2,742 research outputs found

    Blow-Up of Test Fields Near Cauchy Horizons

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    The behaviour of test fields near a compact Cauchy horizon is investigated. It is shown that solutions of nonlinear wave equations on Taub spacetime with generic initial data cannot be continued smoothly to both extensions of the spacetime through the Cauchy horizon. This is proved using an energy method. Similar results are obtained for the spacetimes of Moncrief containing a compact Cauchy horizon and for more general matter models.Comment: 10 pages, Plain TeX, MPA-AR-92-

    Crystal Symmetry Lowering in Chiral Multiferroic Ba3_3TaFe3_3Si2_2O14_{14} observed by X-Ray Magnetic Scattering

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    Chiral multiferroic langasites have attracted attention due to their doubly-chiral magnetic ground state within an enantiomorphic crystal. We report on a detailed resonant soft X-ray diffraction study of the multiferroic Ba3_3TaFe3_3Si2_2O14_{14} at the Fe L2,3L_{2,3} and oxygen KK edges. Below TNT_N (≈27K\approx27K) we observe the satellite reflections (0,0,τ)(0,0,\tau), (0,0,2τ)(0,0,2\tau), (0,0,3τ)(0,0,3\tau) and (0,0,1−3τ)(0,0,1-3\tau) where τ≈0.140±0.001\tau \approx 0.140 \pm 0.001. The dependence of the scattering intensity on X-ray polarization and azimuthal angle indicate that the odd harmonics are dominated by the out-of-plane (c^\mathbf{\hat{c}}-axis) magnetic dipole while the (0,0,2τ)(0,0,2\tau) originates from the electron density distortions accompanying magnetic order. We observe dissimilar energy dependences of the diffraction intensity of the purely magnetic odd-harmonic satellites at the Fe L3L_3 edge. Utilizing first-principles calculations, we show that this is a consequence of the loss of threefold crystal symmetry in the multiferroic phase

    Tunable fiber source of entangled UV-C and infrared photons

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    Pairs of entangled photons -- biphotons -- are indispensable in quantum applications. However, some important spectral ranges, like ultraviolet, have been inaccessible to them so far. Here, we use four-wave mixing in a xenon-filled single-ring photonic crystal fiber to generate biphotons with one of the photons in the ultraviolet and its entangled partner in the infrared spectral range. We tune the biphotons in frequency by varying the gas pressure inside the fiber and thus tailoring the fiber dispersion landscape. The ultraviolet photons are tunable from 271 nm to 235 nm and their entangled partners, from 764 nm to 1342 nm, respectively. The tunability up to 170 THz is achieved by adjusting the gas pressure by only 0.57 bar. At 1.32 bar, the photons of a pair are separated by more than 2 octaves. The access to ultraviolet wavelengths opens the possibility for spectroscopy and sensing with undetected photons in this spectral range.Comment: 6 pages, 6 figure

    Resonant inelastic x-ray scattering probes the electron-phonon coupling in the spin-liquid kappa-(BEDT-TTF)2Cu2(CN)3

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    Resonant inelastic x-ray scattering at the N K edge reveals clearly resolved harmonics of the anion plane vibrations in the kappa-(BEDT-TTF)2Cu2(CN)3 spin-liquid insulator. Tuning the incoming light energy at the K edge of two distinct N sites permits to excite different sets of phonon modes. Cyanide CN stretching mode is selected at the edge of the ordered N sites which are more strongly connected to the BEDT-TTF molecules, while positionally disordered N sites show multi-mode excitation. Combining measurements with calculations on an anion plane cluster permits to estimate the sitedependent electron-phonon coupling of the modes related to nitrogen excitation

    Energy and position resolution of a CdZnTe gamma-ray detector with orthogonal coplanar anodes

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    We report on the simulation, construction and performance of prototype CZT imaging detectors employing orthogonal coplanar anodes. These detectors employ a novel electrode geometry with non-collecting anode strips in 1D and collecting anode pixels, interconnected in rows, in the orthogonal dimensions. These detectors retain the spectroscopic and detection efficiency advantages of single carried charge sensing devices as well as the principal advantage of conventional strip detectors with orthogonal anode and cathode strips, i.e. an N X N array of imagin pixels are realized with only 2N electronic channels. Charge signals induced on the various electrodes of a prototype detector with 8 X 8 unit cells are in good agreement with the simulations. The position resolution is about 1 mm in the direction perpendicular to the pixel lines while it is of the order of 100 micrometers in the other direction. Energy resolutions of 0.9 percent at 662 keV, 2.6 percent at 122 keV and 5.7 percent at 60 keV have been obtained at room temperature

    Observation of Magnetic Edge State and Dangling Bond State on Nanographene in Activated Carbon Fibers

