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Berlin Brandenburg

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  • IOPscience (Institute of Physics)  (21)
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  • 1
    Language: English
    In: Journal of Physics: Condensed Matter, 2012, Vol.24(31), p.314208 (6pp)
    Description: We show here that Br 2 intercalation is an efficient method to enable exfoliation of epitaxial graphene on metals by adhesive tape. We exemplify this method for high-quality graphene of macroscopic extension on Ir(111). The sample quality and the transfer process are monitored using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), scanning electron microscopy (SEM) and Raman spectroscopy. The developed process provides an opportunity for preparing graphene of strictly monatomic thickness and well-defined orientation including the transfer to poly(ethylene terephthalate) (PET) foil.
    Keywords: Scanning Electron Microscopy ; Graphene ; Polyethylene Terephthalates ; Exfoliation ; Scanning Tunneling Microscopy ; Intercalation ; Condensed Matter ; Epitaxy ; Condensed Matter Physics (General) (So);
    ISSN: 0953-8984
    E-ISSN: 1361-648X
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  • 2
    Language: English
    In: Japanese Journal of Applied Physics, 2013, Vol.52(8S)
    Description: We have decreased the dislocation density in Al x Ga 1- x N epitaxial layers grown on sapphire wafers by introducing an in-situ deposited SiN nano-mask layer. Taking together results obtained by transmission electron microscopy, photoluminescence, cathodoluminescence, and X-ray diffraction, we were able to derive a schematic model about the AlGaN growth on the SiN nanomask: On the open pores of the nano-mask, Ga-rich AlGaN hillocks develop, whereas on the SiN layer Al-rich AlGaN nucleates owing to the reduced selectivity of Al-containing material. Once the hillocks are formed, Ga-rich material is more efficiently incorporated on the inclined side-facets leading to an Al-rich coverage of the central c -plane part of the hillocks. We observed a bending of the dislocations towards the side-facets of the hillocks, which eventually leads to dislocation bundles with increased probability of dislocation annihilation, separated by fairly defect-free regions. Thus, we could achieve a significant reduction of the edge-type dislocation density in these epitaxial layers.
    Keywords: Aluminum Gallium Nitrides ; Reduction ; Bundles ; Dislocations ; Epitaxial Layers ; Aluminum ; Dislocation Density ; Nanostructure ; Materials Selection ; Defects and Transport (Wc) ; Applied Physics (General) (So) ; Microstructure (EC) ; Constitution/Crystal Structure/Microstructure (AI) ; Physics (General) (Ah);
    ISSN: 00214922
    E-ISSN: 13474065
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  • 3
    Language: English
    In: The Astrophysical Journal, 2013, Vol.773(1), p.63 (15pp)
    Description: We have combined optical and NIR photometry from Pan-STARRS 1 and UKIDSS to search the young (5-10 Myr) star-forming region of Upper Scorpius for wide (400-4000 AU) substellar companions down to 5 M Jup . Our search is 4 mag deeper than previous work based on the Two Micron All Sky Survey. We identified several candidates around known stellar members using a combination of color selection and spectral energy distribution fitting. Our follow-up spectroscopy has identified two new companions as well as confirmed two companions previously identified from photometry, with spectral types of M7.5-M9 and masses of 15-60 M Jup , indicating a frequency for such wide substellar companions of 0.6% ± 0.3%. Both USco 16101913B and USco 16121800B are more luminous than expected for their spectral type compared with known members of Upper Sco. HIP 77900B has an extreme mass ratio ( M 2 / M 1 0.005) and an extreme separation of 3200 AU. USco 16022401B also has a very large separation of 1000 AU. We have also confirmed a low-mass stellar companion, USco 16102502B (730 AU, M5.5). Our substellar companions appear both non-coeval with their primary stars according to evolutionary models and, as a group, are systematically more luminous than the Upper Sco cluster sequence. One possible reason for these luminosity discrepancies could be different formation processes or accretion histories for these objects.
    Keywords: Astrophysics, Cosmology And Astronomy ; Color ; Energy Spectra ; Luminosity ; Mass ; Near Infrared Radiation ; Photometry ; Sky ; Spectroscopy ; Stars;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 4
    Language: English
    In: The Astronomical Journal, 2012, Vol.143(6), p.142 (8pp)
    Description: We present the discovery of the first high-redshift ( z 〉 5.7) quasar from the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1 or PS1). This quasar was initially detected as an i P1 dropout in PS1, confirmed photometrically with the SAO Wide-field InfraRed Camera at Arizona's Multiple Mirror Telescope (MMT) and the Gamma-Ray Burst Optical/Near-Infrared Detector at the MPG 2.2 m telescope in La Silla. The quasar was verified spectroscopically with the MMT Spectrograph, Red Channel and the Cassegrain Twin Spectrograph at the Calar Alto 3.5 m telescope. Its near-infrared spectrum was taken at the Large Binocular Telescope Observatory (LBT) with the LBT Near-Infrared Spectroscopic Utility with Camera and Integral Field Unit for Extragalactic Research. It has a redshift of 5.73, an AB z P1 magnitude of 19.4, a luminosity of 3.8 × 10 47  erg s 1 , and a black hole mass of 6.9 × 10 9 M . It is a broad absorption line quasar with a prominent Ly peak and a very blue continuum spectrum. This quasar is the first result from the PS1 high-redshift quasar search that is projected to discover more than 100  i P1 dropout quasars and could potentially find more than 10 z P1 dropout ( z 〉 6.8) quasars.
