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At least 1,747 records · Page 97
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

THE STRUCTURE AND EMISSION MODEL OF THE RELATIVISTIC JET IN THE QUASAR 3C 279 INFERRED FROM RADIO TO HIGH-ENERGY {gamma}-RAY OBSERVATIONS IN 2008-2010

We present time-resolved broadband observations of the quasar 3C 279 obtained from multi-wavelength campaigns conducted during the first two years of the Fermi Gamma-ray Space Telescope mission. While investigating the previously reported {gamma}-ray/optical flare accompanied by a change in optical polarization, we found that the optical emission appears to be delayed with respect to the {gamma}-ray emission by about 10 days. X-ray observations reveal a pair of 'isolated' flares separated by {approx}90 days, with only weak {gamma}-ray/optical counterparts. The spectral structure measured by Spitzer reveals a synchrotron component peaking in the mid-infrared band with a sharp break at the far-infrared band during the {gamma}-ray flare, while the peak appears in the millimeter (mm)/submillimeter (sub-mm) band in the low state. Selected spectral energy distributions are fitted with leptonic models including Comptonization of external radiation produced in a dusty torus or the broad-line region. Adopting the interpretation of the polarization swing involving propagation of the emitting region along a curved trajectory, we can explain the evolution of the broadband spectra during the {gamma}-ray flaring event by a shift of its location from {approx}1 pc to {approx}4 pc from the central black hole. On the other hand, if the {gamma}-ray flare is generated instead at sub-pc distance from the central black hole, the far-infrared break can be explained by synchrotron self-absorption. We also model the low spectral state, dominated by the mm/sub-mm peaking synchrotron component, and suggest that the corresponding inverse-Compton component explains the steady X-ray emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

SIMULATIONS OF EARLY BARYONIC STRUCTURE FORMATION WITH STREAM VELOCITY. II. THE GAS FRACTION

Understanding the gas content of high-redshift halos is crucial for studying the formation of the first generation of galaxies and reionization. Recently, Tseliakhovich and Hirata showed that the relative 'stream' velocity between the dark matter and baryons at the time of recombination-formally a second-order effect, but an unusually large one-can influence the later structure formation history of the universe. We quantify the effect of the stream velocity on the so-called characteristic mass-the minimum mass of a dark matter halo capable of retaining most of its baryons throughout its formation epoch-using three different high-resolution sets of cosmological simulations (with separate transfer functions for baryons and dark matter) that vary in box size, particle number, and the value of the relative velocity between the dark matter and baryons. In order to understand this effect theoretically, we generalize the linear theory filtering mass to properly account for the difference between the dark matter and baryonic density fluctuation evolution induced by the stream velocity. We show that the new filtering mass provides an accurate estimate for the characteristic mass, while other theoretical ansatzes for the characteristic mass are substantially less precise.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The {sup 13}C-pocket structure in AGB models: constraints from zirconium isotope abundances in single mainstream SiC grains

We present postprocess asymptotic giant branch (AGB) nucleosynthesis models with different {sup 13}C-pocket internal structures to better explain zirconium isotope measurements in mainstream presolar SiC grains by Nicolussi et al. and Barzyk et al. We show that higher-than-solar {sup 92}Zr/{sup 94}Zr ratios can be predicted by adopting a {sup 13}C-pocket with a flat {sup 13}C profile, instead of the previous decreasing-with-depth {sup 13}C profile. The improved agreement between grain data for zirconium isotopes and AGB models provides additional support for a recent proposal of a flat {sup 13}C profile based on barium isotopes in mainstream SiC grains by Liu et al.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Helical Magnetism and Structural Anomalies in Triangular lattice alpha-SrCr2O4

{alpha}-SrCr{sub 2}O{sub 4} has a triangular planar lattice of d{sup 3} Cr{sup 3+} made from edge sharing CrO{sub 6} octahedra; the plane shows a very small orthorhombic distortion from hexagonal symmetry. With a Weiss temperature of -596 K and a three-dimensional magnetic ordering temperature of 43 K, the magnetic system is quasi-two-dimensional and frustrated. Neutron powder diffraction shows that the ordered state is an incommensurate helical magnet, with an in-plane propagation vector of k = (0, 0.3217(8), 0). Temperature dependent synchrotron powder diffraction characterization of the structure shows an increase in the inter-plane spacing on cooling below 100 K and an inflection in the cell parameters at the magnetic ordering temperature. These anomalies indicate the presence of a moderate degree of magnetostructural coupling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The Lagrangian-space Effective Field Theory of large scale structures

