62,353 research outputs found
Cellular solid behaviour of liquid crystal colloids. 2. Mechanical properties
This paper presents the results of a rheological study of thermotropic
nematic colloids aggregated into cellular structures. Small sterically
stabilised PMMA particles dispersed in a liquid crystal matrix densely pack on
cell interfaces, but reversibly mix with the matrix when the system is heated
above Tni. We obtain a remarkably high elastic modulus, G'~10^5 Pa, which is a
nearly linear function of particle concentration. A characteristic yield stress
is required to disrupt the continuity of cellular structure and liquify the
response. The colloid aggregation in a ``poor nematic'' MBBA has the same
cellular morphology as in the ``good nematic'' 5CB, but the elastic strength is
at least an order of magnitude lower. These findings are supported by
theoretical arguments based on the high surface tension interfaces of a
foam-like cellular system, taking into account the local melting of nematic
liquid and the depletion locking of packed particles on interfaces.Comment: Latex 2e (EPJ style) EPS figures included (poor quality to comply
with space limitations
Transverse modulational instability of partially incoherent soliton stripes
Based on the Wigner distribution approach, an analysis of the effect of
partial incoherence on the transverse instability of soliton structures in
nonlinear Kerr media is presented. It is explicitly shown, that for a
Lorentzian incoherence spectrum the partial incoherence gives rise to a damping
which counteracts, and tends to suppress, the transverse instability growth.
However, the general picture is more complicated and it is shown that the
effect of the partial incoherence depends crucially on the form of the
incoherence spectrum. In fact, for spectra with finite rms-width, the partial
incoherence may even increase both the growth rate and the range of unstable,
transverse wave numbers.Comment: 5 pages, submitted to Phys. Rev.
Coating for prevention of titanium combustion
A limited number of coating options for titanium gas turbine engine components were explored with the objective of minimizing potential combustion initiation and propagation without adversely affecting component mechanical properties. Objectives were met by two of the coatings, ion-plated platinum plus electroplated copper plus electroplated nickel and ion vapor deposited aluminum
The Screen representation of spin networks. Images of 6j symbols and semiclassical features
This article presents and discusses in detail the results of extensive exact
calculations of the most basic ingredients of spin networks, the Racah
coefficients (or Wigner 6j symbols), exhibiting their salient features when
considered as a function of two variables - a natural choice due to their
origin as elements of a square orthogonal matrix - and illustrated by use of a
projection on a square "screen" introduced recently. On these screens, shown
are images which provide a systematic classification of features previously
introduced to represent the caustic and ridge curves (which delimit the
boundaries between oscillatory and evanescent behaviour according to the
asymptotic analysis of semiclassical approaches). Particular relevance is given
to the surprising role of the intriguing symmetries discovered long ago by
Regge and recently revisited; from their use, together with other newly
discovered properties and in conjunction with the traditional combinatorial
ones, a picture emerges of the amplitudes and phases of these discrete
wavefunctions, of interest in wide areas as building blocks of basic and
applied quantum mechanics.Comment: 16 pages, 13 figures, presented at ICCSA 2013 13th International
Conference on Computational Science and Applicatio
Dynamical Instability in a Trimeric Chain of Interacting Bose-Einstein Condensates
We analyze thoroughly the mean-field dynamics of a linear chain of three
coupled Bose-Einstein condensates, where both the tunneling and the
central-well relative depth are adjustable parameters. Owing to its
nonintegrability, entailing a complex dynamics with chaos occurrence, this
system is a paradigm for longer arrays whose simplicity still allows a thorough
analytical study.We identify the set of dynamics fixed points, along with the
associated proper modes, and establish their stability character depending on
the significant parameters. As an example of the remarkable operational value
of our analysis, we point out some macroscopic effects that seem viable to
experiments.Comment: 5 pages, 3 figure
Bering's proposal for boundary contribution to the Poisson bracket
It is shown that the Poisson bracket with boundary terms recently proposed by
Bering (hep-th/9806249) can be deduced from the Poisson bracket proposed by the
present author (hep-th/9305133) if one omits terms free of Euler-Lagrange
derivatives ("annihilation principle"). This corresponds to another definition
of the formal product of distributions (or, saying it in other words, to
another definition of the pairing between 1-forms and 1-vectors in the formal
variational calculus). We extend the formula (initially suggested by Bering
only for the ultralocal case with constant coefficients) onto the general
non-ultralocal brackets with coefficients depending on fields and their spatial
derivatives. The lack of invariance under changes of dependent variables (field
redefinitions) seems a drawback of this proposal.Comment: 18 pages, LaTeX, amssym
Exact and asymptotic computations of elementary spin networks: classification of the quantum-classical boundaries
Increasing interest is being dedicated in the last few years to the issues of
exact computations and asymptotics of spin networks. The large-entries regimes
(semiclassical limits) occur in many areas of physics and chemistry, and in
particular in discretization algorithms of applied quantum mechanics. Here we
extend recent work on the basic building block of spin networks, namely the
Wigner 6j symbol or Racah coefficient, enlightening the insight gained by
exploiting its self-dual properties and studying it as a function of two
(discrete) variables. This arises from its original definition as an
(orthogonal) angular momentum recoupling matrix. Progress also derives from
recognizing its role in the foundation of the modern theory of classical
orthogonal polynomials, as extended to include discrete variables. Features of
the imaging of various regimes of these orthonormal matrices are made explicit
by computational advances -based on traditional and new recurrence relations-
which allow an interpretation of the observed behaviors in terms of an
underlying Hamiltonian formulation as well. This paper provides a contribution
to the understanding of the transition between two extreme modes of the 6j,
corresponding to the nearly classical and the fully quantum regimes, by
studying the boundary lines (caustics) in the plane of the two matrix labels.
This analysis marks the evolution of the turning points of relevance for the
semiclassical regimes and puts on stage an unexpected key role of the Regge
symmetries of the 6j.Comment: 15 pages, 11 figures. Talk presented at ICCSA 2012 (12th
International Conference on Computational Science and Applications, Salvador
de Bahia (Brazil) June 18-21, 2012
Generalized Elliptic Integrals and the Legendre M-function
We study monotonicity and convexity properties of functions arising in the
theory of elliptic integrals, and in particular in the case of a
Schwarz-Christoffel conformal mapping from a half-plane to a trapezoid. We
obtain sharp monotonicity and convexity results for combinations of these
functions, as well as functional inequalities and a linearization property.Comment: 28 page
Gravitational-Wave Stochastic Background from Kinks and Cusps on Cosmic Strings
We compute the contribution of kinks on cosmic string loops to stochastic
background of gravitational waves (SBGW).We find that kinks contribute at the
same order as cusps to the SBGW.We discuss the accessibility of the total
background due to kinks as well as cusps to current and planned gravitational
wave detectors, as well as to the big bang nucleosynthesis (BBN), the cosmic
microwave background (CMB), and pulsar timing constraints. As in the case of
cusps, we find that current data from interferometric gravitational wave
detectors, such as LIGO, are sensitive to areas of parameter space of cosmic
string models complementary to those accessible to pulsar, BBN, and CMB bounds.Comment: 24 pages, 3 figure
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