2,588 research outputs found
Generation of spin-wave dark solitons with phase engineering
We generate experimentally spin-wave envelope dark solitons from rectangular
high-frequency dark input pulses with externally introduced phase shifts in
yttrium-iron garnet magnetic fims. We observe the generation of both odd and
even numbers of magnetic dark solitons when the external phase shift varies.
The experimental results are in a good qualitative agreement with the theory of
the dark-soliton generation in magnetic films developed earlier [Phys. Rev.
Lett. 82, 2583 (1999)].Comment: 6 pages, including 7 figures, submitted to Phys. Rev.
Electron-hole correlation effects in the emission of light from quantum wires
We present a self-consistent treatment of the electron-hole correlations in
optically excited quantum wires within the ladder approximation, and using a
contact potential interaction. The limitations of the ladder approximation to
the excitonic low-density region are largely overcome by the introduction of
higher order correlations through self consistency. We show relevance of these
correlations in the low-temperature emission, even for high density relevant in
lasing, when large gain replaces excitonic absorption.Comment: 4 paes 3 figure
The Next-Generation Multimission U.S. Surveillance Radar Network
The U.S. Government operates seven distinct radar networks, providing weather and aircraft surveillance for public weather services, air traffic control, and homeland defense. In this paper, we describe a next-generation multimission phased-array radar (MPAR) concept that could provide enhanced weather and aircraft surveillance services with potentially lower life cycle costs than multiple single-function radar networks. We describe current U.S. national weather and aircraft surveillance radar networks and show that by reducing overlapping airspace coverage, MPAR could reduce the total number of radars required by approximately one-third. A key finding is that weather surveillance requirements dictate the core parameters of a multimission radar—airspace coverage, aperture size, radiated power, and angular resolution. Aircraft surveillance capability can be added to a phased array weather radar at low incremental cost because the agile, electronically steered beam would allow the radar to achieve the much more rapid scan update rates needed for aircraft volume search missions, and additionally to support track modes for individual aircraft targets. We describe an MPAR system design that includes multiple transmit–receive channels and a highly digitized active phased array to generate independently steered beam clusters for weather, aircraft volume search, and aircraft track modes. For each of these modes, we discuss surveillance capability improvements that would be realized relative to today's radars. The Federal Aviation Administration (FAA) has initiated the development of an MPAR “preprototype” that will demonstrate critical subsystem technologies and multimission operational capabilities. Initial subsystem designs have provided a solid basis for estimating MPAR costs for comparison with existing, mechanically scanned operational surveillance radars.United States. Federal Aviation Administration (FA8721-05-C-0002
The pursuit of isotopic and molecular fire tracers in the polar atmosphere and cryosphere
We present an overview of recent multidisciplinary, multi-institutional efforts to identify and date major sources of combustion aerosol in the current and paleoatmospheres. The work was stimulated, in part, by an atmospheric particle \u27sample of opportunity\u27 collected at Summit, Greenland in August 1994, that bore the 14C imprint of biomass burning. During the summer field seasons of 1995 and 1996, we collected air filter, surface snow and snowpit samples to investigate chemical and isotopic evidence of combustion particles that had been transported from distant fires. Among the chemical tracers employed for source identification are organic acids, potassium and ammonium ions, and elemental and organic components of carbonaceous particles. Ion chromatography, performed by members of the Climate Change Research Center (University of New Hampshire), has been especially valuable in indicating periods at Summit that were likely to have been affected by the long range transport of biomass burning aerosol. Univariate and multivariate patterns of the ion concentrations in the snow and ice pinpointed surface and snowpit samples for the direct analysis of particulate (soot) carbon and carbon isotopes. The research at NIST is focusing on graphitic and polycyclic aromatic carbon, which serve as almost certain indicators of fire, and measurements of carbon isotopes, especially 14C, to distinguish fossil and biomass combustion sources. Complementing the chemical and isotopic record, are direct \u27visual\u27 (satellite imagery) records and less direct backtrajectory records, to indicate geographic source regions and transport paths. In this paper we illustrate the unique way in which the synthesis of the chemical, isotopic, satellite and trajectory data enhances our ability to develop the recent history of the formation and transport of soot deposited in the polar snow and ice
