3,417 research outputs found
Making big steps in trajectories
We consider the solution of initial value problems within the context of
hybrid systems and emphasise the use of high precision approximations (in
software for exact real arithmetic). We propose a novel algorithm for the
computation of trajectories up to the area where discontinuous jumps appear,
applicable for holomorphic flow functions. Examples with a prototypical
implementation illustrate that the algorithm might provide results with higher
precision than well-known ODE solvers at a similar computation time
The Hubbard Model at Infinite Dimensions: Thermodynamic and Transport Properties
We present results on thermodynamic quantities, resistivity and optical
conductivity for the Hubbard model on a simple hypercubic lattice in infinite
dimensions. Our results for the paramagnetic phase display the features
expected from an intuitive analysis of the one-particle spectra and
substantiate the similarity of the physics of the Hubbard model to those of
heavy fermion systems. The calculations were performed using an approximate
solution to the single-impurity Anderson model, which is the key quantity
entering the solution of the Hubbard model in this limit. To establish the
quality of this approximation we compare its results, together with those
obtained from two other widely used methods, to essentially exact quantum Monte
Carlo results.Comment: 29 pages, 16 figure
Optical fibers with interferometric path length stability by controlled heating for transmission of optical signals and as components in frequency standards
We present a simple method to stabilize the optical path length of an optical
fiber to an accuracy of about 1/100 of the laser wavelength. We study the
dynamic response of the path length to modulation of an electrically conductive
heater layer of the fiber. The path length is measured against the laser
wavelength by use of the Pound-Drever-Hall method; negative feedback is applied
via the heater. We apply the method in the context of a cryogenic resonator
frequency standard.Comment: Expanded introduction and outlook. 9 pages, 5 figure
Three-Dimensional Simulations of Mixing Instabilities in Supernova Explosions
We present the first three-dimensional (3D) simulations of the large-scale
mixing that takes place in the shock-heated stellar layers ejected in the
explosion of a 15.5 solar-mass blue supergiant star. The outgoing supernova
shock is followed from its launch by neutrino heating until it breaks out from
the stellar surface more than two hours after the core collapse. Violent
convective overturn in the post-shock layer causes the explosion to start with
significant asphericity, which triggers the growth of Rayleigh-Taylor (RT)
instabilities at the composition interfaces of the exploding star. Deep inward
mixing of hydrogen (H) is found as well as fast-moving, metal-rich clumps
penetrating with high velocities far into the H-envelope of the star as
observed, e.g., in the case of SN 1987A. Also individual clumps containing a
sizeable fraction of the ejected iron-group elements (up to several 0.001 solar
masses) are obtained in some models. The metal core of the progenitor is
partially turned over with Ni-dominated fingers overtaking oxygen-rich bullets
and both Ni and O moving well ahead of the material from the carbon layer.
Comparing with corresponding 2D (axially symmetric) calculations, we determine
the growth of the RT fingers to be faster, the deceleration of the dense
metal-carrying clumps in the He and H layers to be reduced, the asymptotic
clump velocities in the H-shell to be higher (up to ~4500 km/s for the
considered progenitor and an explosion energy of 10^{51} ergs, instead of <2000
km/s in 2D), and the outward radial mixing of heavy elements and inward mixing
of hydrogen to be more efficient in 3D than in 2D. We present a simple argument
that explains these results as a consequence of the different action of drag
forces on moving objects in the two geometries. (abridged)Comment: 15 pages, 8 figures, 30 eps files; significantly extended and more
figures added after referee comments; accepted by The Astrophysical Journa
Computational Complexity of Iterated Maps on the Interval (Extended Abstract)
The exact computation of orbits of discrete dynamical systems on the interval
is considered. Therefore, a multiple-precision floating point approach based on
error analysis is chosen and a general algorithm is presented. The correctness
of the algorithm is shown and the computational complexity is analyzed. As a
main result, the computational complexity measure considered here is related to
the Ljapunow exponent of the dynamical system under consideration
Inversionless gain in a three-level system driven by a strong field and collisions
Inversionless gain in a three-level system driven by a strong external field
and by collisions with a buffer gas is investigated. The mechanism of
