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Search Results: 1 - 10 of 461799 matches for " A. Janossy "
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Mg5C60: A stable two dimensional conducting polymer
D. Quintavalle,F. Borondics,G. Klupp,A. Baserga,F. Simon,A. Janossy,K. Kamaras,S. Pekker
Physics , 2008,
Abstract: We present a study on the structural, spectroscopic, conducting and magnetic properties of Mg5C60, a two dimensional (2D) fulleride polymer. The polymer phase is stable up to the exceptionally high temperature of 823 K. Infrared and Raman studies suggest the formation of single bonds between fulleride ions and possibly Mg - C60 covalent bonds. Mg5C60 is a metal at ambient temperature as shown by electron spin resonance and microwave conductivity measurements. The smooth transition from a metallic to a paramagnetic insulator state below 200 K is attributed to Anderson localization driven by structural disorder.
Influence of Nd on the magnetic properties of Nd1-xCaxMnO3
F. Dupont,F. Millange,S. de Brion,A. Janossy,G. Chouteau
Physics , 2001, DOI: 10.1103/PhysRevB.64.220403
Abstract: The role played by the Nd ions in the magnetic properties of Nd0.5Ca0.5MnO3 and Nd0.7Ca0.3MnO3 is studied using static magnetization, neutron diffraction and high frequency (9.4-475GHz) Electron Spin Resonance. We show that the Nd ions are weakly coupled to the Mn ions via ferromagnetic exchange and are responsible for the peculiar ferromagnetic resonance observed in the FM phase of both compounds (ground state below 120K for x=0.3, high field state for x=0.5). We then use ESR to look for magnetic phase separation in the low field, CO phase of Nd0.5Ca0.5MnO3. We show that there is no trace of the FM phase imbedded in the CO phase, contrary to what is observed in La0.5Ca0.5MnO3 or Pr0.5Sr0.5MnO3.
Phase separation effects in charge-ordered Pr0.5Ca0.5MnO3 thin film
S. de Brion,G. Storch,G. Chouteau,A. Janossy,W. Prellier,E. Rauwel Buzin
Physics , 2003, DOI: 10.1140/epjb/e2003-00182-7
Abstract: Compressed Pr0.5Ca0.5MnO3 films (250nm) deposited on LaAlO3 have been studied by Electron Spin Resonance technique under high frequency and high magnetic field. We show evidences for the presence of a ferromagnetic phase (FM) embedded in the charge-order phase (CO), in form of thin layers which size depends on the strength and orientation of the magnetic field (parallel or perpendicular to the substrate plane). This FM phase presents an easy plane magnetic anisotropy with an anisotropy constant 100 times bigger than typical bulk values. When the magnetic field is applied perpendicular to the substrate plane, the FM phase is strongly coupled to the CO phase whereas for the parallel orientation it keeps an independent ferromagnetic resonance even when the CO phase becomes antiferromagnetic.
Generalized Elliott-Yafet theory of electron spin relaxation in metals: the origin of the anomalous electron spin life-time in MgB2
F. Simon,B. Dora,F. Muranyi,A. Janossy,S. Garaj,L. Forro,S. Bud'ko,C. Petrovic,P. C. Canfield
Physics , 2008, DOI: 10.1103/PhysRevLett.101.177003
Abstract: The temperature dependence of the electron spin relaxation time in MgB2 is anomalous as it does not follow the temperature dependence of the resistivity above 150 K, it has a maximum around 400 K, and it decreases for higher temperatures. This violates the well established Elliot-Yafet theory of electron spin relaxation in metals. We show that the anomaly occurs when the quasi-particle scattering rate (in energy units) becomes comparable to the energy difference between the conduction- and a neighboring band. We find that the anomalous behavior is related to the unique band structure of MgB$_2$ and the large electron-phonon coupling. The saturating spin-lattice relaxation can be regarded as the spin transport analogue of the Ioffe-Regel criterion of electron transport.
