of in-plane electric fields on the director structure of cholesteric liquid crystals has been imaged in three dimensions using fluorescence confocal polarizing microscopy. The results show that a liquid crystal lying outside the electrode gap can be significantly affected by stray fields occurring above the electrode surface, resulting in a 90° rotation of the cholesteric helix. Distinct differences between the behavior of cholesterics with positive and negative dielectric anisotropies are observed
Helium irradiation effects in polycrystalline Si, silica, and single crystal Si
Transmission electron microscopy (TEM) has been used to investigate the effects of room temperature 6 keV helium ion irradiation of a thin (≈55 nm thick) tri-layer consisting of polycrystalline Si, silica, and single-crystal Si. The ion irradiation was carried out in situ within the TEM under conditions where approximately 24% of the incident ions came to rest in the specimen. This paper reports on the comparative development of irradiation-induced defects (primarily helium bubbles) in the polycrystalline Si and single-crystal Si under ion irradiation and provides direct measurement of a radiation-induced increase in the width of the polycrystalline layer and shrinkage of the silica layer. Analysis using TEM and electron energy-loss spectroscopy has led to the hypothesis that these result from helium-bubble-induced swelling of the silicon and radiation-induced viscoelastic flow processes in the silica under the influence of stresses applied by the swollen Si layers. The silicon and silica layers are sputtered as a result of the helium ion irradiation; however, this is estimated to be a relatively minor effect with swelling and stress-related viscoelastic flow being the dominant mechanisms of dimensional change
Multi-interaction mean-field renormalization group
We present an extension of the previously proposed mean-field renormalization
method to model Hamiltonians which are characterized by more than just one type
of interaction. The method rests on scaling assumptions about the magnetization
of different sublattices of the given lattice and it generates as many flow
equations as coupling constants without arbitrary truncations on the
renormalized Hamiltonian. We obtain good results for the test case of Ising
systems with an additional second-neighbor coupling in two and three
dimensions. An application of the method is also done to a morphological model
of interacting surfaces introduced recenlty by Likos, Mecke and Wagner [J.
Chem. Phys. {\bf{102}}, 9350 (1995)].
PACS: 64.60.Ak, 64.60.Fr, 05.70.JkComment: Tex file and three macros appended at the end. Five figures available
upon request to: [email protected], Fax: [+]39-40-224-60
The Block Spin Renormalization Group Approach and Two-Dimensional Quantum Gravity
A block spin renormalization group approach is proposed for the dynamical
triangulation formulation of two-dimensional quantum gravity. The idea is to
update link flips on the block lattice in response to link flips on the
original lattice. Just as the connectivity of the original lattice is meant to
be a lattice representation of the metric, the block links are determined in
such a way that the connectivity of the block lattice represents a block
metric. As an illustration, this approach is applied to the Ising model coupled
to two-dimensional quantum gravity. The correct critical coupling is
reproduced, but the critical exponent is obscured by unusually large finite
size effects.Comment: 10 page
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Stability of High Voltage Modulators for Nonlinear Loads
OAK-B135 Gyrotrons have a nonlinear voltage--current characteristic such that the small signal or ac impedance changes as operational voltage and currents are reached. The ac impedance determines the stability of a voltage or current control system. this can become particularly challenging when several gyrotron are connected in parallel to a single modulator. With all gyrotrons hooked to a common ground, large current loops can be generated as well as non-canceling currents in individual coaxial lines. These inequalities can provide the required feedback impulse to start an oscillation condition in the power system for the tubes. Recent operation of two CPI 110 GHz gyrotrons in the MN class from a single modulator on DIII-D has shown instability in the power system. An oscillation in the drive current occurs at various points in the ramp up and flat top portions of the 80 kV voltage pulse with each tube drawing 40 A at full voltage. Efforts to stabilize these instabilities are presented along with some modeling and examination of the issues for gyrotron modulators
Monte Carlo Renormalization of the 3-D Ising model: Analyticity and Convergence
We review the assumptions on which the Monte Carlo renormalization technique
is based, in particular the analyticity of the block spin transformations. On
this basis, we select an optimized Kadanoff blocking rule in combination with
the simulation of a d=3 Ising model with reduced corrections to scaling. This
is achieved by including interactions with second and third neighbors. As a
consequence of the improved analyticity properties, this Monte Carlo
renormalization method yields a fast convergence and a high accuracy. The
results for the critical exponents are y_H=2.481(1) and y_T=1.585(3).Comment: RevTeX, 4 PostScript file
Ice XII in its second regime of metastability
We present neutron powder diffraction results which give unambiguous evidence
for the formation of the recently identified new crystalline ice phase[Lobban
et al.,Nature, 391, 268, (1998)], labeled ice XII, at completely different
conditions. Ice XII is produced here by compressing hexagonal ice I_h at T =
77, 100, 140 and 160 K up to 1.8 GPa. It can be maintained at ambient pressure
in the temperature range 1.5 < T < 135 K. High resolution diffraction is
carried out at T = 1.5 K and ambient pressure on ice XII and accurate
structural properties are obtained from Rietveld refinement. At T = 140 and 160
K additionally ice III/IX is formed. The increasing amount of ice III/IX with
increasing temperature gives an upper limit of T ~ 150 K for the successful
formation of ice XII with the presented procedure.Comment: 3 Pages of RevTeX, 3 tables, 3 figures (submitted to Physical Review
Letters
The Iteratively Regularized Gau{\ss}-Newton Method with Convex Constraints and Applications in 4Pi-Microscopy
This paper is concerned with the numerical solution of nonlinear ill-posed
operator equations involving convex constraints. We study a Newton-type method
which consists in applying linear Tikhonov regularization with convex
constraints to the Newton equations in each iteration step. Convergence of this
iterative regularization method is analyzed if both the operator and the right
hand side are given with errors and all error levels tend to zero. Our study
has been motivated by the joint estimation of object and phase in 4Pi
microscopy, which leads to a semi-blind deconvolution problem with
nonnegativity constraints. The performance of the proposed algorithm is
illustrated both for simulated and for three-dimensional experimental data
Ab-o'th-yate at the Isle of Man
Literatura dialectal. -- Lancashire. -- Pertenece a la colección 1800-1950 del Salamanca Corpus. -- Prosa. -- Benjamin Brierley. -- Ab-o'th-yate at the Isle of Man. -- 1869. -- Primera edición.[EN] Fiction letters written in the Lancashire dialect.
[ES] Cartas escritas en el dialecto de Lancashire
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