2,430 research outputs found

    Silsesquioxanes as precursors to ceramic composites

    Get PDF
    Silsesquioxanes having the general structure RSiO sub 1.5, where R = methyl, propyl, or phenyl, melt flow at 70 to 100 C. Above 100 C, free -OH groups condense. At 225 C further crosslinking occurs, and the materials form thermosets. Pyrolysis, with accompanying loss of volatiles, takes place at nominally 525 C. At higher temperatures, the R group serves as an internal carbon soruce for carbo-thermal reduction to SiC accompanied by the evolution of CO. By blending silsesquioxanes with varying R groups, both the melt rheology and composition of the fired ceramic can be controlled. Fibers can be spun from the melt which are stable in argon in 1400 C. The silsesquioxanes also were used as matrix precursors for Nicalon and alpha-SiC platelet reinforced composites

    Alternative Solution of the Path Integral for the Radial Coulomb Problem

    Get PDF
    In this Letter I present an alternative solution of the path integral for the radial Coulomb problem which is based on a two-dimensional singular version of the Levi-Civita transformation.Comment: 7 pages, Late

    String Branchings on Complex Tori and Algebraic Representations of Generalized Krichever-Novikov Algebras

    Full text link
    The propagation differential for bosonic strings on a complex torus with three symmetric punctures is investigated. We study deformation aspects between two point and three point differentials as well as the behaviour of the corresponding Krichever-Novikov algebras. The structure constants are calculated and from this we derive a central extension of the Krichever-Novikov algebras by means of b-c systems. The defining cocycle for this central extension deforms to the well known Virasoro cocycle for certain kinds of degenerations of the torus. AMS subject classification (1991): 17B66, 17B90, 14H52, 30F30, 81T40Comment: 11 pages, amste

    Back-action ground state cooling of a micromechanical membrane via intensity-dependent interaction

    Full text link
    We propose a theoretical scheme to show the possibility of achieving the quantum ground state cooling of a vibrating micromechanical membrane inside a high finesse optical cavity by back-action cooling approach. The scheme is based on an intensity-dependent coupling of the membrane to the intracavity radiation pressure field. We find the exact expression for the position and momentum variances of the membrane by solving the linearized quantum Langevin equations in the steady-state, conditioned by the Routh-Hurwitz criterion. We show that by varying the Lamb-Dicke parameter and the membrane's reflectivity one can effectively control the mean number of excitations of vibration of the membrane and also cool down the system to micro-Kelvin temperatures

    Why Might Stratospheric Sudden Warmings Occur with Similar Frequency in El Niño and La Niña Winters?

    Get PDF
    The effect of El Niño-Southern Oscillation (ENSO) on the frequency and character of Northern Hemisphere major mid-winter stratospheric sudden warmings (SSWs) is evaluated using a meteorological reanalysis data set and comprehensive chemistry-climate models. There is an apparent inconsistency between the impact of opposite phases of ENSO on the seasonal mean vortex and on SSWs: El Niño leads to an anomalously warm, and La Niña leads to an anomalously cool, seasonal mean polar stratospheric state, but both phases of ENSO lead to an increased SSW frequency. A resolution to this apparent paradox is here proposed: the region in the North Pacific most strongly associated with precursors of SSWs is not strongly influenced by El Niño and La Niña teleconnections. In the observational record, both La Niña and El Niño lead to similar anomalies in the region associated with precursors of SSWs and, consistent with this, there is a similar SSW frequency in La Niña and El Niño winters. A similar correspondence between the penetration of ENSO teleconnections into the SSW precursor region and SSW frequency is found in the comprehensive chemistry-climate models. The inability of some of the models to capture the observed relationship between La Niña and SSW frequency appears related to whether the modeled ENSO teleconnections result in extreme anomalies in the region most closely associated with SSWs. Finally, it is confirmed that the seasonal mean polar vortex response to ENSO is only weakly related to the relative frequency of SSWs during El Niño and La Niña

    Deconfined SU(2) vector fields at zero temperature

    Full text link
    Markov chain Monte Carlo simulations of pure SU(2)xU(1) lattice gauge theory show a (zero temperature) deconfining phase transition in the SU(2) gluon sector when a term is added to the SU(2) and U(1) Wilson actions, which requires joint U(2) gauge transformations of the SU(2) and U(1) vector fields. Investigations of this deconfined phase are of interest as it could provide an alternative to the Higgs mechanism.Comment: 5 pages, 8 figures, revised for inaccuracie

    Effect of bevacizumab in older patients with metastatic colorectal cancer: pooled analysis of four randomized studies