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    The electronic structure of nanographene in pristine and fluorinated activated carbon fibers (ACFs) have been investigated with near-edge x-ray absorption fine structure (NEXAFS) and compared with magnetic properties we reported on previously. In pristine ACFs in which magnetic properties are governed by non-bonding edge states of the \pi-electron, a pre-peak assigned to the edge state was observed below the conduction electron {\pi}* peak close to the Fermi level in NEXAFS. Via the fluorination of the ACFs, an extra peak, which was assigned to the \sigma-dangling bond state, was observed between the pre-peak of the edge state and the {\pi}* peak in the NEXAFS profile. The intensities of the extra peak correlate closely with the spin concentration created upon fluorination. The combination of the NEXAFS and magnetic measurement results confirms the coexistence of the magnetic edge states of \pi-electrons and dangling bond states of \sigma-electrons on fluorinated nanographene sheets.Comment: 4 figures, to appear in Phys. Rev.

    Predicted FeII Emission-Line Strengths from Active Galactic Nuclei

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    We present theoretical FeII emission line strengths for physical conditions typical of Active Galactic Nuclei with Broad-Line Regions. The FeII line strengths were computed with a precise treatment of radiative transfer using extensive and accurate atomic data from the Iron Project. Excitation mechanisms for the FeII emission included continuum fluorescence, collisional excitation, self-fluorescence amoung the FeII transitions, and fluorescent excitation by Lyman-alpha and Lyman-beta. A large FeII atomic model consisting of 827 fine structure levels (including states to E ~ 15 eV) was used to predict fluxes for approximately 23,000 FeII transitions, covering most of the UV, optical, and IR wavelengths of astrophysical interest. Spectral synthesis for wavelengths from 1600 Angstroms to 1.2 microns is presented. Applications of present theoretical templates to the analysis of observations are described. In particular, we discuss recent observations of near-IR FeII lines in the 8500 Angstrom -- 1 micron region which are predicted by the Lyman-alpha fluorescence mechanism. We also compare our UV spectral synthesis with an empirical iron template for the prototypical, narrow-line Seyfert galaxy I Zw 1. The theoretical FeII template presented in this work should also applicable to a variety of objects with FeII spectra formed under similar excitation conditions, such as supernovae and symbiotic stars.Comment: 33 pages, 15 postscript figure

    Infections du site opĂ©ratoire sur ablation de varices d’aprĂšs une sĂ©rie continue de 408 interventions rĂ©alisĂ©es dans un centre hospitalier universitaire

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    But de l’étudeNotre objectif Ă©tait de dĂ©terminer l’incidence des infections du site opĂ©ratoire (ISO) consĂ©cutives Ă  des interventions sur varices dans le service de chirurgie vasculaire d’un centre hospitalier universitaire. Patients et mĂ©thodes Une surveillance prospective des ISO a Ă©tĂ© mise en place pendant un an, avec un suivi postopĂ©ratoire des patients de 30 jours. Le diagnostic d’ISO a Ă©tĂ© rĂ©alisĂ© suivant les dĂ©finitions des Centers for Disease Control (CDC, États-Unis). La saisie et l’analyse des donnĂ©es ont Ă©tĂ© rĂ©alisĂ©es Ă  l’aide du logiciel Epi-Info des CDC. RĂ©sultats Les trois-quarts des 408 interventions incluses Ă©taient caractĂ©risĂ©es par un score de NNISS Ă©gal Ă  0. Une dĂ©pilation a Ă©tĂ© effectuĂ©e pour tous les patients, avec des modalitĂ©s trĂšs variables et souvent non conformes (rasage mĂ©canique dans 44,6 % des cas) aux recommandations nationales. L’incidence des ISO Ă©tait de 1,2 % (intervalle de confiance Ă  95 % = [0,2–2,2]). Toutes les infections ont Ă©tĂ© diagnostiquĂ©es aprĂšs retour au domicile. Quatre patients infectĂ©s sur cinq prĂ©sentaient des problĂšmes de surpoids ou d’obĂ©sitĂ© et deux d’entre eux Ă©taient diabĂ©tiques. L’ñge moyen des patients Ă©tait plus Ă©levĂ© chez les patients infectĂ©s (70,4 ans versus 52,0 ; p < 0,01). Les cinq ISO ont eu des consĂ©quences (rĂ©hospitalisation et/ou reprise chirurgicale et/ou antibiothĂ©rapie). Conclusion D’aprĂšs ces rĂ©sultats, les ISO consĂ©cutives Ă  des interventions sur varices sont rares et concernent principalement des patients Ă  haut risque. Dans un but de prĂ©vention, il semble cependant nĂ©cessaire d’homogĂ©nĂ©iser les pratiques de dĂ©pilation dans ce service
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