65,740 research outputs found
Photon Stars
We discuss numerical solutions of Einstein's field equation describing
static, spherically symmetric conglomerations of a photon gas. These equations
imply a back reaction of the metric on the energy density of the photon gas
according to Tolman's equation. The 3-fold of solutions corresponds to a class
of physically different solutions which is parameterized by only two
quantities, e.g. mass and surface temperature. The energy density is typically
concentrated on a shell because the center contains a repelling singularity,
which can, however, not be reached by timelike or null geodesics. The physical
relevance of these solutions is completely open, although their existence may
raise some doubts w.r. to the stability of black holes.Comment: 10 pages, 5 figures, talk at the DPG Spring Meeting 199
Sidewall depletion in nano-patterned LAO/STO heterostructures
We report the fabrication of nanostructures from the quasi-two-dimensional
electron gas (q2DEG) formed at the LaAlO/ SrTiO (LAO/STO)
interface. The process uses electron beam lithography in combination with
reactive ion etching. This technique allows to pattern high-quality structures
down to lateral dimensions as small as nm while maintaining the conducting
properties without inducing conductivity in the STO substrate. Temperature
dependent transport properties of patterned Hall bars of various widths show
only a small size dependence of conductivity at low temperature as well as at
room temperature. The deviation can be explained by a narrow lateral depletion
region. All steps of the patterning process are fully industry compatible.Comment: 5 pages, 4 figure
Luminosity segregation in galaxy clusters as an indication of dynamical evolution
Theoretical models describing the dynamical evolution of self-gravitating systems predict a spatial mass segregation for more evolved systems, with the more massive objects concentrated toward the center of the configuration. From the observational point of view, however, the existence of mass segregation in galaxy clusters seems to be a matter of controversy. A special problem in this connection is the formation of cD galaxies in the centers of galaxy clusters. The most promising scenarios of their formation are galaxy cannibalism (merger scenario) and growing by cooling flows. It seems to be plausible to consider the swallowing of smaller systems by a dominant galaxy as an important process in the evolution of a cD galaxy. The stage of the evolution of the dominant galaxy should be reflected by the surrounding galaxy population, especially by possible mass segregation effects. Assuming that mass segregation is tantamount to luminosity segregation we analyzed luminosity segregation in roughly 40 cD galaxy clusters. Obviously there are three different groups of clusters: (1) clusters with luminosity segregation, (2) clusters without luminosity segregation, and (3) such objects exhibiting a phenomenon which we call antisegregation in luminosity, i.e. a deficiency of bright galaxies in the central regions of clusters. This result is interpreted in the sense of different degrees of mass segregation and as an indication for different evolution stages of these clusters. The clusters are arranged in the three segregation classes 2, 1, and 0 (S2 = strong mass segregation, S1 = moderate mass segregation, S0 = weak or absent mass segregation). We assume that a galaxy cluster starts its dynamical evolution after virialization without any radial mass segregation. Energy exchange during encounters of cluster members as well as merger processes between cluster galaxies lead to an increasing radial mass segregation in the cluster (S1). If a certain degree of segregation (S2) has been established, an essential number of slow-moving and relative massive cluster members in the center will be cannibalized by the initial brightest cluster galaxy. This process should lead to the growing of the predominate galaxy, which is accompanied by a diminution of the mass segregation (transition to S1 and S0, respectively) in the neighborhood of the central very massive galaxy. An increase of the areal density of brighter galaxies towards the outer cluster regions (antisegregation of luminosity), i.e. an extreme low degree of mass segregation was estimated for a substantial percentage of cD clusters. This result favors the cannibalism scenario for the formation of cD galaxies
Numerical analysis of nanostructures for enhanced light extraction from OLEDs
Nanostructures, like periodic arrays of scatters or low-index gratings, are
used to improve the light outcoupling from organic light-emitting diodes
(OLED). In order to optimize geometrical and material properties of such
structures, simulations of the outcoupling process are very helpful. The finite
element method is best suited for an accurate discretization of the geometry
and the singular-like field profile within the structured layer and the
emitting layer. However, a finite element simulation of the overall OLED stack
is often beyond available computer resources. The main focus of this paper is
the simulation of a single dipole source embedded into a twofold infinitely
periodic OLED structure. To overcome the numerical burden we apply the Floquet
transform, so that the computational domain reduces to the unit cell. The
relevant outcoupling data are then gained by inverse Flouqet transforming. This
step requires a careful numerical treatment as reported in this paper
Finite-Element Simulations of Light Propagation through Circular Subwavelength Apertures
Light transmission through circular subwavelength apertures in metallic films
with surrounding nanostructures is investigated numerically. Numerical results
are obtained with a frequency-domain finite-element method. Convergence of the
obtained observables to very low levels of numerical error is demonstrated.
