An efficient numerical algorithm for the transient analysis of high-frequency non-linear circuits

The paper proposes a new approach for the discrete-time integration of non-linear differential equations that describe the behaviour of high-frequency circuits, in particular those containing complex equivalent-circuit models of microwave transistor devices. The proposed approach reformulates a conventional predictor-corrector method in terms of Pad�© approximates about each function sample. The method is especially suited to the kind of non-linear stiff differential equations that arise frequently in high-frequency analysis

Effective Hamiltonian for transition-metal compounds. Application to Na_xCoO_2

We describe a simple scheme to construct a low-energy effective Hamiltonian H_eff for highly correlated systems containing non-metals like O, P or As (O in what follows) and a transition-metal (M) as the active part in the electronic structure, eliminating the O degrees of freedom from a starting Hamiltonian that contains all M d orbitals and all non-metal p orbitals. We calculate all interaction terms between d electrons originating from Coulomb repulsion, as a function of three parameters (F_0, F_2 and F_4) and write them in a basis of orbitals appropriate for cubic, tetragonal, tetrahedral or hexagonal symmetry around M. The approach is based on solving exactly (numerically if necessary) a MO_n cluster containing the transition-metal atom and its n nearest O atoms (for example a CoO_6 cluster in the case of the cobaltates, or a CuO_n cluster in the case of the cuprates, in which n depends on the number of apical O atoms), and mapping them into many-body states of the same symmetry containing d holes only. We illustrate the procedure for the case of Na_xCoO_2. The resulting H_eff, including a trigonal distortion D, has been studied recently and its electronic structure agrees well with angle-resolved photoemission spectra [A. Bourgeois, A. A. Aligia, and M. J. Rozenberg, Phys. Rev. Lett. 102, 066402 (2009)]. Although H_eff contains only 3d t_2g holes, the highly correlated states that they represent contain an important amount not only of O 2p holes but also of 3d e_g holes. When more holes are added, a significant redistribution of charge takes place. As a consequence of these facts, the resulting values of the effective interactions between t_2g states are smaller than previously assumed, rendering more important the effect of D in obtaining only one sheet around the center of the Brillouin zone for the Fermi surface (without additional pockets).Comment: 11 pages, 1 figure, accepted for publication in Phys.Rev.

Resolving the Radio Source Background: Deeper Understanding Through Confusion

We used the Karl G. Jansky Very Large Array (VLA) to image one primary beam area at 3 GHz with 8 arcsec FWHM resolution and 1.0 microJy/beam rms noise near the pointing center. The P(D) distribution from the central 10 arcmin of this confusion-limited image constrains the count of discrete sources in the 1 < S(microJy/beam) < 10 range. At this level the brightness-weighted differential count S^2 n(S) is converging rapidly, as predicted by evolutionary models in which the faintest radio sources are star-forming galaxies; and ~96$% of the background originating in galaxies has been resolved into discrete sources. About 63% of the radio background is produced by AGNs, and the remaining 37% comes from star-forming galaxies that obey the far-infrared (FIR) / radio correlation and account for most of the FIR background at lambda = 160 microns. Our new data confirm that radio sources powered by AGNs and star formation evolve at about the same rate, a result consistent with AGN feedback and the rough correlation of black hole and bulge stellar masses. The confusion at centimeter wavelengths is low enough that neither the planned SKA nor its pathfinder ASKAP EMU survey should be confusion limited, and the ultimate source detection limit imposed by "natural" confusion is < 0.01 microJy at 1.4 GHz. If discrete sources dominate the bright extragalactic background reported by ARCADE2 at 3.3 GHz, they cannot be located in or near galaxies and most are < 0.03 microJy at 1.4 GHz.Comment: 28 pages including 16 figures. ApJ accepted for publicatio

RBSC-NVSS Sample. I. Radio and Optical Identifications of a Complete Sample of 1500 Bright X-ray Sources