Here, we introduce a Lagrangian-space Effective Field Theory (LEFT) formalism for the study of cosmological large scale structures. Unlike the previous Eulerian-space construction, it is naturally formulated as an effective field theory of extended objects in Lagrangian space. In LEFT the resulting finite size effects are described using a multipole expansion parameterized by a set of time dependent coefficients and organized in powers of the ratio of the wavenumber of interest k over the non-linear scale k NL . The multipoles encode the effects of the short distance modes on the long-wavelength universe and absorb UV divergences when present. There are no IR divergences in LEFT. Some of the parameters that control the perturbative approach are not assumed to be small and can be automatically resummed. We present an illustrative one-loop calculation for a power law universe. We describe the dynamics both at the level of the equations of motion and through an action formalism.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

On the statistics of biased tracers in the Effective Field Theory of Large Scale Structures

With the completion of the Planck mission, in order to continue to gather cosmological information it has become crucial to understand the Large Scale Structures (LSS) of the universe to percent accuracy. The Effective Field Theory of LSS (EFTofLSS) is a novel theoretical framework that aims to develop an analytic understanding of LSS at long distances, where inhomogeneities are small. We further develop the description of biased tracers in the EFTofLSS to account for the effect of baryonic physics and primordial non-Gaussianities, finding that new bias coefficients are required. Then, restricting to dark matter with Gaussian initial conditions, we describe the prediction of the EFTofLSS for the one-loop halo-halo and halo-matter two-point functions, and for the tree-level halo-halo-halo, matter-halo-halo and matter-matter-halo three-point functions. Several new bias coefficients are needed in the EFTofLSS, even though their contribution at a given order can be degenerate and the same parameters contribute to multiple observables. We develop a method to reduce the number of biases to an irreducible basis, and find that, at the order at which we work, seven bias parameters are enough to describe this extremely rich set of statistics. We then compare with the output of an N-body simulation where the normalization parameter of the linear power spectrum is set to σ 8 = 0.9. For the lowest mass bin, we find percent level agreement up to k ≃ 0.3 h Mpc –1 for the one-loop two-point functions, and up to k ≃ 0.15 h Mpc –1 for the tree-level three-point functions, with the k-reach decreasing with higher mass bins. In conclusion, this is consistent with the theoretical estimates, and suggests that the cosmological information in LSS amenable to analytical control is much more than previously believed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

RECALIBRATION OF PAGEL'S METHOD FOR H II REGIONS CONSIDERING THE THERMAL STRUCTURE, THE IONIZATION STRUCTURE, AND THE DEPLETION OF O INTO DUST GRAINS

Using a sample of 28 H II regions from the literature with measured temperature inhomogeneity parameter, t {sup 2}, we present a statistical correction to the chemical abundances determined with the T{sub e} (4363/5007) method. We used the t {sup 2} values to correct the oxygen gaseous abundances and consider the oxygen depletion into dust to calculate the total abundances for these objects. This correction is used to obtain a new calibration of Pagel's strong-line method, R{sub 23}, to determine oxygen abundances in H II regions. Our new calibration simultaneously considers the temperature structure, the ionization structure, and the fraction of oxygen depleted into dust grains. Previous calibrations in the literature have included one or two of these factors; this is the first time all three are taken into account. This recalibration reconciles the systematic differences among the temperatures found from different methods. We find that the total correction due to thermal inhomogeneities and dust depletion amounts to an increase in the O/H ratio of H II regions by factors of 1.7-2.2 (or 0.22-0.35 dex). This result has important implications in various areas of astrophysics such as the study of the higher end of the initial mass function, the star formation rate, and the mass-metallicity relation of galaxies, among others.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Local structural effects in Sr 3 NiRhO 6 across magnetic transitions

Here, we investigate the temperature dependence of the structural parameters of quasi-one-dimensional Sr 3 NiRhO 6 across the region of magnetic phase transitions using Ni K-edge and Sr K-edge x-ray absorption spectroscopy (XAS). The features in the x-ray absorption near-edge region are identified using multiple scattering calculations. The temperature-dependent extended x-ray absorption fine structure (EXAFS) studies show that the setting of the intra-chain super exchange interaction starts at ~200 K, which is well above the first transition temperature (45 K) revealed by magnetic susceptibility studies. The onset of the inter-chain super–super exchange interaction appears to be at ~125 K. Interestingly, the role played by direct exchange interaction between the Ni 3d and Rh 4d states in stabilising the magnetic interaction is less significant. The present results shed light on the generic features exhibited by isostructural compounds and may help in identifying the magnetic exchange pathways useful for understanding the unusual properties exhibited by such compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Quantum decimation in Hilbert space: Coarse graining without structure

We introduce a technique to coarse grain quantum states in a finite-dimensional Hilbert space. Our method is distinguished from other approaches by not relying on structures such as a preferred factorization of Hilbert space or a preferred set of operators (local or otherwise) in an associated algebra. Rather, we use the data corresponding to a given set of states, either specified independently or constructed from a single state evolving in time. Our method is based on principle component analysis (PCA), and the resulting coarse-grained quantum states live in a lower-dimensional Hilbert space whose basis is defined using the underlying (isometric embedding) transformation of the set of fine-grained states we wish to coarse grain. Physically, the transformation can be interpreted to be an “entanglement coarse-graining” scheme that retains most of the global, useful entanglement structure of each state, while needing fewer degrees of freedom for its reconstruction. This scheme could be useful for efficiently describing collections of states whose number is much smaller than the dimension of Hilbert space, or a single state evolving over time.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Linking local structure and properties in perovskites containing equal concentrations of manganese and ruthenium

