344 research outputs found

    The Magnetic Field of the Solar Corona from Pulsar Observations

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    We present a novel experiment with the capacity to independently measure both the electron density and the magnetic field of the solar corona. We achieve this through measurement of the excess Faraday rotation due to propagation of the polarised emission from a number of pulsars through the magnetic field of the solar corona. This method yields independent measures of the integrated electron density, via dispersion of the pulsed signal and the magnetic field, via the amount of Faraday rotation. In principle this allows the determination of the integrated magnetic field through the solar corona along many lines of sight without any assumptions regarding the electron density distribution. We present a detection of an increase in the rotation measure of the pulsar J1801-2304 of approximately 160 \rad at an elongation of 0.95^\circ from the centre of the solar disk. This corresponds to a lower limit of the magnetic field strength along this line of sight of >393μG> 393\mu\mathrm{G}. The lack of precision in the integrated electron density measurement restricts this result to a limit, but application of coronal plasma models can further constrain this to approximately 20mG, along a path passing 2.5 solar radii from the solar limb. Which is consistent with predictions obtained using extensions to the Source Surface models published by Wilcox Solar ObservatoryComment: 16 pages, 4 figures (1 colour): Submitted to Solar Physic

    Who doesn't receive carotid endarterectomy when appropriate?

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    AbstractObjectiveThe purpose of this study was to identify clinical and nonclinical factors associated with failure to perform carotid endarterectomy (CEA) in patients with clinically appropriate indications. We analyzed data from a prospective cohort study performed at five Veterans Affairs medical centers. Patients were referred for carotid artery evaluation if they had at least 50% stenosis in one carotid artery, had no history of CEA, and were independently classified preoperatively as appropriate candidates for CEA, according to clinical criteria. The primary outcome was receipt of CEA within 6 months of evaluation. Data were collected by medical record review and interview regarding clinical status, and patient and physician perception of the risks and benefits of CEA.ResultsAmong clinically appropriate candidates for CEA, 66.8% (n = 233) did not undergo the operation. Compared with patients who did undergo CEA, a greater proportion of these patients had no symptoms (68.7% vs 45.7%; P < .001). A twofold greater proportion of patients who did not undergo CEA were in the highest quartile of reported aversion to surgery. Moreover, a fourfold greater proportion were perceived by their physicians to be at less than 5% risk for future stroke without the operation, and more than a twofold greater proportion were believed to experience less than 5% efficacy from the operation by their providers (P < .01). In multivariable analyses, four characteristics were significantly associated with whether an appropriate candidate did not receive CEA: asymptomatic disease, less than 70% stenosis, high expressed aversion to surgery score, and low (<5%) provider-perceived efficacy of the operation.ConclusionAmong patients in the Veterans Affairs health care system who are clinically appropriate candidates for CEA, those who did not receive the operation were less likely to have symptomatic disease or high-grade carotid artery stenosis, but were more likely to report high aversion to surgery and to have a provider who believed CEA would not be efficacious

    Bosonic Excitations in Random Media

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    We consider classical normal modes and non-interacting bosonic excitations in disordered systems. We emphasise generic aspects of such problems and parallels with disordered, non-interacting systems of fermions, and discuss in particular the relevance for bosonic excitations of symmetry classes known in the fermionic context. We also stress important differences between bosonic and fermionic problems. One of these follows from the fact that ground state stability of a system requires all bosonic excitation energy levels to be positive, while stability in systems of non-interacting fermions is ensured by the exclusion principle, whatever the single-particle energies. As a consequence, simple models of uncorrelated disorder are less useful for bosonic systems than for fermionic ones, and it is generally important to study the excitation spectrum in conjunction with the problem of constructing a disorder-dependent ground state: we show how a mapping to an operator with chiral symmetry provides a useful tool for doing this. A second difference involves the distinction for bosonic systems between excitations which are Goldstone modes and those which are not. In the case of Goldstone modes we review established results illustrating the fact that disorder decouples from excitations in the low frequency limit, above a critical dimension dcd_c, which in different circumstances takes the values dc=2d_c=2 and dc=0d_c=0. For bosonic excitations which are not Goldstone modes, we argue that an excitation density varying with frequency as ρ(ω)ω4\rho(\omega) \propto \omega^4 is a universal feature in systems with ground states that depend on the disorder realisation. We illustrate our conclusions with extensive analytical and some numerical calculations for a variety of models in one dimension

    Charge order and low frequency spin dynamics in lanthanum cuprates revealed by Nuclear Magnetic Resonance

