35,206 research outputs found

    Mechanical detection of nuclear spin relaxation in a micron-size crystal

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    A room temperature nuclear magnetic resonance force microscope (MRFM), fitted in a 1^1Tesla electromagnet, is used to measure the nuclear spin relaxation of 1^1H in a micron-size (70ng) crystal of ammonium sulfate. NMR sequences, combining both pulsed and continuous wave r.f. fields, have allowed us to measure mechanically T2T_2 and T1T_1, the transverse and longitudinal spin relaxation times. Because two spin species with different T1T_1 values are measured in our 7ÎŒm7\mu{\rm m} thick crystal, magnetic resonance imaging of their spatial distribution inside the sample section are performed. To understand quantitatively the measured signal, we carefully study the influence of the spin-lattice relaxation and the non-adiabaticity of the c.w. sequence on the intensity and time dependence of the detected signal.Comment: latex drafteps.tex, 13 files, 12 pages [SPEC-S00/010], submitted to Eur. Phys. J.

    Superposition of a static perfect fluid and a radial elecric field

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    We obtain a two-parameter set of solutions, which represents a spherically symmetric space-time with a superposition of a neutral fluid and an electric field. The electromagnetic four-potential of this Einstein-Maxwell space-time is taken in the form A=(q/n)(r^n)dt, when n=/0 and A=q*ln(r)dt, when n=0 (where q and n are arbitrary constants)Comment: 12 pages, RevTeX, no figure

    Market Transparency, Adverse Selection, and Moral Hazard

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    We study the effects of improvements in market transparency on eBay on seller exit and continuing sellers’ behavior. An improvement in market transparency by reducing strategic bias in buyer ratings led to a significant increase in buyer valuation especially of sellers rated poorly prior to the change, but not to an increase in seller exit. When sellers had the choice between exiting—a reduction in adverse selection—and improved behavior—a reduction in moral hazard—, they preferred the latter because of lower cost. Increasing market transparency improves on market outcomes

    Reconstructing the geometric structure of a Riemannian symmetric space from its Satake diagram

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    The local geometry of a Riemannian symmetric space is described completely by the Riemannian metric and the Riemannian curvature tensor of the space. In the present article I describe how to compute these tensors for any Riemannian symmetric space from the Satake diagram, in a way that is suited for the use with computer algebra systems. As an example application, the totally geodesic submanifolds of the Riemannian symmetric space SU(3)/SO(3) are classified. The submission also contains an example implementation of the algorithms and formulas of the paper as a package for Maple 10, the technical documentation for this implementation, and a worksheet carrying out the computations for the space SU(3)/SO(3) used in the proof of Proposition 6.1 of the paper.Comment: 23 pages, also contains two Maple worksheets and technical documentatio

    The Diffraction Model and its Applicability for Wakefield Calculations

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    The operation of a Free Electron Laser (FEL) in the ultraviolet or in the X-ray regime requires the acceleration of electron bunches with an rms length of 25 to 50 micro meters. The wakefields generated by these sub picosecond bunches extend into the frequency range well beyond the threshold for Cooper pair breakup (about 750 GHz) in superconducting niobium at 2 K. It is shown, that the superconducting cavities can indeed be operated with 25 micro meter bunches without suffering a breakdown of superconductivity (quench), however at the price of a reduced quality factor and an increased heat transfer to the superfluid helium bath. This was first shown by wakefield calculations based on the diffraction model. In the meantime a more conventional method of computing wake fields in the time domain by numerical methods was developed and used for the wakefield calculations. Both methods lead to comparable results: the operation of TESLA with 25 micro meter bunches is possible but leads to an additional heat load due to the higher order modes (HOMs). Therefore HOM dampers for these high frequencies are under construction. These dampers are located in the beam pipes between the 9-cell cavities. So it is of interest, if there are trapped modes in the cavity due to closed photon orbits. In this paper we investigate the existence of trapped modes and the distribution of heat load over the surface of the TESLA cavity by numerical photon tracking.Comment: Linac2000 conference paper ID No. MOE0

    Solar Energetic Particle Events in the 23rd Solar Cycle: Interplanetary Magnetic Field Configuration and Statistical Relationship with Flares and CMEs

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    We study the influence of the large-scale interplanetary magnetic field configuration on the solar energetic particles (SEPs) as detected at different satellites near Earth and on the correlation of their peak intensities with the parent solar activity. We selected SEP events associated with X and M-class flares at western longitudes, in order to ensure good magnetic connection to Earth. These events were classified into two categories according to the global interplanetary magnetic field (IMF) configuration present during the SEP propagation to 1AU: standard solar wind or interplanetary coronal mass ejections (ICMEs). Our analysis shows that around 20% of all particle events are detected when the spacecraft is immersed in an ICME. The correlation of the peak particle intensity with the projected speed of the SEP-associated coronal mass ejection is similar in the two IMF categories of proton and electron events, ≈0.6\approx 0.6. The SEP events within ICMEs show stronger correlation between the peak proton intensity and the soft X-ray flux of the associated solar flare, with correlation coefficient r= r=\,0.67±\pm0.13, compared to the SEP events propagating in the standard solar wind, r= r=\,0.36±\pm0.13. The difference is more pronounced for near-relativistic electrons. The main reason for the different correlation behavior seems to be the larger spread of the flare longitude in the SEP sample detected in the solar wind as compared to SEP events within ICMEs. We discuss to which extent observational bias, different physical processes (particle injection, transport, etc.), and the IMF configuration can influence the relationship between SEPs and coronal activity.Comment: http://adsabs.harvard.edu.ezproxy.obspm.fr/abs/2013SoPh..282..579