We cross-identified the ROSAT Bright Source Catalog (RBSC) and the NRAO VLA Sky Survey (NVSS) to construct the RBSC-NVSS sample of the brightest X-ray sources (>= 0.1 counts/s or ~1E-12 ergs/cm/cm/s in the 0.1-2.4 keV band) that are also radio sources (S >= 2.5 mJy at 1.4 GHz) in the 7.8 sr of extragalactic sky with |b| > 15 degrees. and delta > -40 degrees. The sky density of NVSS sources is low enough that they can be reliably identified with RBSC sources having average rms positional uncertainties = 10 arcsec. We used the more accurate radio positions to make reliable X-ray/radio/optical identifications down to the POSS plate limits. We obtained optical spectra for many of the bright identifications lacking published redshifts. The resulting X-ray/radio sample is unique in its size (N ~ 1500 objects), composition (a mixture of nearly normal galaxies, Seyfert galaxies, quasars, and clusters), and low average redshift ( ~ 0.1).Comment: 35 LaTeX pages including 6 eps figures + 40 LaTeX page table2 (landscape) w/ AASTeX 5.0; accepted to ApJ

Radio continuum and far-infrared emission from the galaxies in the Eridanus group

The Eridanus galaxies follow the well-known radio-FIR correlation. Majority (70%) of these galaxies have their star formation rates below that of the Milky Way. The galaxies having a significant excess of radio emission are identified as low luminosity AGNs based on their radio morphologies obtained from the GMRT observations. There are no powerful AGNs (L{20cm} > 10^{23} W Hz^{-1}) in the group. The two most far-infrared and radio luminous galaxies in the group have optical and HI morphologies suggestive of recent tidal interactions. The Eridanus group also has two far-infrared luminous but radio-deficient galaxies. It is believed that these galaxies are observed within a few Myr of the onset of an intense star formation episode after being quiescent for at least a 100 Myr. The upper end of the radio luminosity distribution of the Eridanus galaxies (L_{20cm} ~ 10^{22} W Hz^{-1}) is consistent with that of the field galaxies, other groups, and late-type galaxies in nearby clusters.Comment: 16 pages; Accepted for publication in Journal of Astroph. & Astron. March, 200

Compact continuum source-finding for next generation radio surveys

We present a detailed analysis of four of the most widely used radio source finding packages in radio astronomy, and a program being developed for the Australian Square Kilometer Array Pathfinder (ASKAP) telescope. The four packages; SExtractor, SFind, IMSAD and Selavy are shown to produce source catalogues with high completeness and reliability. In this paper we analyse the small fraction (~1%) of cases in which these packages do not perform well. This small fraction of sources will be of concern for the next generation of radio surveys which will produce many thousands of sources on a daily basis, in particular for blind radio transients surveys. From our analysis we identify the ways in which the underlying source finding algorithms fail. We demonstrate a new source finding algorithm Aegean, based on the application of a Laplacian kernel, which can avoid these problems and can produce complete and reliable source catalogues for the next generation of radio surveys.Comment: 14 pages, 12 figures, accepted for publication in MNRA

Geodynamic setting and origin of the Oman/UAE ophiolite

The ~500km-long mid-Cretaceous Semail nappe of the Sultanate of Oman and UAE (henceforth referred to as the Oman ophiolite) is the largest and best-preserved ophiolite complex known. It is of particular importance because it is generally believed to have an internal structure and composition closely comparable to that of crust formed at the present-day East Pacific Rise (EPR), making it our only known on-land analogue for ocean lithosphere formed at a fast spreading rate. On the basis of this assumption Oman has long played a pivotal role in guiding our conceptual understanding of fast-spreading ridge processes, as modern fast-spread ocean crust is largely inaccessible

Polarization dependence of x-ray absorption spectra in Na_xCoO_2

In order to shed light on the electronic structure of Na_xCoO_2, and motivated by recent Co L-edge X-ray absorption spectra (XAS) experiments with polarized light, we calculate the electronic spectrum of a CoO_6 cluster including all interactions between 3d orbitals. We obtain the ground state for two electronic occupations in the cluster that correspond nominally to all O in the O^{-2} oxidation state, and Co^{+3} or Co^{+4}. Then, all excited states obtained by promotion of a Co 2p electron to a 3d electron, and the corresponding matrix elements are calculated. A fit of the observed experimental spectra is good and points out a large Co-O covalency and cubic crystal field effects, that result in low spin Co 3d configurations. Our results indicate that the effective hopping between different Co atoms plays a major role in determining the symmetry of the ground state in the lattice. Remaining quantitative discrepancies with the XAS experiments are expected to come from composition effects of itineracy in the ground and excited states.Comment: 10 pages, 4 figure