29,583 research outputs found
Superantenna made of transformation media
We show how transformation media can make a superantenna that is either
completely invisible or focuses incoming light into a needle-sharp beam. Our
idea is based on representating three-dimensional space as a foliage of sheets
and performing two-dimensional conformal maps on each shee
A two-species continuum model for aeolian sand transport
Starting from the physics on the grain scale, we develop a simple continuum
description of aeolian sand transport. Beyond popular mean-field models, but
without sacrificing their computational efficiency, it accounts for both
dominant grain populations, hopping (or "saltating") and creeping (or
"reptating") grains. The predicted stationary sand transport rate is in
excellent agreement with wind tunnel experiments simulating wind conditions
ranging from the onset of saltation to storms. Our closed set of equations thus
provides an analytically tractable, numerically precise, and computationally
efficient starting point for applications addressing a wealth of phenomena from
dune formation to dust emission.Comment: 23 pages, 9 figure
Structural Characteristics Of Sr1-xLaxTi 3+δ As A Function Of Oxygen Partial Pressure At 1400°C
The structural characteristics of Sr1-x Lax TiO3+δ (0≤x≤0.4) at 1400°C have been investigated as a function of ambient oxygen partial pressure. A modified Rietveld pattern-fitting structure-refinement program [H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969)] was used to determine the nature of the distortions of the fundamental perovskite unit cell, the degree of lattice perfection, and the cation vacancy concentrations. Specimens equilibrated in forming gas displayed a linear relation between x, the lattice parameters, and the degree of lattice perfection while those samples annealed in air and oxygen deviated significantly from linearity. A monoclinic distortion of the perovskite structure was seen in the samples at low oxygen partial pressures while a second phase or layer type of distortion appeared in samples with x\u3e0.2 under oxidizing conditions
Mycobacterium marinum: MR imaging and clinical course of a rare soft tissue infection
Mycobacterium marinum is a rare cause of soft tissue infections. The imposing MR appearance of the soft tissue involvement is in contrast to the chronic painless clinical manifestatio
Stewart-Treves syndrome: MR imaging of a postmastectomy upper-limb chronic lymphedema with angiosarcoma
The rare occurrence of angiosarcoma in postmastectomy upper-limb lymphedema with magnetic resonance (MR) imaging is discussed. Unfamiliarity with this aggressive vascular tumor and its harmless appearance often leads to delayed diagnosis. Angiosarcoma complicating chronic lymphedema may be low in signal intensity on T2-weighting and short tau inversion recovery (STIR) imaging reflecting the densely cellular, fibrous stroma, and sparsely vascularized tumor histology. Additional administration of intravenous contrast medium revealed significant enhancement of the tumorous lesions. Awareness of angiosarcoma and its MR imaging appearance in patients with chronic lymphedema may be a key to early diagnosis or allow at least inclusion in the differential diagnosi
New interpretation of variational principles for gauge theories. I. Cyclic coordinate alternative to ADM split
I show how there is an ambiguity in how one treats auxiliary variables in
gauge theories including general relativity cast as 3 + 1 geometrodynamics.
Auxiliary variables may be treated pre-variationally as multiplier coordinates
or as the velocities corresponding to cyclic coordinates. The latter treatment
works through the physical meaninglessness of auxiliary variables' values
applying also to the end points (or end spatial hypersurfaces) of the
variation, so that these are free rather than fixed. [This is also known as
variation with natural boundary conditions.] Further principles of dynamics
workings such as Routhian reduction and the Dirac procedure are shown to have
parallel counterparts for this new formalism. One advantage of the new scheme
is that the corresponding actions are more manifestly relational. While the
electric potential is usually regarded as a multiplier coordinate and Arnowitt,
Deser and Misner have regarded the lapse and shift likewise, this paper's
scheme considers new {\it flux}, {\it instant} and {\it grid} variables whose
corresponding velocities are, respectively, the abovementioned previously used
variables. This paper's way of thinking about gauge theory furthermore admits
interesting generalizations, which shall be provided in a second paper.Comment: 11 page
Stability of bicontinuous cubic phases in ternary amphiphilic systems with spontaneous curvature
We study the phase behavior of ternary amphiphilic systems in the framework
of a curvature model with non-vanishing spontaneous curvature. The amphiphilic
monolayers can arrange in different ways to form micellar, hexagonal, lamellar
and various bicontinuous cubic phases. For the latter case we consider both
single structures (one monolayer) and double structures (two monolayers). Their
interfaces are modeled by the triply periodic surfaces of constant mean
curvature of the families G, D, P, C(P), I-WP and F-RD. The stability of the
different bicontinuous cubic phases can be explained by the way in which their
universal geometrical properties conspire with the concentration constraints.
For vanishing saddle-splay modulus , almost every phase considered
has some region of stability in the Gibbs triangle. Although bicontinuous cubic
phases are suppressed by sufficiently negative values of the saddle-splay
modulus , we find that they can exist for considerably lower
values than obtained previously. The most stable bicontinuous cubic phases with
decreasing are the single and double gyroid structures since
they combine favorable topological properties with extreme volume fractions.Comment: Revtex, 23 pages with 10 Postscript files included, to appear in J.
Chem. Phys. 112 (6) (February 2000
Linear independence of Gamma values in positive characteristic
We investigate the arithmetic nature of special values of Thakur's function
field Gamma function at rational points. Our main result is that all linear
independence relations over the field of algebraic functions are consequences
of the known relations of Anderson and Thakur arising from the functional
equations of the Gamma function.Comment: 51 page
Renormalization approach to many-particle systems
This paper presents a renormalization approach to many-particle systems. By
starting from a bare Hamiltonian with an
unperturbed part and a perturbation ,we define an
effective Hamiltonian which has a band-diagonal shape with respect to the
eigenbasis of . This means that all transition matrix elements are
suppressed which have energy differences larger than a given cutoff
that is smaller than the cutoff of the original Hamiltonian. This
property resembles a recent flow equation approach on the basis of continuous
unitary transformations. For demonstration of the method we discuss an exact
solvable model, as well as the Anderson-lattice model where the well-known
quasiparticle behavior of heavy fermions is derived.Comment: 11 pages, final version, to be published in Phys. Rev.
- …