1,958 research outputs found
Stokes-vector evolution in a weakly anisotropic inhomogeneous medium
Equation for evolution of the four-component Stokes vector in weakly
anisotropic and smoothly inhomogeneous media is derived on the basis of
quasi-isotropic approximation of the geometrical optics method, which provides
consequent asymptotic solution of Maxwell equations. Our equation generalizes
previous results, obtained for the normal propagation of electromagnetic waves
in stratified media. It is valid for curvilinear rays with torsion and is
capable to describe normal modes conversion in the inhomogeneous media.
Remarkably, evolution of the Stokes vector is described by the
Bargmann-Michel-Telegdi equation for relativistic spin precession, whereas the
equation for the three-component Stokes vector resembles the Landau-Lifshitz
equation for spin precession in ferromegnetic systems. General theory is
applied for analysis of polarization evolution in a magnetized plasma. We also
emphasize fundamental features of the non-Abelian polarization evolution in
anisotropic inhomogeneous media and illustrate them by simple examples.Comment: 16 pages, 3 figures, to appear in J. Opt. Soc. Am.
Photoinduced Changes of Reflectivity in Single Crystals of YBa2Cu3O6.5 (Ortho II)
We report measurements of the photoinduced change in reflectivity of an
untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay
rate of the transient change in reflectivity is found to decrease rapidly with
decreasing temperature and, below Tc, with decreasing laser intensity. We
interpret the decay as a process of thermalization of antinodal quasiparticles,
whose rate is determined by an inelastic scattering rate of quasiparticle
pairs.Comment: 4 pages, 4 figure
Levi umbilical surfaces in complex space
We define a complex connection on a real hypersurface of \C^{n+1} which is
naturally inherited from the ambient space. Using a system of Codazzi-type
equations, we classify connected real hypersurfaces in \C^{n+1}, ,
which are Levi umbilical and have non zero constant Levi curvature. It turns
out that such surfaces are contained either in a sphere or in the boundary of a
complex tube domain with spherical section.Comment: 18 page
The Finite Field Kakeya Problem
A Besicovitch set in AG(n,q) is a set of points containing a line in every
direction. The Kakeya problem is to determine the minimal size of such a set.
We solve the Kakeya problem in the plane, and substantially improve the known
bounds for n greater than 4.Comment: 13 page
Monte-Carlo simulation of events with Drell-Yan lepton pairs from antiproton-proton collisions
The complete knowledge of the nucleon spin structure at leading twist
requires also addressing the transverse spin distribution of quarks, or
transversity, which is yet unexplored because of its chiral-odd nature.
Transversity can be best extracted from single-spin asymmetries in fully
polarized Drell-Yan processes with antiprotons, where valence contributions are
involved anyway. Alternatively, in single-polarized Drell-Yan the transversity
happens convoluted with another chiral-odd function, which is likely to be
responsible for the well known (and yet unexplained) violation of the Lam-Tung
sum rule in the corresponding unpolarized cross section. We present Monte-Carlo
simulations for the unpolarized and single-polarized Drell-Yan at different center-of-mass energies in both
configurations where the antiproton beam hits a fixed proton target or it
collides on another proton beam. The goal is to estimate the minimum number of
events needed to extract the above chiral-odd distributions from future
measurements at the HESR ring at GSI. It is important to study the feasibility
of such experiments at HESR in order to demonstrate that interesting spin
physics can be explored already using unpolarized antiprotons.Comment: Deeply revised text with improved discussion of kinematics and
results; added one table; 12 figures. Accepted for publication in Phys. Rev.
Shear-induced quench of long-range correlations in a liquid mixture
A static correlation function of concentration fluctuations in a (dilute)
binary liquid mixture subjected to both a concentration gradient and uniform
shear flow is investigated within the framework of fluctuating hydrodynamics.
It is shown that a well-known long-range correlation at
large wave numbers crosses over to a weaker divergent one for wave numbers
satisfying , while an asymptotic shear-controlled
power-law dependence is confirmed at much smaller wave numbers given by , where , , and are the
mass concentration, the rate of the shear, the mass diffusivity and the
kinematic viscosity of the mixture, respectively. The result will provide for
the first time the possibility to observe the shear-induced suppression of a
long-range correlation experimentally by using, for example, a low-angle light
scattering technique.Comment: 8pages, 2figure
Pfaffian representations of cubic surfaces
Let K be a field of characteristic zero. We describe an algorithm which
requires a homogeneous polynomial F of degree three in K[x_0,x_1,x_2,x_3] and a
zero A of F in P^3_K and ensures a linear pfaffian representation of V(F) with
entries in K[x_0,x_1,x_2,x_3], under mild assumptions on F and A. We use this
result to give an explicit construction of (and to prove the existence of) a
linear pfaffian representation of V(F), with entries in K'[x_0,x_1,x_2,x_3],
being K' an algebraic extension of K of degree at most six. An explicit example
of such a construction is given.Comment: 17 pages. Expanded with some remarks. Published with minor
corrections in Geom. Dedicat
Mixing by polymers: experimental test of decay regime of mixing
By using high molecular weight fluorescent passive tracers with different
diffusion coefficients and by changing the fluid velocity we study dependence
of a characteristic mixing length on the Peclet number, , which controls
the mixing efficiency. The mixing length is found to be related to by a
power law, , and increases faster than
expected for an unbounded chaotic flow. Role of the boundaries in the mixing
length abnormal growth is clarified. The experimental findings are in a good
quantitative agreement with the recent theoretical predictions.Comment: 4 pages,5 figures. accepted for publication in PR
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