Spin excitations in the antiferromagnet NaNiO2
Sophie De Brion,C. Darie,M. Hopzapfel,D. Talbayev,L. Mihaly,F. Simon,A. Janossy,G. Chouteau
Physics , 2006, DOI: 10.1103/PhysRevB.75.094402
Abstract: In NaNiO2, Ni3+ ions form a quasi two dimensional triangular lattice of S = 1=2 spins. The magnetic order observed below 20K has been described as an A type antiferromagnet with ferro- magnetic layers weakly coupled antiferromagnetically. We studied the magnetic excitations with the electron spin resonance for frequencies 1-20 cm-1, in magnetic fields up to 14 T. The bulk of the results are interpreted in terms of a phenomenological model involving bi-axial anisotropy for the spins: a strong easy-plane term, and a weaker anisotropy within the plane. The direction of the easy plane is constrained by the collective Jahn-Teller distortion occurring in this material at 480 K.
Magnetic fullerenes inside single-wall carbon nanotubes
F. Simon,H. Kuzmany,B. Nafradi,T. Feher,L. Forro,F. Fulop,A. Janossy,L. Korecz,A. Rockenbauer,F. Hauke,A. Hirsch
Physics , 2006, DOI: 10.1103/PhysRevLett.97.136801
Abstract: C59N magnetic fullerenes were formed inside single-wall carbon nanotubes by vacuum annealing functionalized C59N molecules encapsulated inside the tubes. A hindered, anisotropic rotation of C59N was deduced from the temperature dependence of the electron spin resonance spectra near room temperature. Shortening of spin-lattice relaxation time, T_1, of C59N indicates a reversible charge transfer toward the host nanotubes above $\sim 350$ K. Bound C59N-C60 heterodimers are formed at lower temperatures when C60 is co-encapsulated with the functionalized C59N. In the 10-300 K range, T_1 of the heterodimer shows a relaxation dominated by the conduction electrons on the nanotubes.
Rapid Diagnosis of Smear-Negative Tuberculosis Using Immunology and Microbiology with Induced Sputum in HIV-Infected and Uninfected Individuals
Ronan A. M. Breen, Gareth A. D. Hardy, Felicity M. R. Perrin, Sara Lear, Sabine Kinloch, Colette J. Smith, Ian Cropley, George Janossy, Marc C. I. Lipman
PLOS ONE , 2007, DOI: 10.1371/journal.pone.0001335
Abstract: Rationale and Objectives Blood-based studies have demonstrated the potential of immunological assays to detect tuberculosis. However lung fluid sampling may prove superior as it enables simultaneous microbiological detection of mycobacteria to be performed. Until now this has only been possible using the expensive and invasive technique of broncho-alveolar lavage. We sought to evaluate an immunoassay using non-invasive induced-sputum to diagnose active tuberculosis. Methods and Results Prospective cohort study of forty-two spontaneous sputum smear-negative or sputum non-producing adults under investigation for tuberculosis. CD4 lymphocytes specific to purified-protein-derivative of Mycobacterium tuberculosis actively synthesising interferon-gamma were measured by flow cytometry and final diagnosis compared to immunoassay using a cut-off of 0.5%. Sixteen subjects (38%) were HIV-infected (median CD4 count [range] = 332 cells/μl [103–748]). Thirty-eight (90%) were BCG-vaccinated. In 27 subjects diagnosed with active tuberculosis, the median [range] percentage of interferon-gamma synthetic CD4+ lymphocytes was 2.77% [0–23.93%] versus 0% [0–2.10%] in 15 negative for active infection (p<0.0001). Sensitivity and specificity of the immunoassay versus final diagnosis of active tuberculosis were 89% (24 of 27) and 80% (12 of 15) respectively. The 3 positive assays in the latter group occurred in subjects diagnosed with quiescent/latent tuberculosis. Assay performance was unaffected by HIV-status, BCG-vaccination or disease site. Combining this approach with traditional microbiological methods increased the diagnostic yield to 93% (25 of 27) alongside acid-fast bacilli smear and 96% (26 of 27) alongside tuberculosis culture. Conclusions These data demonstrate for the first time that a rapid immunological assay to diagnose active tuberculosis can be performed successfully in combination with microbiological methods on a single induced-sputum sample.