    Get PDF
    Background: Bevacizumab is frequently combined with 5-fluorouracil-based chemotherapy for patients with metastatic colorectal cancer (mCRC). The relative benefit of bevacizumab in older patients has not been widely studied and is of interest. Patients and methods: This retrospective analysis used data from three first-line randomized controlled studies and one second-line randomized controlled study of bevacizumab plus chemotherapy in medically fit (Eastern Cooperative Oncology Group performance status 0 or 1) patients with mCRC. Overall survival (OS) and on-treatment progression-free survival (PFS) were assessed in patients aged greater than 65, greater than or equal to 65, and greater than or equal to 70 years. Results were compared using unstratified hazard ratios (HRs). Grade 3-5 adverse events were also assessed. Results: Bevacizumab statistically significantly improved PFS [HR 0.58; 95% confidence interval (CI) 0.49-0.68] and OS (HR 0.85; 95% CI 0.74-0.97) in patients aged greater than or equal to 65 years; patients aged greater than or equal to 70 years had similar improvements. Benefits were consistent across the studies, irrespective of setting, bevacizumab dose, or chemotherapy regimen. Increases in thromboembolic events were observed in patients aged greater than or equal to 65 and greater than or equal to 70 years in the bevacizumab group compared with the control group, mainly as a result of increases in arterial thromboembolic events. No other substantial age-related increases in grade 3-5 adverse events were observed. Conclusions: In medically fit older patients, bevacizumab provides similar PFS and OS benefits as in younger patients

    Renal homotransplantation in pediatric patients.

    Get PDF
    Fifty-seven patients were treated with renal homotransplantation from 1½ to 7⅔ years ago; 23 patients were 12 years or younger and the other 34 patients were 13 to 18. Family members (usually parents) were the primary donors in 45 cases. Unrelated volunteers or cadavers donated the other 12 homografts. Immunosuppression was with azathioprine and prednisone, and in some cases also with ALG. Forty-two of the 57 recipients survived for at least 1 year. Additional deaths occurred at 17½ and 19 months leaving 40 recipients (70.2%) alive. Six survivors had successful retransplantation following late failure of their original homografts. Control of rejection was not particularly different than in adult cases. “Homograft glomerulonephritis” was found in chronically tolerated transplants, but no more frequently than in older patients. Many postoperative problems in the pediatric age group were the consequence of retardation of growth caused either by pre-existing uremia or by the need for high dose postoperative steroid therapy, orthopedic accidents such as femoral and vertebral fractures, and psychiatric complications which led to two suicides. In spite of these difficulties, the meaningful rehabilitation that was obtained in the chronic survivors makes us regard pediatric patients as favorable candidates for therapy with renal transplantation

    Connections Between the Spring Breakup of the Southern Hemisphere Polar Vortex, Stationary Waves, and Air-sea Roughness

    Get PDF
    A robust connection between the drag on surface-layer winds and the stratospheric circulation is demonstrated in NASA's Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM). Specifically, an updated parameterization of roughness at the air-sea interface, in which surface roughness is increased for moderate wind speeds (4ms to 20ms), leads to a decrease in model biases in Southern Hemispheric ozone, polar cap temperature, stationary wave heat flux, and springtime vortex breakup. A dynamical mechanism is proposed whereby increased surface roughness leads to improved stationary waves. Increased surface roughness leads to anomalous eddy momentum flux convergence primarily in the Indian Ocean sector (where eddies are strongest climatologically) in September and October. The localization of the eddy momentum flux convergence anomaly in the Indian Ocean sector leads to a zonally asymmetric reduction in zonal wind and, by geostrophy, to a wavenumber-1 stationary wave pattern. This tropospheric stationary wave pattern leads to enhanced upwards wave activity entering the stratosphere. The net effect is an improved Southern Hemisphere vortex: the vortex breaks up earlier in spring (i.e., the spring late-breakup bias is partially ameliorated) yet is no weaker in mid-winter. More than half of the stratospheric biases appear to be related to the surface wind speed biases. As many other chemistry climate models use a similar scheme for their surface layer momentum exchange and have similar biases in the stratosphere, we expect that results from GEOSCCM may be relevant for other climate models

    Steady-state entanglement and normal-mode splitting in an atom-assisted optomechanical system with intensity-dependent coupling

    Full text link
    In this paper, we study theoretically the bipartite and tripartite continuous variable entanglement as well as the normal-mode splitting in a single-atom cavity optomechanical system with intensity-dependent coupling. The system under consideration is formed by a Fabry-Perot cavity with a thin vibrating end mirror and a two-level atom in the Gaussian standing-wave of the cavity mode. We first derive the general form of Hamiltonian describing the tripartite intensity-dependent atom-field-mirror coupling due to the presence of cavity mode structure. We then restrict our treatment to the first vibrational sideband of the mechanical resonator and derive a novel form of tripartite atom-field-mirror Hamiltonian. We show that when the optical cavity is intensely driven one can generate bipartite entanglement between any pair of the tripartite system, and that, due to entanglement sharing, the atom-mirror entanglement is efficiently generated at the expense of optical-mechanical and optical-atom entanglement. We also find that in such a system, when the Lamb-Dicke parameter is large enough one can simultaneously observe the normal mode splitting into three modes
    • …
    corecore