The HITRAN 2008 molecular spectroscopic database
This paper describes the status of the 2008 edition of the HITRAN molecular spectroscopic database. The new edition is the first official public release since the 2004 edition, although a number of crucial updates had been made available online since 2004. The HITRAN compilation consists of several components that serve as input for radiative-transfer calculation codes: individual line parameters for the microwave through visible spectra of molecules in the gas phase; absorption cross-sections for molecules having dense spectral features, i.e. spectra in which the individual lines are not resolved; individual line parameters and absorption cross-sections for bands in the ultraviolet; refractive indices of aerosols, tables and files of general properties associated with the database; and database management software. The line-by-line portion of the database contains spectroscopic parameters for 42 molecules including many of their isotopologues. (c) 2009 Elsevier Ltd. All rights reserved
The HITRAN2012 molecular spectroscopic database
This paper describes the status of the 2012 edition of the HITRAN molecular spectroscopic compilation. The new edition replaces the previous HITRAN edition of 2008 and its updates during the intervening years. The HITRAN molecular absorption compilation is comprised of six major components structured into folders that are freely accessible on the internet. These folders consist of the traditional line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, infrared absorption cross-sections for molecules not yet amenable to representation in a line-by-line form, ultraviolet spectroscopic parameters, aerosol indices of refraction, ision-induced absorption data, and general tables such as partition sums that apply globally to the data. The new HITRAN is greatly extended in terms of accuracy, spectral coverage, additional absorption phenomena, and validity. Molecules and isotopologues have been added that address the issues of atmospheres beyond the Earth. Also discussed is a new initiative that casts HITRAN into a relational database format that offers many advantages over the long-standing sequential text-based structure that has existed since the initial release of HITRAN in the early 1970s. (C) 2013 Elsevier Ltd. All rights reserved
Band gap renormalization in photoexcited semiconductor quantum wire structures in the GW approximation
We investigate the dynamical self-energy corrections of the electron-hole
plasma due to electron-electron and electron-phonon interactions at the band
edges of a quasi-one dimensional (1D) photoexcited electron-hole plasma. The
leading-order dynamical screening approximation is used in the calculation
by treating electron-electron Coulomb interaction and electron-optical phonon
Fr\"{o}hlich interaction on an equal footing. We calculate the
exchange-correlation induced band gap renormalization (BGR) as a function of
the electron-hole plasma density and the quantum wire width. The calculated BGR
shows good agreement with existing experimental results, and the BGR normalized
by the effective quasi-1D excitonic Rydberg exhibits an approximate
one-parameter universality.Comment: 11 pages, 3 figure
Engineering a rare-cutting restriction enzyme: genetic screening and selection of NotI variants
Restriction endonucleases (REases) with 8-base specificity are rare specimens in nature. NotI from Nocardia otitidis-caviarum (recognition sequence 5′-GCGGCCGC-3′) has been cloned, thus allowing for mutagenesis and screening for enzymes with altered 8-base recognition and cleavage activity. Variants possessing altered specificity have been isolated by the application of two genetic methods. In step 1, variant E156K was isolated by its ability to induce DNA-damage in an indicator strain expressing M.EagI (to protect 5′-NCGGCCGN-3′ sites). In step 2, the E156K allele was mutagenized with the objective of increasing enzyme activity towards the alternative substrate site: 5′-GCTGCCGC-3′. In this procedure, clones of interest were selected by their ability to eliminate a conditionally toxic substrate vector and induce the SOS response. Thus, specific DNA cleavage was linked to cell survival. The secondary substitutions M91V, F157C and V348M were each found to have a positive effect on specific activity when paired with E156K. For example, variant M91V/E156K cleaves 5′-GCTGCCGC-3′ with a specific activity of 8.2 × 10(4) U/mg, a 32-fold increase over variant E156K. A comprehensive analysis indicates that the cleavage specificity of M91V/E156K is relaxed to a small set of 8 bp substrates while retaining activity towards the NotI sequence
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