51,631 research outputs found

    Decrease in the high energy X-ray flux from Cen XR-2. Search for X-rays from the large and small Magellanic clouds

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    High energy X ray sky survey data on decreased intensity of Cen XR-2, and X ray energy flux from Magellanic clouds - Mildura, Australia, October 15 and 24, 196

    1^{1}H-NMR spin-echo measurements of the static and dynamic spin properties in λ\lambda-(BETS)2_{2}FeCl4_{4}

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    1^{1}H-NMR spin-echo measurements of the spin-echo decay M(2τ)M(2\tau) with a decay rate 1/T2T_{2} and the frequency shift Δν/ν0\Delta\nu/\nu_{0} under applied magnetic field B\mathbf{B}0_{0} = 9 T along the a-axis over a temperature range 2.0-180 K are reported for a single crystal of the organic conductor λ\lambda-(BETS)2_{2}FeCl4_{4}. It provides the spin dynamic and static properties in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI) states as well as across the PM-AFI phase transition. A large slow beat structure in the spin-echo decay is observed with a typical beat frequency of ff \sim 7 kHz and it varies across the spectrum. Its origin is attributed to the 1^{1}H-1^{1}H dipole interactions rather than to the much larger dipolar field contribution from the Fe3+^{3+} electrons (spin SS = 5/2). A simple phenomenological model provides an excellent fit to the data. The dominant 1^{1}H-NMR frequency shift comes from the dipolar field from the 3d Fe3+^{3+} ions, and the Fe3+^{3+} - Fe3+^{3+} exchange interactions (J0J_{0}) (J0J_{0} includes the d-d exchange interactions through the π\pi-electrons) have a substantial effect to the local field at the proton sites expecially at low temperatures. A good fit is obtained with J0J_{0} = - 1.7 K. The data of the spin-echo decay rate 1/T2T_{2} indicates that there is a significant change in the slow fluctuations of the local magnetic field at the 1^{1}H-sites on traversing the PM to AFI phase. This evidence supports earlier reports that the PM-AFI phase transition in λ\lambda-(BETS)2_{2}% FeCl4_{4} is driven magnetically and first order.Comment: 9 pages, 10 figures, resubmitted to Phys. Rev. B in response to comments of Editor and reviewers on March 23, 200

    Suboptimal compensation of gyroscopic coupling for inertia-wheel attitude control

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    Suboptimal compensation of gyroscopic coupling for inertia-wheel attitude control by mathematical technique

    Switching and Extension of a [c2]Daisy-Chain Dimer Polymer

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    We report the synthesis of a [c2]daisy-chain dimer via ruthenium-catalyzed ring-closing olefin metathesis. Confirmation of the interlocked nature of the structure was achieved through single-crystal X-ray diffraction analysis. The dimer could be readily switched from the bound to the unbound conformation by treatment with 3.0 equiv of KOH and subsequently reprotonated by treatment with 3.0 equiv of HPF_6. Azide functionalization of the dimer enabled incorporation in linear step-growth polymer chains using the alkyne-azide “click” reaction. Gel permeation chromatography coupled with multiangle laser light scattering analysis showed the polymers contained 22 dimers and had a radius of gyration of 14.8 nm. Acylation of the amines of the dimers sterically forced elongation of the interlocked units, and MALLS analysis of the polymer showed a 48% increase in the R_g (21.4 nm)

    Polarizabilities of Si^{2+}: a benchmark test of theory and experiment

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    We have calculated electric-dipole polarizabilities of the 3s^2 ^1S_0, 3s3p ^3P_0, and 3s3p ^1P_1 states of the Si^{2+} ion using recently developed configuration interaction + all-order method. Detailed evaluation of the uncertainties of the final results is carried out. Our value for the ground state electric-dipole polarizability 11.670(13) a.u. is in excellent agreement with the resonant excitation Stark ionization spectroscopy value 11.669(9) a.u. [Komara et al., J. Phys. B 38, 87 (2005); Mitroy, Phys. Rev. A 78, 052515 (2008)]. This work represents the most precise benchmark test to date of theory and experiment in divalent atoms. The near cancellation of the ns^2 ^1S_0 ground state and the lowest nsnp ^3P_0 polarizabilities previously observed in B+, Al+, In+, Tl+, and Pb^{2+} is also found in Si^{2+} ion.Comment: 6 page

    Microscopic Study of 1S0{}^1{S_0} Superfluidity in Dilute Neutron Matter

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    Singlet SS-wave superfluidity of dilute neutron matter is studied within the correlated BCS method, which takes into account both pairing and short-range correlations. First, the equation of state (EOS) of normal neutron matter is calculated within the Correlated Basis Function (CBF) method in lowest cluster order using the 1S0{}^1{S_0} and 3P{}^3P components of the Argonne V18V_{18} potential, assuming trial Jastrow-type correlation functions. The 1S0{}^1{S_0} superfluid gap is then calculated with the corresponding component of the Argonne V18V_{18} potential and the optimally determined correlation functions. The dependence of our results on the chosen forms for the correlation functions is studied, and the role of the PP-wave channel is investigated. Where comparison is meaningful, the values obtained for the 1S0{}^1{S_0} gap within this simplified scheme are consistent with the results of similar and more elaborate microscopic methods.Comment: 9 pages, 6 figure

    Subglacial floods beneath ice sheets.

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    Subglacial floods (jökulhlaups) are well documented as occurring beneath present day glaciers and ice caps. In addition, it is known that massive floods have occurred from ice-dammed lakes proximal to the Laurentide ice sheet during the last ice age, and it has been suggested that at least one such flood below the waning ice sheet was responsible for a dramatic cooling event some 8000 years ago. We propose that drainage of lakes from beneath ice sheets will generally occur in a time-periodic fashion, and that such floods can be of severe magnitude. Such hydraulic eruptions are likely to have caused severe climatic disturbances in the past, and may well do so in the future
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