1,524,833 research outputs found
A differential method of maximum entropy
We consider a differential method of maximum entropy that is based on the
linearity of Fourier transform and involves reconstruction of images from the
differences of the visibility function. The efficiency of the method is
demonstrated with respect to the recovery of source images with bright
components against the background of a sufficiently weak extended base. The
simulation results are given along with the maps of an extragalactic radio
source 0059+581, which were obtained using the standard and differential
methods of maximum entropy for three observation dates and show that the
principle of differential mapping allows us to increase considerably the
dynamic interval of images.Comment: Latex, 6 pages with 4 Postscript figure
High multipole transitions in NIXS: valence and hybridization in 4f systems
Momentum-transfer (q) dependent non-resonant inelastic x-ray scattering
measurements were made at the N4,5 edges for several rare earth compounds. With
increasing q, giant dipole resonances diminish, to be replaced by strong
multiplet lines at lower energy transfer. These multiplets result from two
different orders of multipole scattering and are distinct for systems with
simple 4f^0 and 4f^1 initial states. A many-body theoretical treatment of the
multiplets agrees well with the experimental data on ionic La and Ce phosphate
reference compounds. Comparing measurements on CeO2 and CeRh3 to the theory and
the phosphates indicates sensitivity to hybridization as observed by a
broadening of 4f^0-related multiplet features. We expect such strong, nondipole
features to be generic for NIXS from f-electron systems
Beer's law in semiconductor quantum dots
The propagation of a coherent optical linear wave in an ensemble of
semiconductor quantum dots is considered. It is shown that a distribution of
transition dipole moments of the quantum dots changes significantly the
polarization and Beer's absorption length of the ensemble of quantum dots.
Explicit analytical expressions for these quantities are presented
Supersolids in the Bose-Hubbard Hamiltonian
We use a combination of numeric and analytic techniques to determine the
groun d state phase diagram of the Bose--Hubbard Hamiltonian with longer range
repulsi ve interactions. At half filling one finds superfluidity and an
insulating solid phase. Depending on the relative sizes of near--neighbor and
next near--neighbor interactions, this solid either follows a checkerboard or a
striped pattern. In neither case is there a coexistence with superfluidity.
However upon doping ``supersolid'' phases appear with simultaneous diagonal and
off--diagonal long range order.Comment: 11 pages, Revtex 3.0, 6 figures (upon request
The light-cone gauge without prescriptions
Feynman integrals in the physical light-cone gauge are harder to solve than
their covariant counterparts. The difficulty is associated with the presence of
unphysical singularities due to the inherent residual gauge freedom in the
intermediate boson propagators constrained within this gauge choice. In order
to circumvent these non-physical singularities, the headlong approach has
always been to call for mathematical devices --- prescriptions --- some
successful ones and others not so much so. A more elegant approach is to
consider the propagator from its physical point of view, that is, an object
obeying basic principles such as causality. Once this fact is realized and
carefully taken into account, the crutch of prescriptions can be avoided
altogether. An alternative third approach, which for practical computations
could dispense with prescriptions as well as prescinding the necessity of
careful stepwise watching out of causality would be of great advantage. And
this third option is realizable within the context of negative dimensions, or
as it has been coined, negative dimensional integration method, NDIM for short.Comment: 9 pages, PTPTeX (included
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