    Keywords: Astrophysics, Cosmology And Astronomy ; Absorption ; Astronomy ; Astrophysics ; Black Holes ; Cameras ; Cosmic Gamma Bursts ; Infrared Spectra ; Luminosity ; Mirrors ; Near Infrared Radiation ; Quasars ; Red Shift ; Telescopes ; Universe ; Astronomy & Astrophysics;
    ISSN: 0004-6256
    E-ISSN: 1538-3881
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  • 5
    Language: English
    In: Journal of Physics: Conference Series, 2011, Vol.312(5), p.052011 (6pp)
    Description: The Focusing Disc DIRC is a novel detector concept for particle identification based on the detection of internally-reflected Cherenkov light. A thin synthetic silica disc of 1 m radius will be used as radiator and for light transportation. At the rim LiF bars are foreseen to passively correct for dispersion. Focusing elements map the angles of the propagating photons to spatial positions on the focal plane covered with position sensitive photon detection devices. This novel 2D(+1t) detector concept will contribute to the outstanding particle identification performance of the general purpose PANDA detector. The aims of the PANDA experiment are to address fundamental questions of the strong force, to explore the structure of the nucleon and to search for new matter. The technical design, the current status of the development and recent results from prototype test experiments for the Focusing Disc DIRC are presented.
    Keywords: Physics;
    ISSN: 1742-6588
    E-ISSN: 1742-6596
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  • 6
    Language: English
    In: The Astrophysical Journal Letters, 2015, Vol.803(2), p.L16 (6pp)
    Description: Supermassive black hole binaries (SMBHBs) should be an inevitable consequence of the hierarchical growth of massive galaxies through mergers and the strongest sirens of gravitational waves (GWs) in the cosmos. Yet, their direct detection has remained elusive due to the compact (sub-parsec) orbital separations of gravitationally bound SMBHBs. Here, we exploit a theoretically predicted signature of an SMBHB in the time domain: periodic variability caused by a mass accretion rate that is modulated by the binary’s orbital motion. We report our first significant periodically varying quasar detection from the systematic search in the Pan-STARRS1 (PS1) Medium Deep Survey. Our SMBHB candidate, PSO J334.2028+01.4075, is a luminous radio-loud quasar at z = 2.060, with extended baseline photometry from the Catalina Real-Time Transient Survey, as well as archival spectroscopy from the FIRST Bright Quasar Survey. The observed period (542 ± 15 days) and estimated black hole mass ( ) correspond to an orbital separation of Schwarzschild radii ( pc), assuming the rest-frame period of the quasar variability traces the orbital period of the binary. This SMBHB candidate, discovered at the peak redshift for SMBH mergers, is in a physically stable configuration for a circumbinary accretion disk and within the regime of GW-driven orbital decay. Our search with PS1 is a benchmark study for the exciting capabilities of LSST, which will have orders of magnitude larger survey power and will potentially pinpoint the locations of thousands of SMBHBs in the variable night sky.
    Keywords: Binary Stars ; Gravity Waves ; Galaxies ; Black Holes ; Spectroscopy ; Quasars ; Stars, Universe (524) ; Quasars: General ; Surveys;
    ISSN: 2041-8205
    E-ISSN: 2041-8213
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  • 7
    Language: English
    In: The Astrophysical Journal, 2016, Vol.821(2), p.120 (31pp)
    Description: Substellar members of young (≲150 Myr) moving groups are valuable benchmarks to empirically define brown dwarf evolution with age and to study the low-mass end of the initial mass function. We have combined Pan-STARRS1 (PS1) proper motions with optical–IR photometry from PS1, Two Micron All Sky Survey (2MASS), and WISE to search for substellar members of the AB Dor Moving Group within ≈50 pc and with spectral types of late M to early L, corresponding to masses down to ≈30 M Jup at the age of the group (≈125 Myr). Including both photometry and proper motions allows us to better select candidates by excluding field dwarfs whose colors are similar to young AB Dor Moving Group members. Our near-IR spectroscopy has identified six ultracool dwarfs (M6–L4; ≈30–100 M Jup ) with intermediate surface gravities ( int-g ) as candidate members of the AB Dor Moving Group. We find another two candidate members with spectra showing hints of youth but consistent with field gravities. We also find four field brown dwarfs unassociated with the AB Dor Moving Group, three of which have int-g gravity classification. While signatures of youth are present in the spectra of our ≈125 Myr objects, neither their J – K nor W 1 – W 2 colors are significantly redder than field dwarfs with the same spectral types, unlike younger ultracool dwarfs. We also determined PS1 parallaxes for eight of our candidates and one previously identified AB Dor Moving Group candidate. Although radial velocities (and parallaxes, for some) are still needed to fully assess membership, these new objects provide valuable insight into the spectral characteristics and evolution of young brown dwarfs.