populating of the upper laser level contributed by the collision transfer as
well as by relaxation caused by a buffer gas is discussed in detail. Explicit
formulae for analysis of optimal conditions are derived. The mechanism
developed here for the incoherent pump could be generalized to other systems.Comment: RevTeX, 9 pages, 4 eps figure
Optimized Herschel/PACS photometer observing and data reduction strategies for moving solar system targets
The "TNOs are Cool!: A survey of the trans-Neptunian region" is a Herschel
Open Time Key Program that aims to characterize planetary bodies at the
outskirts of the Solar System using PACS and SPIRE data, mostly taken as
scan-maps. In this paper we summarize our PACS data reduction scheme that uses
a modified version of the standard pipeline for basic data reduction, optimized
for faint, moving targets. Due to the low flux density of our targets the
observations are confusion noise limited or at least often affected by bright
nearby background sources at 100 and 160\,m. To overcome these problems we
developed techniques to characterize and eliminate the background at the
positions of our targets and a background matching technique to compensate for
pointing errors. We derive a variety of maps as science data products that are
used depending on the source flux and background levels and the scientific
purpose. Our techniques are also applicable to a wealth of other Herschel solar
system photometric observations, e.g. comets and near-Earth asteroids. The
principles of our observing strategies and reduction techniques for moving
targets will also be applicable for similar surveys of future infrared space
projects.Comment: Accepted for publication in Experimental Astronom
From ferromagnetism to spin-density wave: Magnetism in the two channel periodic Anderson model
The magnetic properties of the two-channel periodic Anderson model for
uranium ions, comprised of a quadrupolar and a magnetic doublet are
investigated through the crossover from the mixed-valent to the stable moment
regime using dynamical mean field theory. In the mixed-valent regime
ferromagnetism is found for low carrier concentration on a hyper-cubic lattice.
The Kondo regime is governed by band magnetism with small effective moments and
an ordering vector \q close to the perfect nesting vector. In the stable
moment regime nearest neighbour anti-ferromagnetism dominates for less than
half band filling and a spin density wave transition for larger than half
filling. is governed by the renormalized RKKY energy scale \mu_{eff}^2
^2 J^2\rho_0(\mu).Comment: 4 pages, RevTeX, 3 eps figure
Dynamical Cluster Approximation Employing FLEX as a Cluster Solver
We employ the Dynamical Cluster Approximation (DCA) in conjunction with the
Fluctuation Exchange Approximation (FLEX) to study the Hubbard model. The DCA
is a technique to systematically restore the momentum conservation at the
internal vertices of Feynman diagrams relinquished in the Dynamical Mean Field
Approximation (DMFA). FLEX is a perturbative diagrammatic approach in which
classes of Feynman diagrams are summed over analytically using geometric
series. The FLEX is used as a tool to investigate the complementarity of the
DCA and the finite size lattice technique with periodic boundary conditions by
comparing their results for the Hubbard model. We also study the microscopic
theory underlying the DCA in terms of compact (skeletal) and non-compact
diagrammatic contributions to the thermodynamic potential independent of a
specific model. The significant advantages of the DCA implementation in
momentum space suggests the development of the same formalism for the frequency
space. However, we show that such a formalism for the Matsubara frequencies at
finite temperatures leads to acausal results and is not viable. However, a real
frequency approach is shown to be feasible.Comment: 15 pages, 24 figures. Submitted to Physical Review B as a Regular
Articl
Magnetic Properties of the t-J Model in the Dynamical Mean-Field Theory
We present a theory for the spin correlation function of the t-J model in the
framework of the dynamical mean-field theory. Using this mapping between the
lattice and a local model we are able to obtain an intuitive expression for the
non-local spin susceptibility, with the corresponding local correlation
function as input. The latter is calculated by means of local Goldstone
diagrams following closely the procedures developed and successfully applied
for the (single impurity) Anderson model.We present a systematic study of the
magnetic susceptibility and compare our results with those of a Hubbard model
at large U. Similarities and differences are pointed out and the magnetic phase
diagram of the t-J model is discussed.Comment: 28 pages LaTeX, postscript figures as compressed and uuencoded file
included fil
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