67,373 research outputs found
Interactions between Membrane Inclusions on Fluctuating Membranes
We model membrane proteins as anisotropic objects characterized by
symmetric-traceless tensors and determine the coupling between these
order-parameters and membrane curvature. We consider the interactions between
transmembrane proteins that respect up-down (reflection) symmetry of bilayer
membranes and that have circular or non-circular cross-sectional areas in the
tangent-plane of membranes. Using a field theoretic approach, we find
non-entropic interactions between reflection-symmetry-breaking
transmembrane proteins with circular cross-sectional area and entropic
interactions between transmembrane proteins with circular
cross-section that do not break up-down symmetry in agreement with previous
calculations. We also find anisotropic interactions between
reflection-symmetry-conserving transmembrane proteins with non-circular
cross-section, anisotropic interactions between
reflection-symmetry-breaking transmembrane proteins with non-circular
cross-section, and non-entropic many-particle interactions among
non-transmembrane proteins. For large , these interactions might provide the
dominant force inducing aggregation of the membrane proteins.Comment: REVTEX, 29 pages with 4 postscript figures compressed using uufiles.
Introduction and Discussion sections revised. To appear in J. Phys. France I
(September
Casimir torque
We develop a formalism for the calculation of the flow of angular momentum
carried by the fluctuating electromagnetic field within a cavity bounded by two
flat anisotropic materials. By generalizing a procedure employed recently for
the calculation of the Casimir force between arbitrary materials, we obtain an
expression for the torque between anisotropic plates in terms of their
reflection amplitude matrices. We evaluate the torque in 1D for ideal and
realistic model materials.Comment: 8 pages, 4 figs, Submitted to Proc. of QFEXT'05, to appear in J.
Phys.
Effect of linear polarisability and local fields on surface SHG
A discrete dipole model has been developed to describe Surface Second Harmonic Generation by centrosymmetric semiconductors. The double cell method, which enables the linear reflection problem to be solved numerically for semi-infinite systems, has been extended for the nonlinear case. It is shown that a single layer of nonlinear electric dipoles at the surface and nonlocal effects allows to describe the angle of incidence dependent anisotropic SHG obtained from oxidised Si(001) wafers. The influence of the linear response, turns out to be essential to understand the anisotropic SHG-process
A Phase-Field Model of Spiral Dendritic Growth
Domains of condensed-phase monolayers of chiral molecules exhibit a variety
of interesting nonequilibrium structures when formed via pressurization. To
model these domain patterns, we add a complex field describing the tilt degree
of freedom to an (anisotropic) complex-phase-field solidification model. The
resulting formalism allows for the inclusion of (in general, non-reflection
symmetric) interactions between the tilt, the solid-liquid interface, and the
bond orientation. Simulations demonstrate the ability of the model to exhibit
spiral dendritic growth.Comment: text plus Four postscript figure file
N=2 boundary supersymmetry in integrable models and perturbed boundary conformal field theory
Boundary integrable models with N=2 supersymmetry are considered. For the
simplest boundary N=2 superconformal minimal model with a Chebyshev bulk
perturbation we show explicitly how fermionic boundary degrees of freedom arise
naturally in the boundary perturbation in order to maintain integrability and
N=2 supersymmetry. A new boundary reflection matrix is obtained for this model
and N=2 boundary superalgebra is studied. A factorized scattering theory is
proposed for a N=2 supersymmetric extension of the boundary sine-Gordon model
with either (i) fermionic or (ii) bosonic and fermionic boundary degrees of
freedom. Exact results are obtained for some quantum impurity problems: the
boundary scaling Lee-Yang model, a massive deformation of the anisotropic Kondo
model at the filling values g=2/(2n+3) and the boundary Ashkin-Teller model.Comment: 14 pages, LaTeX file with amssymb; v2: typos corrected, references
added, to appear in Nucl. Phys.
Determination of crystal orientation fabric from seismic wideangle data
It is known from ice core analyses that the crystal orientation fabric (COF) of ice sheets is anisotropic and changes over depth. A better understanding of these anisotropies as well as their remote detection is important to optimize flow models for ice. Here we show how seismic wideangle measurements can be used to determine the COF remotely. We demonstrate the principle formalism how observed seismic traveltimes can be related to COF properties by a forward model and then apply the formalism to field data.
The eigenvalues that describe the ice fabric of the ice core EDML (Dronning Maud Land, Antarctca) are set into a relationship with the elasticity tensor. From the elasticity tensor the expected seismic velocities and reflection coefficients are calculated. Additionally we calculate the value eta from the Thomsen-parameters epsilon and delta. The value eta gives a measure of the anisotropy of vertical transverse isotropic (VTI)-media and is an important tool for the NMO-correction of anisotropic data. The approximation of reflection horizons as hyperbolas is not valid anymore in anisotropic media. The calculation of the moveout is therefore performed by a 4th order NMO-correction with the rms-velocity and the effective eta value as variables.
This approach is applied to data from a wideangle survey shot at Halvfarryggen, Dronning Maud Land, Antarctica. From this data we derived rms-velocities and effective eta values. These values were than recalculated to interval velocities and interval eta values to give a hint on the measure of anisotropy of the different layers. The results give first insight into the anisotropies at Halvfarryggen
Light scattering from an isotropic layer between uniaxial crystals
We develop a model for the reflection and transmission of plane waves by an
isotropic layer sandwiched between two uniaxial crystals of arbitrary
orientation. In the laboratory frame, reflection and transmission coefficients
corresponding to the principal polarization directions in each crystal are
given explicitly in terms of the c-axis and propagation directions. The
solution is found by first deriving explicit expressions for reflection and
transmission amplitude coefficients for waves propagating from an arbitrarily
oriented uniaxial anisotropic material into an isotropic material. By combining
these results with Lekner's (1991) earlier treatment of waves propagating from
isotropic media to anisotropic media and employing a matrix method we determine
a solution to the general form of the multiple reflection case. The example
system of a wetted interface between two ice crystals is used to contextualize
the results.Comment: 18 pages, 9 figures,updated with changes made to published versio
Analysis of fluid substitution in a porous and fractured medium
To improve quantitative interpretation of seismic data, we analyze the effect of fluid substitution in a porous and fractured medium on elastic properties and reflection coefficients. This analysis uses closed-form expressions suitable for fluid substitution in transversely isotropic media with a horizontal symmetry axis (HTI). For the HTI medium, the effect of changing porosity and water saturation on (1) P-wave moduli, (2) horizontal and vertical velocities, (3) anisotropic parameters, and (4) reflection coefficients are examined. The effects of fracture density on these four parameters are also studied. For the model used in this study, a 35% increase in porosity lowers the value of P-wave moduli by maximum of 45%. Consistent with the reduction in P-wave moduli, P-wave velocities also decrease by maximum of 17% with a similar increment in porosity. The reduction is always larger for the horizontal P-wave modulus than for the vertical one and is nearly independent of fracture density. The magnitude of the anisotropic parameters of the fractured medium also changes with increased porosity depending on the changes in the value of P-wave moduli. The reflection coefficients at an interface of the fractured medium with an isotropic medium change in accordance with the above observations and lead to an increase in anisotropic amplitude variation with offset (AVO) gradient with porosity.Additionally, we observe a maximum increase in P-wave modulus and velocity by 30% and 8%, respectively, with a 100% increase in water saturation. Water saturation also changes the anisotropic parameters and reflection coefficients. Increase in water saturation considerably increases the magnitude of the anisotropic AVO gradient irrespective of fracture density. From this study, we conclude that porosity and water saturation have a significant impact on the four studied parameters and the impacts are seismically detectable
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