    ISSN: 0004-637X
    E-ISSN: 1538-4357
    Source: IOPscience (IOP Publishing)
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  • 8
    Language: English
    In: The Astrophysical Journal, 2018, Vol.859(1), p.31 (32pp)
    Description: We characterize the spatial density of the Pan-STARRS1 (PS1) sample of Rrab stars to study the properties of the old Galactic stellar halo. This sample, containing 44,403 sources, spans galactocentric radii of 0.55 kpc ≤  R gc  ≤ 141 kpc with a distance precision of 3% and thus is able to trace the halo out to larger distances than most previous studies. After excising stars that are attributed to dense regions such as stellar streams, the Galactic disk and bulge, and halo globular clusters, the sample contains ∼11,000 sources within 20 kpc ≤  R gc  ≤ 131 kpc. We then apply forward modeling using Galactic halo profile models with a sample selection function. Specifically, we use ellipsoidal stellar density models ρ ( l , b , R gc ) with a constant and a radius-dependent halo flattening q ( R gc ). Assuming constant flattening q , the distribution of the sources is reasonably well fit by a single power law with and and comparably well fit by an Einasto profile with , an effective radius r eff  = 1.07 ± 0.10 kpc, and a halo flattening of q  = 0.923 ± 0.007. If we allow for a radius-dependent flattening q ( R gc ), we find evidence for a distinct flattening of q  ∼ 0.8 of the inner halo at ∼25 kpc. Additionally, we find that the south Galactic hemisphere is more flattened than the north Galactic hemisphere. The results of our work are largely consistent with many earlier results (e.g., Watkins et al.; Iorio et al.). We find that the stellar halo, as traced in RR Lyrae stars, exhibits a substantial number of further significant over- and underdensities, even after masking all known overdensities.
    Keywords: Astrophysics - Astrophysics Of Galaxies;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 9
    Language: English
    In: The Astrophysical Journal, 2015, Vol.810(1), p.25 (23pp)
    Description: We present a three-dimensional map of interstellar dust reddening, covering three-quarters of the sky out to a distance of several kiloparsecs, based on Pan-STARRS 1 (PS1) and 2MASS photometry. The map reveals a wealth of detailed structure, from filaments to large cloud complexes. The map has a hybrid angular resolution, with most of the map at an angular resolution of – , and a maximum distance resolution of . The three-dimensional distribution of dust is determined in a fully probabilistic framework, yielding the uncertainty in the reddening distribution along each line of sight, as well as stellar distances, reddenings, and classifications for 800 million stars detected by PS1. We demonstrate the consistency of our reddening estimates with those of two-dimensional emission-based maps of dust reddening. In particular, we find agreement with the Planck -based reddening map to within in to a depth of , and explore systematics at reddenings less than . We validate our per-star reddening estimates by comparison with reddening estimates for stars with both Sloan Digital Sky Survey photometry and Sloan Extension for Galactic Understanding and Exploration spectral classifications, finding per-star agreement to within out to a stellar of 1 mag. We compare our map to two existing three-dimensional dust maps, by Marshall et al. and Lallement et al., demonstrating our finer angular resolution, and better distance resolution compared to the former within . The map can be queried or downloaded at http://argonaut.skymaps.info . We expect the three-dimensional reddening map presented here to find a wide range of uses, among them correcting for reddening and extinction for objects embedded in the plane of the Galaxy, studies of Galactic structure, calibration of future emission-based dust maps, and determining distances to objects of known reddening.
    Keywords: Astrophysics - Astrophysics Of Galaxies;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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  • 10
    Language: English
    In: The Astrophysical Journal, 2018, Vol.854(2), p.79 (20pp)
    Description: We present a simple but effective technique for measuring angular variation in R V across the sky. We divide stars from the Pan-STARRS1 catalog into Healpix pixels and determine the posterior distribution of reddening and R V for each pixel using two independent Monte Carlo methods. We find the two methods to be self-consistent in the limits where they are expected to perform similarly. We also find some agreement with high-precision photometric studies of R V in Perseus and Ophiuchus, as well as with a map of reddening near the Galactic plane based on stellar spectra from APOGEE. While current studies of R V are mostly limited to isolated clouds, we have developed a systematic method for comparing R V values for the majority of observable dust. This is a proof of concept for a more rigorous Galactic reddening map.
    Keywords: Astronomy & Astrophysics ; Physics;
    ISSN: 0004-637X
    E-ISSN: 1538-4357
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