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41.
This paper reports that monitoring the composition of the c(0 0 0 1), a(11–20) and m(10–10) sapphire surfaces is essential for a proper interpretation of the surface morphologies obtained after annealing at 1253 and 1473 K in ArH2 or ArO2 atmospheres. Our experimental investigations, which have used Auger electron spectroscopy (AES) and atomic force microscopy (AFM) on the surfaces of 99.99% pure sapphire wafers, have led to the following original conclusions: (i) Calcium segregates at the c-surface of sapphire both under ArO2 and ArH2. (ii) Potassium adsorption enhances the kinetics of step-bunching on the c-surface under ArO2. (iii) The step edges on the a-surface may develop a comb-like morphology made of parallel strips along the [10–10] direction. (iv) At 1253 K, clean m-surfaces may be stable. (v) Under ArH2, alumina surface diffusion is much slower than under ArO2 for all surface orientations, the surface concentration of impurities is low, and the Al–O ratio of the AES signals at the surface is significantly larger. 相似文献
42.
We show that, in low-dimensional photonic bandgaps, wave diffraction resulting from localization in the translational-invariant directions is strongly influenced by the photonic band structure of the periodic crystal, leading to new kinds of wave localization. In particular, for a periodic layered structure we show that, close to a bandgap edge, diffraction is enhanced, with a transition from a parabolic diffraction curve-typical of isotropic media and supporting Gaussian beams-to hyperbolic or elliptic diffraction curves. In the last two cases localization in the form of stationary X-shaped or sinc-shaped waves is possible. 相似文献
43.
Galzerano G Sani E Toncelli A Della Valle G Taccheo S Tonelli M Laporta P 《Optics letters》2004,29(7):715-717
Continuous-wave laser action at approximately 2 microm is demonstrated in a Tm-Ho:KYF4 single crystal at room temperature. Crystal growth, spectroscopic measurements, and laser results are presented. An output power in excess of 250 mW is obtained with a tuning range of 99 nm, the largest ever published, to our knowledge, for Tm-Ho in any crystalline host. 相似文献
44.
Villoresi P Bonora S Pascolini M Poletto L Tondello G Vozzi C Nisoli M Sansone G Stagira S De Silvestri S 《Optics letters》2004,29(2):207-209
We present a method for the optimization of high-order harmonic generation based on wave-front correction of the driving laser beam. The technique exploits wave-front adaptive control by means of a deformable mirror, governed by an optimization procedure. 相似文献
45.
We introduce a new protocol for a lossy data compression algorithm which is based on constraint satisfaction gates. We show that the theoretical capacity of algorithms built from standard parity-check gates converges exponentially fast to the Shannon's bound when the number of variables seen by each gate increases. We then generalize this approach by introducing random gates. They have theoretical performances nearly as good as parity checks, but they offer the great advantage that the encoding can be done in linear time using the survey inspired decimation algorithm, a powerful algorithm for constraint satisfaction problems derived from statistical physics. 相似文献
46.
We consider higher-order QCD corrections to the production of colorless high-mass systems (lepton pairs, vector bosons, Higgs bosons, etc.) in hadron collisions. We propose a new formulation of the subtraction method to numerically compute arbitrary infrared-safe observables for this class of processes. To cancel the infrared divergences, we exploit the universal behavior of the associated transverse-momentum (qT) distributions in the small-qT region. The method is illustrated in general terms up to the next-to-next-to-leading order in QCD perturbation theory. As a first explicit application, we study Higgs-boson production through gluon fusion. Our calculation is implemented in a parton level Monte Carlo program that includes the decay of the Higgs boson into two photons. We present selected numerical results at the CERN Large Hadron Collider. 相似文献
47.
We present results of a Monte Carlo study over the ferromagnetism of Co-doped ZnO. The magnetic interaction has the form of the donor impurity band exchange model, where the Co magnetic moments are exchange coupled to band electrons. These are assumed to occupy large hydrogenic orbitals and originate from shallow intrinsic ZnO defects. A number of parameters of this model remain uncertain and here we investigate the dependence of the Curie temperature on the strength of the magnetic coupling. We find an unusual concave upward shape in the magnetization curves consistent with other Monte Carlo studies for dilute systems and we predict high temperature ferromagnetism for sufficiently strong coupling. 相似文献
48.
Egidio Di Gialleonardo Francesco Braghin Stefano Bruni 《Journal of sound and vibration》2012,331(19):4246-4258
This paper investigates the effect of different models for track flexibility on the simulation of railway vehicle running dynamics on tangent and curved track. To this end, a multi-body model of the rail vehicle is defined including track flexibility effects on three levels of detail: a perfectly rigid pair of rails, a sectional track model and a three-dimensional finite element track model. The influence of the track model on the calculation of the nonlinear critical speed is pointed out and it is shown that neglecting the effect of track flexibility results in an overestimation of the critical speed by more than 10%. Vehicle response to stochastic excitation from track irregularity is also investigated, analysing the effect of track flexibility models on the vertical and lateral wheel–rail contact forces. Finally, the effect of the track model on the calculation of dynamic forces produced by wheel out-of-roundness is analysed, showing that peak dynamic loads are very sensitive to the track model used in the simulation. 相似文献
49.
Stefano Catani 《Zeitschrift fur Physik C Particles and Fields》1997,75(4):665-678
I present a theoretical discussion of the uncertainties related to the QCD analysis of the proton structure function F 2(x,Q 2) at small x. The role played by the ‘unphysical’ gluon density is pointed out. It is shown how the study of more observables can reduce the theoretical uncertainty and, in particular, an alternative method of analysis, based on the introduction of physical anomalous dimensions, is suggested. 相似文献
50.
Precision and sensitivity optimization of quantitative measurements in solid state NMR 总被引:1,自引:1,他引:1
Ziarelli F Viel S Sanchez S Cross D Caldarelli S 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2007,188(2):260-266
This work presents a methodology for optimizing the precision, accuracy and sensitivity of quantitative solid state NMR measurements based on the external reference method. It is shown that the sample must be exclusively located within and completely span the coil region where the NMR response is directly proportional to the sample amount. We describe two methods to determine this "quantitative" coil volume, based on whether the probe is equipped or not with a gradient coil. In addition, to improve the sensitivity and the accuracy, an optimum rotor packing design is described, which allows the sample volume of the rotor to be matched to the quantitative coil volume. Experiments conducted on adamantane and NaCl, which are representative of a soft and hard material, respectively, show that one order of magnitude increase in experimental precision can be achieved with this methodology. Interestingly, the precision can be further improved by using the ERETIC method in order to compensate for most instrumental instabilities. 相似文献