The local structures of six perovskite compounds containing equal amounts of manganese and ruthenium on the B-site have been investigated by neutron and x-ray pair distribution function analysis. The compounds SrMn{sub 0.5}Ru{sub 0.5}O{sub 3}, Sr{sub 0.5}Ca{sub 0.5}Mn{sub 0.5}Ru{sub 0.5}O{sub 3}, and CaMn{sub 0.5}Ru{sub 0.5}O{sub 3} were studied to investigate the effects of pure chemical pressure on the local structure and valency ratio between Mn{sup 3+}/Ru{sup 5+} and Mn{sup 4+}/Ru{sup 4+}. Reverse Monte Carlo simulations confirm that there is a shift in the B-site cation charge distribution from nearly equal amounts of Mn{sup 3+}, Ru{sup 5+}, Mn{sup 4+}, and Ru{sup 4+} for SrMn{sub 0.5}Ru{sub 0.5}O{sub 3} to primarily Mn{sup 4+} and Ru{sup 4+} for CaMn{sub 0.5}Ru{sub 0.5}O{sub 3}. The compounds Ba{sub 0.5}La{sub 0.5}Mn{sub 0.5}Ru{sub 0.5}O{sub 3}, Ca{sub 0.5}La{sub 0.5}Mn{sub 0.5}Ru{sub 0.5}O{sub 3}, and Sr{sub 0.5}Ca{sub 0.25}La{sub 0.25}Mn{sub 0.5}Ru{sub 0.5}O{sub 3} were also investigated to study the effects of changing the charge of the A-site cation. Although substitution of La{sup 3+} for a divalent alkaline earth ion increases the Mn{sup 3+} content, this series of compounds also shows a relative increase in the concentration of Mn{sup 4+} as the average size of the A-site cation is decreased. In all compounds the octahedra containing Mn{sup 3+} are found to be Jahn-Teller distorted regardless of whether or not long-range orbital ordering is observed, while the Ru-centered octahedra are symmetric. No evidence for short-range cation ordering at either the A- or B-sites was found for any of the compositions. This study also reports that locally the A-site cations lie closer to the Mn ions than to the Ru ions and this asymmetry appears to be correlated to the degree of octahedral tilting.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Band Structure of Strain-Balanced GaAsBi/GaAsN Super-lattices on GaAs

GaAs alloys with dilute content of Bi and N provide a large reduction in band-gap energy with increasing alloy composition. GaAsBi/GaAsN heterojunctions have a type-II band alignment, where superlattices based on these materials offer a wide range for designing effective band-gap energy by varying superlattice period and alloy composition. The miniband structure and effective band gap for strain-balanced GaAsBi/GaAsN superlattices with effective lattice match to GaAs are calculated for alloy compositions up to 5% Bi and N using the k·p method. The effective band gap for these superlattices is found to vary between 0.89 and 1.32 eV for period thickness ranging from 10 to 100 Å. The joint density of states and optical absorption of a 40/40 Å GaAs0.96Bi0.04/GaAs0.98N0.02 superlattice are reported demonstrating a ground-state transition at 1.005 eV and first excited transition at 1.074 eV. The joint density of states is similar in magnitude to GaAs, while the optical absorption is approximately one order of magnitude lower due to the spatially indirect optical transition in the type-II structure. The GaAsBi/GaAsN system may provide a new material system with lattice match to GaAs in a spectral range of high importance for optoelectronic devices including solar cells, photodetectors, and light emitters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

High-pressure Structural Stability of Multiferroic Hexagonal RMnO3 (R=Y Ho Lu)

Structural changes in RMnO{sub 3} (R = Y, Ho, Lu) under high pressure were examined by synchrotron x-ray diffraction methods at room temperature. Compression occurs more readily in the ab plane than along the c axis. With increased pressure, a pressure-induced hexagonal to orthorhombic phase transition was observed starting at {approx}22 GPa for Lu(Y)MnO{sub 3}. When the pressure is increased to 35 GPa, a small volume fraction of Lu(Y)MnO{sub 3} is converted to the orthorhombic phase and the orthorhombic phase is maintained on pressure release. High-pressure infrared absorption spectroscopy and Mn K-edge near-edge x-ray absorption spectroscopy confirm that the hexagonal P6{sub 3}cm structure is stable below {approx}20 GPa and the environment around the Mn ion is not changed. Shifts in the unoccupied p-band density of states with pressure are observed in the Mn K-edge spectra. A schematic pressure-temperature phase diagram is given for the small ion RMnO{sub 3} system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