1,718 research outputs found
Inward and Outward Integral Equations and the KKR Method for Photons
In the case of electromagnetic waves it is necessary to distinguish between
inward and outward on-shell integral equations. Both kinds of equation are
derived. A correct implementation of the photonic KKR method then requires the
inward equations and it follows directly from them. A derivation of the KKR
method from a variational principle is also outlined. Rather surprisingly, the
variational KKR method cannot be entirely written in terms of surface integrals
unless permeabilities are piecewise constant. Both kinds of photonic KKR method
use the standard structure constants of the electronic KKR method and hence
allow for a direct numerical application. As a by-product, matching rules are
obtained for derivatives of fields on different sides of the discontinuity of
permeabilities.
Key words: The Maxwell equations, photonic band gap calculationsComment: (to appear in J. Phys. : Cond. Matter), Latex 17 pp, PRA-HEP 93/10
(exclusively English and unimportant misprints corrected
Entropic Elasticity of Twist-Storing Polymers
We investigate the statistical mechanics of a torsionally constrained
polymer. The polymer is modeled as a fluctuating rod with bend stiffness A kT
and twist stiffness C kT. In such a model, thermal bend fluctuations couple
geometrically to an applied torque through the relation Lk = Tw + Wr. We
explore this coupling and find agreement between the predictions of our model
and recent experimental results on single lambda-DNA molecules. This analysis
affords an experimental determination of the microscopic twist stiffness
(averaged over a helix repeat). Quantitative agreement between theory and
experiment is obtained using C=109 nm. The theory further predicts a thermal
reduction of the effective twist rigidity induced by bend fluctuations.
Finally, we find a small reflection of molecular chirality in the experimental
data and interpret it in terms of a twist-stretch coupling of the DNA duplex.Comment: 37 pages RevTeX, 2 postscript figures. Revisions include the analysis
of new data and an investigation of non-perturbative effects. Postscript also
available at http://www.physics.upenn.edu/~moro
Dynamically-Stabilized Pores in Bilayer Membranes
Zhelev and Needham have recently created large, quasi-stable pores in
artificial lipid bilayer vesicles [Biochim. Biophys. Acta 1147 (1993) 89].
Initially created by electroporation, the pores remain open for up to several
seconds before quickly snapping shut. This result is surprising in light of the
large line tension for holes in bilayer membranes and the rapid time scale for
closure of large pores. We show how pores can be dynamically stabilized via a
new feedback mechanism. We also explain quantitatively the observed sudden pore
closure as a tangent bifurcation. Finally we show how Zhelev and Needham's
experiment can be used to measure accurately the pore line tension, an
important material parameter. For their SOPC/CHOL mixture we obtain a line
tension of 2.6 10^{-6} erg/cm.Comment: 7 pages RevTeX, 3 included eps figures (published version).
Postscript also available at http://dept.physics.upenn.edu/~moro
Pulling hairpinned polynucleotide chains: Does base-pair stacking interaction matter?
Force-induced structural transitions both in relatively random and in
designed single-stranded DNA (ssDNA) chains are studied theoretically. At high
salt conditions, ssDNA forms compacted hairpin patterns stabilized by
base-pairing and base-pair stacking interactions, and a threshold external
force is needed to pull the hairpinned structure into a random coiled one. The
base-pair stacking interaction in the ssDNA chain makes this hairpin-coil
conversion a discontinuous (first-order) phase transition process characterized
by a force plateau in the force-extension curve, while lowering this potential
below some critical level turns this transition into continuous (second-order)
type, no matter how strong the base-pairing interaction is. The phase diagram
(including hairpin-I, -II, and random coil) is discussed as a function of
stacking potential and external force. These results are in quantitative
agreement with recent experimental observations of different ssDNA sequences,
and they reveal the necessity to consider the base-pair stacking interactions
in order to understand the structural formation of RNA, a polymer designed by
nature itself. The theoretical method used may be extended to study the
long-range interaction along double-stranded DNA caused by the topological
constraint of fixed linking number.Comment: 8 pages using Revte
Voice morphing using the generative topographic mapping
In this paper we address the problem of Voice Morphing. We attempt to transform the spectral characteristics of a source speaker's speech signal so that the listener would believe that the speech was uttered by a target speaker. The voice morphing system transforms the spectral envelope as represented by a Linear Prediction model. The transformation is achieved by codebook mapping using the Generative Topographic Mapping, a non-linear, latent variable, parametrically constrained, Gaussian Mixture Model
Wavelet-based voice morphing
This paper presents a new multi-scale voice morphing algorithm. This algorithm enables a user to transform one person's speech pattern into another person's pattern with distinct characteristics, giving it a new identity, while preserving the original content. The voice morphing algorithm performs the morphing at different subbands by using the theory of wavelets and models the spectral conversion using the theory of Radial Basis Function Neural Networks. The results obtained on the TIMIT speech database demonstrate effective transformation of the speaker identity
How many orthonormal bases are needed to distinguish all pure quantum states?
We collect some recent results that together provide an almost complete
answer to the question stated in the title. For the dimension d=2 the answer is
three. For the dimensions d=3 and d>4 the answer is four. For the dimension d=4
the answer is either three or four. Curiously, the exact number in d=4 seems to
be an open problem
Resonance-Induced Effects in Photonic Crystals
For the case of a simple face-centered-cubic photonic crystal of homogeneous
dielectric spheres, we examine to what extent single-sphere Mie resonance
frequencies are related to band gaps and whether the width of a gap can be
enlarged due to nearby resonances. Contrary to some suggestions, no spectacular
effects may be expected. When the dielectric constant of the spheres
is greater than the dielectric constant of the
background medium, then for any filling fraction there exists a critical
above which the lowest lying Mie resonance frequency falls inside
the lowest stop gap in the (111) crystal direction, close to its midgap
frequency. If , the correspondence between Mie
resonances and both the (111) stop gap and a full gap does not follow such a
regular pattern. If the Mie resonance frequency is close to a gap edge, one can
observe a resonance-induced widening of a relative gap width by .Comment: 14 pages, 3 figs., RevTex. For more info look at
http://www.amolf.nl/external/wwwlab/atoms/theory/index.htm
Correlation functions for 1d interacting fermions with spin-orbit coupling
We compute correlation functions for one-dimensional electron systems which
spin and charge degrees of freedom are coupled through spin-orbit coupling.
Charge density waves, spin density waves, singlet- triplet- superconducting
fluctuations are studied. We show that the spin-orbit interaction modify the
exponents and the phase diagram of the system, changing the dominant
fluctuations and making new susceptibilities diverge for low temperature.Comment: 5 pages, 3 figures. Accepted for publication in Phys. Rev.
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