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    We report detailed 17O, 139La, and 63Cu Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered La1.875Ba0.125CuO4 single crystal and in oriented powder samples of La1.8-xEu0.2SrxCuO4. We observe a partial wipeout of the 17O NMR intensity and a simultaneous drop of the 17O electric field gradient (EFG) at low temperatures where the spin stripe order sets in. In contrast, the 63Cu intensity is completely wiped out at the same temperature. The drop of the 17O quadrupole frequency is compatible with a charge stripe order. The 17O spin lattice relaxation rate shows a peak similar to that of the 139La, which is of magnetic origin. This peak is doping dependent and is maximal at x ~ 1/8.Comment: submitted to European Physical Journal Special Topic

    Angle-resolved photoemission in doped charge-transfer Mott insulators

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    A theory of angle-resolved photoemission (ARPES) in doped cuprates and other charge-transfer Mott insulators is developed taking into account the realistic (LDA+U) band structure, (bi)polaron formation due to the strong electron-phonon interaction, and a random field potential. In most of these materials the first band to be doped is the oxygen band inside the Mott-Hubbard gap. We derive the coherent part of the ARPES spectra with the oxygen hole spectral function calculated in the non-crossing (ladder) approximation and with the exact spectral function of a one-dimensional hole in a random potential. Some unusual features of ARPES including the polarisation dependence and spectral shape in YBa2Cu3O7 and YBa2Cu4O8 are described without any Fermi-surface, large or small. The theory is compatible with the doping dependence of kinetic and thermodynamic properties of cuprates as well as with the d-wave symmetry of the superconducting order parameter.Comment: 8 pages (RevTeX), 10 figures, submitted to Phys. Rev.

    Low energy excitations and dynamic Dzyaloshinskii-Moriya interaction in α\alpha'-NaV2_2O5_5 studied by far infrared spectroscopy

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    We have studied far infrared transmission spectra of alpha'-NaV2O5 between 3 and 200cm-1 in polarizations of incident light parallel to a, b, and c crystallographic axes in magnetic fields up to 33T. The triplet origin of an excitation at 65.4cm-1 is revealed by splitting in the magnetic field. The magnitude of the spin gap at low temperatures is found to be magnetic field independent at least up to 33T. All other infrared-active transitions appearing below Tc are ascribed to zone-folded phonons. Two different dynamic Dzyaloshinskii-Moriya (DM) mechanisms have been discovered that contribute to the oscillator strength of the otherwise forbidden singlet to triplet transition. 1. The strongest singlet to triplet transition is an electric dipole transition where the polarization of the incident light's electric field is parallel to the ladder rungs, and is allowed by the dynamic DM interaction created by a high frequency optical a-axis phonon. 2. In the incident light polarization perpendicular to the ladder planes an enhancement of the singlet to triplet transition is observed when the applied magnetic field shifts the singlet to triplet resonance frequency to match the 68cm-1 c-axis phonon energy. The origin of this mechanism is the dynamic DM interaction created by the 68cm-1 c-axis optical phonon. The strength of the dynamic DM is calculated for both mechanisms using the presented theory.Comment: 21 pages, 22 figures. Version 2 with replaced fig. 18 were labels had been los

    Plasma lipid profiles discriminate bacterial from viral infection in febrile children

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    Fever is the most common reason that children present to Emergency Departments. Clinical signs and symptoms suggestive of bacterial infection are often non-specific, and there is no definitive test for the accurate diagnosis of infection. The 'omics' approaches to identifying biomarkers from the host-response to bacterial infection are promising. In this study, lipidomic analysis was carried out with plasma samples obtained from febrile children with confirmed bacterial infection (n = 20) and confirmed viral infection (n = 20). We show for the first time that bacterial and viral infection produces distinct profile in the host lipidome. Some species of glycerophosphoinositol, sphingomyelin, lysophosphatidylcholine and cholesterol sulfate were higher in the confirmed virus infected group, while some species of fatty acids, glycerophosphocholine, glycerophosphoserine, lactosylceramide and bilirubin were lower in the confirmed virus infected group when compared with confirmed bacterial infected group. A combination of three lipids achieved an area under the receiver operating characteristic (ROC) curve of 0.911 (95% CI 0.81 to 0.98). This pilot study demonstrates the potential of metabolic biomarkers to assist clinicians in distinguishing bacterial from viral infection in febrile children, to facilitate effective clinical management and to the limit inappropriate use of antibiotics
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