Magnetic Field Induced Density of States in Superconducting MgB$_{2}$: Measurement of Conduction Electron Spin-Susceptibility
F. Simon,A. Janossy,T. Feher,F. Muranyi,S. Garaj,L. Forro,C. Petrovic,S. Bud'ko,R. A. Ribeiro,P. C. Canfield
Physics , 2003, DOI: 10.1103/PhysRevB.72.012511
Abstract: The magnetic field dependence of the spin-susceptibility, $\chi_{s}$ was measured in the superconducting state of high purity MgB$_{2}$ fine powders below 1.3 T. $\chi_{s}$ was determined from the intensity of the conduction electron spin resonance spectra at 3.8, 9.4, and 35 GHz. At the lowest magnetic fields (0.14 T), a gap opens in the density of states at the Fermi energy and, accordingly, $\chi_{s}(T)$ is small at low temperatures. Fields above 0.2 T (about 15 % of $H^{c}_{c2}$, the minimum upper critical field), destroy the gap. The field induced $\chi_{s}$ is much larger than expected from current superconductor models of MgB$_{2}$.
T lymphocytes among HIV-infected and -uninfected infants: CD4/CD8 ratio as a potential tool in diagnosis of infection in infants under the age of 2 years
Lynn S Zijenah, David A Katzenstein, Kusum J Nathoo, Simbarashe Rusakaniko, Ocean Tobaiwa, Christine Gwanzura, Arsene Bikoue, Margaret Nhembe, Petronella Matibe, George Janossy
Journal of Translational Medicine , 2005, DOI: 10.1186/1479-5876-3-6
Abstract: Whole blood was collected in EDTA from 137 infants aged 0 to 18 months. DNA polymerase chain reaction was used as the reference standard for diagnosis of HIV-1 infection. T-cell subset profiles were determined by flow cytometry.Seventy-six infants were DNA PCR positive while 61 were negative. The median CD4 counts of PCR negative infants were significantly higher than those of the PCR positive infants, p < 0.001. The median CD4/CD8 ratio and the %CD4 of the PCR positive infants were both significantly lower than those of the negative infants, p < 0.001. The CD4/CD8 ratio had a >98% sensitivity for diagnosis of HIV-1 infection and a specificity of >98%.The CD4/CD8 ratio appears useful in identifying HIV-infected infants. The development of lower cost and more robust flow cytometric methods that provide both CD4/CD8 ratio and %CD4 may be cost-effective for HIV-1 diagnosis and identification of infants for cotrimoxazole prophylaxis and/or highly active antiretroviral therapy.Endemic HIV infection, in sub-Saharan Africa, where in many countries more than 20% of pregnant women are HIV seropositive leads to a diagnostic problem in the evaluation of their infants. Without intervention, more than 25% of infants born to seropositive women will acquire HIV infection in the first year of life. HIV testing with enzyme immunoassay-based rapid tests have expanded capacity to identify seropositive women and provide interventions, but even with single dose Nevirapine and other antiretrovirals, infection of infants still exceeds 10% in the first year of life. Serologic tests for HIV do not accurately identify those infants who have acquired infection within the first 18 months of life because of transplacentally acquired maternal IgG antibodies. As antimicrobial and antiviral interventions are developed to reduce morbidity and mortality, among infants born to seropositive women, the early diagnosis of infection is increasingly important. The gold standard for diagnosis of HIV-1 infe
Anisotropy of superconducting MgB2 as seen in electron spin resonance and magnetization data
F. Simon,A. Janossy,T. Feher,F. Muranyi,S. Garaj,L. Forro,C. Petrovic,S. L. Bud'ko,G. Lapertot,V. G. Kogan,P. C. Canfield
Physics , 2001, DOI: 10.1103/PhysRevLett.87.047002
Abstract: We have observed the conduction electron spin resonance (CESR) in fine powders of MgB2 both in the superconducting and normal states. The Pauli susceptibility is chi_s=2.0*10^{-5} emu/mole in the temperature range of 450 to 600 K. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below T_c at several frequencies suggests that MgB_2 is a strongly anisotropic superconductor with the upper critical field, H_c2, ranging between 2 and 16 T. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that MgB_2 is an anisotropic superconductor with H_c2^{ab} / H_{c2}^{c} \approx 6--9.
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