Very good agreement to experimental results from the literature is reached, and
the utility of the method is demonstrated in the investigation of the influence
of geometrical parameters on enhanced transmission through the apertures
A General-applications Direct Global Matrix Algorithm for Rapid Seismo-acoustic Wavefield Computations
A new matrix method for rapid wave propagation modeling in generalized stratified media, which has recently been applied to numerical simulations in diverse areas of underwater acoustics, solid earth seismology, and nondestructive ultrasonic scattering is explained and illustrated. A portion of recent efforts jointly undertaken at NATOSACLANT and NORDA Numerical Modeling groups in developing, implementing, and testing a new fast general-applications wave propagation algorithm, SAFARI, formulated at SACLANT is summarized. The present general-applications SAFARI program uses a Direct Global Matrix Approach to multilayer Green's function calculation. A rapid and unconditionally stable solution is readily obtained via simple Gaussian ellimination on the resulting sparsely banded block system, precisely analogous to that arising in the Finite Element Method. The resulting gains in accuracy and computational speed allow consideration of much larger multilayered air/ocean/Earth/engineering material media models, for many more source-receiver configurations than previously possible. The validity and versatility of the SAFARI-DGM method is demonstrated by reviewing three practical examples of engineering interest, drawn from ocean acoustics, engineering seismology and ultrasonic scattering
Effective spin model for the spin-liquid phase of the Hubbard model on the triangular lattice
We show that the spin liquid phase of the half-filled Hubbard model on the
triangular lattice can be described by a pure spin model. This is based on a
high-order strong coupling expansion (up to order 12) using perturbative
continuous unitary transformations. The resulting spin model is consistent with
a transition from three-sublattice long-range magnetic order to an insulating
spin liquid phase, and with a jump of the double occupancy at the transition.
Exact diagonalizations of both models show that the effective spin model is
quantitatively accurate well into the spin liquid phase, and a comparison with
the Gutzwiller projected Fermi sea suggests a gapless spectrum and a spinon
Fermi surface.Comment: 4 pages, 4 figures, published versions with additional dat
Simulation of Thematic Mapper performance as a function of sensor scanning parameters
The investigation and results of the Thematic Mapper Instrument Performance Study are described. The Thematic Mapper is the advanced multispectral scanner initially planned for the Earth Observation Satellite and now planned for LANDSAT D. The use of existing digital airborne scanner data obtained with the Modular Multispectral Scanner (M2S) at Bendix provided an opportunity to simulate the effects of variation of design parameters of the Thematic Mapper. Analysis and processing of this data on the Bendix Multispectral Data Analysis System were used to empirically determine categorization performance on data generated with variations of the sampling period and scan overlap parameters of the Thematic Mapper. The Bendix M2S data, with a 2.5 milliradian instantaneous field of view and a spatial resolution (pixel size) of 10-m from 13,000 ft altitude, allowed a direct simulation of Thematic Mapper data with a 30-m resolution. The flight data chosen were obtained on 30 June 1973 over agricultural test sites in Indiana
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