2,948 research outputs found

    Carbon Dioxide Observational Platform System (CO-OPS), feasibility study

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    The Carbon Dioxide Observational Platform System (CO-OPS) is a near-space, geostationary, multi-user, unmanned microwave powered monitoring platform system. This systems engineering feasibility study addressed identified existing requirements such as: carbon dioxide observational data requirements, communications requirements, and eye-in-the-sky requirements of other groups like the Defense Department, the Forestry Service, and the Coast Guard. In addition, potential applications in: earth system science, space system sciences, and test and verification (satellite sensors and data management techniques) were considered. The eleven month effort is summarized. Past work and methods of gathering the required observational data were assessed and rough-order-of magnitude cost estimates have shown the CO-OPS system to be most cost effective (less than $30 million within a 10 year lifetime). It was also concluded that there are no technical, schedule, or obstacles that would prevent achieving the objectives of the total 5-year CO-OPS program

    Anisotropic properties of MgB2 by torque magnetometry

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    Anisotropic properties of superconducting MgB2 obtained by torque magnetometry are compared to theoretical predictions, concentrating on two issues. Firstly, the angular dependence of Hc2 is shown to deviate close to Tc from the dependence assumed by anisotropic Ginzburg-Landau theory. Secondly, from the evaluation of torque vs angle curves it is concluded that the anisotropy of the penetration depth gamma_lambda has to be substantially higher at low temperature than theoretical estimates, at least in fields higher than 0.2 T.Comment: 2 p.,2 Fig., submitted to Physica C (M2S-Rio proceedings); v2: 1 ref adde

    Specific heat of MgB_2 after irradiation

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    We studied the effect of disorder on the superconducting properties of polycrystalline MgB_2 by specific-heat measurements. In the pristine state, these measurements give a bulk confirmation of the presence of two superconducting gaps with 2 Delta 0 / k_B T_c = 1.3 and 3.9 with nearly equal weights. The scattering introduced by irradiation suppresses T_c and tends to average the two gaps although less than predicted by theory. We also found that by a suitable irradiation process by fast neutrons, a substantial bulk increase of dH_{c2}/dT at T_c can be obtained without sacrificing more than a few degrees in T_c. The upper critical field of the sample after irradiation exceeds 28 T at T goes to 0 K.Comment: 11 pages text, 6 figures, accepted by Journal of Physics: Condensed Matte

    Noether symmetries for two-dimensional charged particle motion

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    We find the Noether point symmetries for non-relativistic two-dimensional charged particle motion. These symmetries are composed of a quasi-invariance transformation, a time-dependent rotation and a time-dependent spatial translation. The associated electromagnetic field satisfy a system of first-order linear partial differential equations. This system is solved exactly, yielding three classes of electromagnetic fields compatible with Noether point symmetries. The corresponding Noether invariants are derived and interpreted

    Infrared properties of Mg1−x_{1-x}Alx(_x(B1−y_{1-y}Cy_{y})2_2 single crystals in the normal and superconducting state

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    The reflectivity R(ω)R (\omega) of abab-oriented Mg1−x_{1-x}Alx_x(B1−y_{1-y }Cy_y)2_2 single crystals has been measured by means of infrared microspectroscopy for 1300<ω<170001300<\omega<17000 cm−1^{-1}. An increase with doping of the scattering rates in the π\pi and σ\sigma bands is observed, being more pronounced in the C doped crystals. The σ\sigma-band plasma frequency also changes with doping due to the electron doping, while the π\pi-band one is almost unchanged. Moreover, a σ→σ\sigma\to\sigma interband excitation, predicted by theory, is observed at ωIB≃0.47\omega_{IB} \simeq 0.47 eV in the undoped sample, and shifts to lower energies with doping. By performing theoretical calculation of the doping dependence ωIB\omega_{IB}, the experimental observations can be explained with the increase with electron doping of the Fermi energy of the holes in the σ\sigma-band. On the other hand, the σ\sigma band density of states seems not to change substantially. This points towards a TcT_c reduction driven mainly by disorder, at least for the doping level studied here. The superconducting state has been also probed by infrared synchrotron radiation for 30<ω<15030<\omega<150 cm−1^{-1} in one pure and one C-doped sample. In the undoped sample (TcT_c = 38.5 K) a signature of the π\pi-gap only is observed. At yy = 0.08 (TcT_c = 31.9 K), the presence of the contribution of the σ\sigma-gap indicates dirty-limit superconductivity in both bands.Comment: 12 pages, 9 figure
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