22,979 research outputs found
Direct 3D Tomographic Reconstruction and Phase-Retrieval of Far-Field Coherent Diffraction Patterns
We present an alternative numerical reconstruction algorithm for direct
tomographic reconstruction of a sample refractive indices from the measured
intensities of its far-field coherent diffraction patterns. We formulate the
well-known phase-retrieval problem in ptychography in a tomographic framework
which allows for simultaneous reconstruction of the illumination function and
the sample refractive indices in three dimensions. Our iterative reconstruction
algorithm is based on the Levenberg-Marquardt algorithm. We demonstrate the
performance of our proposed method with simulation studies
Multi- nuclei and kaon condensation
We extend previous relativistic mean-field (RMF) calculations of multi- nuclei, using vector boson fields with SU(3) PPV coupling constants and
scalar boson fields constrained phenomenologically. For a given core nucleus,
the resulting separation energy , as well as the
associated nuclear and -meson densities, saturate with the number
of mesons for . Saturation
appears robust against a wide range of variations, including the RMF nuclear
model used and the type of boson fields mediating the strong interactions.
Because generally does not exceed 200 MeV, it is argued that
multi- nuclei do not compete with multihyperonic nuclei in providing
the ground state of strange hadronic configurations and that kaon condensation
is unlikely to occur in strong-interaction self-bound strange hadronic matter.
Last, we explore possibly self-bound strange systems made of neutrons and
mesons, or protons and mesons, and study their properties.Comment: 21 pages, 8 figures, revised text and reference
The treatment of zero eigenvalues of the matrix governing the equations of motion in many-body Green's function theory
The spectral theorem of many-body Green's function theory relates
thermodynamic correlations to Green's functions. More often than not, the
matrix governing the equations of motion has zero eigenvalues. In this case,
the standard text-book approach requires both commutator and anti-commutator
Green's functions to obtain equations for that part of the correlation which
does not lie in the null space of the matrix. In this paper, we show that this
procedure fails if the projector onto the null space is dependent on the
momentum vector. We propose an alternative formulation of the theory in terms
of the non-null space alone and we show that a solution is possible if one can
find a momentum-independent projector onto some subspace of the non-null space.
To do this, we enlist the aid of the singular value decomposition (SVD) of the
equation of motion matrix in order to project out the null space, thus reducing
the size of the matrix and eliminating the need for the anti-commutator Green's
function. We extend our previous work, dealing with a ferromagnetic Heisenberg
monolayer and a momentum-independent projector onto the null space, where both
multilayer films and a momentum-dependent projector are considered. We develop
the numerical methods capable of handling these cases and offer a computational
algorithmus that should be applicable to any similar problem arising in Green's
function theory.Comment: 16 pages, 7 figure
Invariant classification and the generalised invariant formalism: conformally flat pure radiation metrics, with zero cosmological constant
Metrics obtained by integrating within the generalised invariant formalism
are structured around their intrinsic coordinates, and this considerably
simplifies their invariant classification and symmetry analysis. We illustrate
this by presenting a simple and transparent complete invariant classification
of the conformally flat pure radiation metrics (except plane waves) in such
intrinsic coordinates; in particular we confirm that the three apparently
non-redundant functions of one variable are genuinely non-redundant, and easily
identify the subclasses which admit a Killing and/or a homothetic Killing
vector. Most of our results agree with the earlier classification carried out
by Skea in the different Koutras-McIntosh coordinates, which required much more
involved calculations; but there are some subtle differences. Therefore, we
also rework the classification in the Koutras-McIntosh coordinates, and by
paying attention to some of the subtleties involving arbitrary functions, we
are able to obtain complete agreement with the results obtained in intrinsic
coordinates. In particular, we have corrected and completed statements and
results by Edgar and Vickers, and by Skea, about the orders of Cartan
invariants at which particular information becomes available.Comment: Extended version of GRG publication, with some typos etc correcte
VISIR-VLT high resolution study of the extended emission of four obscured post-AGB candidates
The onset of the asymmetry of planetary nebulae (PNe) is expected to occur
during the late Asymptotic Giant Branch (AGB) and early post-AGB phases of low-
and intermediate-mass stars. Among all post-AGB objects, the most heavily
obscured ones might have escaped the selection criteria of previous studies
detecting extreme axysimmetric structures in young PNe. Since the most heavily
obscured post-AGB sources can be expected to descend from the most massive PN
progenitors, these should exhibit clear asymmetric morphologies. We have
obtained VISIR-VLT mid-IR images of four heavily obscured post-AGB objects
barely resolved in previous Spitzer IRAC observations to analyze their
morphology and physical conditions across the mid-IR. The VISIR-VLT images have
been deconvolved, flux calibrated, and used to construct RGB composite pictures
as well as color and optical depth maps that allow us to study the morphology
and physical properties of the extended emission of these sources. We have
detected extended emission from the four objects in our sample and resolved it
into several structural components that are greatly enhanced in the temperature
and optical depth maps. They reveal the presence of asymmetry in three young
PNe (IRAS 15534-5422, IRAS 17009-4154, and IRAS 18454+0001), where the
asymmetries can be associated with dusty torii and slightly bipolar outflows.
The fourth source (IRAS 18229-1127), a possible post-AGB star, is better
described as a rhomboidal detached shell. The heavily obscured sources in our
sample do not show extreme axisymmetric morphologies. This is at odds with the
expectation of highly asymmetrical morphologies in post-AGB sources descending
from massive PN progenitors. The sources presented in this paper may be
sampling critical early phases in the evolution of massive PN progenitors,
before extreme asymmetries develop.Comment: 9 pages, 4 figure
Equilibrium onions?
We demonstrate the possibility of a stable equilibrium multi-lamellar ("onion") phase in pure lamellar systems (no excess solvent) due to a sufficiently negative Gaussian curvature modulus. The onion phase is stabilized by non-linear elastic moduli coupled to a polydisperse size distribution (Apollonian packing) to allow space-filling without appreciable elastic distortion. This model is compared to experiments on copolymer-decorated lamellar surfactant systems, with reasonable qualitative agreement
The U(5)-O(6) transition in the Interacting Boson Model and the E(5) critical point symmetry
The relation of the recently proposed E(5) critical point symmetry with the
interacting boson model is investigated. The large-N limit of the interacting
boson model at the critical point in the transition from U(5) to O(6) is
obtained by solving the Richardson equations. It is shown explicitly that this
algebraic calculation leads to the same results as the solution of the Bohr
differential equation with a potential.Comment: Accepted in PR
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