47,592 research outputs found

    Temporal and Spectral Correlations of Cyg X-1

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    Temporal and spectral properties of X-ray rapid variability of Cyg X-1 are studied by an approach of correlation analysis in the time domain on different time scales. The correlation coefficients between the total intensity in 2-60 keV and the hardness ratio of 13-60 keV to 2-6 keV band on the time scale of about 1 ms are always negative in all states. For soft states, the correlation coefficients are positive on all the time scales from about 0.01 s to 100 s, which is significantly different with that for transition and low states. Temporal structures in high energy band are narrower than that in low energy band in quite a few cases. The delay of high energy photons relative to low energy ones in the X-ray variations has also been revealed by the correlation analysis. The implication of observed temporal and spectral characteristics to the production region and mechanism of Cyg X-1 X-ray variations is discussed.Comment: 17 pages, 6 figures included, to appear in Ap

    Strain-stress study of AlxGa1-xN/AlN heterostructures on c-plane sapphire and related optical properties

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    This work presents a systematic study of stress and strain of AlxGa1-xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1-xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1-xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1-xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1-xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1-xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain is also discussed on the optical phonon energies of the epitaxial layers. The techniques discussed here can be used to study other similar materials.Comment: 14 pages, 5 figures, 2 table

    Surface Adsorbate Fluctuations and Noise in Nanoelectromechanical Systems

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    Physisorption on solid surfaces is important in both fundamental studies and technology. Adsorbates can also be critical for the performance of miniature electromechanical resonators and sensors. Advances in resonant nanoelectromechanical systems (NEMS), particularly mass sensitivity attaining the single-molecule level, make it possible to probe surface physics in a new regime, where a small number of adatoms cause a detectable frequency shift in a high quality factor (Q) NEMS resonator, and adsorbate fluctuations result in resonance frequency noise. Here we report measurements and analysis of the kinetics and fluctuations of physisorbed xenon (Xe) atoms on a high-Q NEMS resonator vibrating at 190.5 MHz. The measured adsorption spectrum and frequency noise, combined with analytic modeling of surface diffusion and adsorption−desorption processes, suggest that diffusion dominates the observed excess noise. This study also reveals new power laws of frequency noise induced by diffusion, which could be important in other low-dimensional nanoscale systems

    Morphological evolution of a 3D CME cloud reconstructed from three viewpoints

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    The propagation properties of coronal mass ejections (CMEs) are crucial to predict its geomagnetic effect. A newly developed three dimensional (3D) mask fitting reconstruction method using coronagraph images from three viewpoints has been described and applied to the CME ejected on August 7, 2010. The CME's 3D localisation, real shape and morphological evolution are presented. Due to its interaction with the ambient solar wind, the morphology of this CME changed significantly in the early phase of evolution. Two hours after its initiation, it was expanding almost self-similarly. CME's 3D localisation is quite helpful to link remote sensing observations to in situ measurements. The investigated CME was propagating to Venus with its flank just touching STEREO B. Its corresponding ICME in the interplanetary space shows a possible signature of a magnetic cloud with a preceding shock in VEX observations, while from STEREO B only a shock is observed. We have calculated three principle axes for the reconstructed 3D CME cloud. The orientation of the major axis is in general consistent with the orientation of a filament (polarity inversion line) observed by SDO/AIA and SDO/HMI. The flux rope axis derived by the MVA analysis from VEX indicates a radial-directed axis orientation. It might be that locally only the leg of the flux rope passed through VEX. The height and speed profiles from the Sun to Venus are obtained. We find that the CME speed possibly had been adjusted to the speed of the ambient solar wind flow after leaving COR2 field of view and before arriving Venus. A southward deflection of the CME from the source region is found from the trajectory of the CME geometric center. We attribute it to the influence of the coronal hole where the fast solar wind emanated from.Comment: ApJ, accepte

    On the tau-functions of the Degasperis-Procesi equation

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    The DP equation is investigated from the point of view of determinant-pfaffian identities. The reciprocal link between the Degasperis-Procesi (DP) equation and the pseudo 3-reduction of the CC_{\infty} two-dimensional Toda system is used to construct the N-soliton solution of the DP equation. The N-soliton solution of the DP equation is presented in the form of pfaffian through a hodograph (reciprocal) transformation. The bilinear equations, the identities between determinants and pfaffians, and the τ\tau-functions of the DP equation are obtained from the pseudo 3-reduction of the CC_{\infty} two-dimensional Toda system.Comment: 27 pages, 4 figures, Journal of Physics A: Mathematical and Theoretical, to be publishe

    Optical and transport properties in doped two-leg ladder antiferromagnet

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    Within the t-J model, the optical and transport properties of the doped two-leg ladder antiferromagnet are studied based on the fermion-spin theory. It is shown that the optical and transport properties of the doped two-leg ladder antiferromagnet are mainly governed by the holon scattering. The low energy peak in the optical conductivity is located at a finite energy, while the resistivity exhibits a crossover from the high temperature metallic-like behavior to the low temperature insulating-like behavior, which are consistent with the experiments.Comment: 13 pages, 5 figures, accepted for publication in Phys. Rev. B65 (2002) (April 15 issue

    Parametric study of cavity length and mirror reflectivity in ultralow threshold quantum well InGaAs/AlGaAs lasers

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    Record low CW threshold currents of 16 μA at-room temperature and 21 μA at cryogenic temperature have been demonstrated in buried heterostructure strained layer, single quantum well InGaAs/AlGaAs lasers with a short cavity length and high reflectivity coatings
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