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211.
We study the domain of existence of a solution to a Riemann problem for the pressure gradient equation in two space dimensions. The Riemann problem is the expansion of a quadrant of gas of constant state into the other three vacuum quadrants. The global existence of a smooth solution was established in Dai and Zhang [Z. Dai, T. Zhang, Existence of a global smooth solution for a degenerate Goursat problem of gas dynamics, Arch. Ration. Mech. Anal. 155 (2000) 277-298] up to the free boundary of vacuum. We prove that the vacuum boundary is the coordinate axes. 相似文献
212.
An infinite row of periodically spaced, identical rigid circularcylinders is excited by an acoustic line source which is parallelto the generators of the cylinders. A method for calculatingthe scattered field accurately and efficiently is presented.When the cylinders are sufficiently close together, Rayleigh–Blochsurface waves that propagate energy to infinity along the arrayare excited. An expression is derived which enables the amplitudesof these surface waves to be computed without requiring thesolution to the full scattering problem. 相似文献
213.
Quantitative H–Al distances in acid sites of two zeolites with MFI and IFR framework topology were obtained by numerical simulation of 1H{27Al} rotational echo adiabatic passage double resonance (REAPDOR) experiments. A 27Al offset-dependent data set yields for each resolved 1H NMR line a corresponding nuclear electric quadrupole coupling constant of the neighboring 27Al site. This information is used for analyzing a second data set for on-resonance irradiation, where the dipolar evolution time (number of rotor cycles) was varied, to yield the 1H–27Al dipolar coupling constant. Numerical simulations indicate that the REAPDOR method does not depend significantly on the polar angles, defining the orientation of the electric field gradient tensor of 27Al with respect to the Al–H dipolar vector. In contrast, the transfer of populations in double resonance sequence is sensitive to these angles, and it can be thus used to measure them. 相似文献
214.
用分子动力学方法模拟了空位在金刚石近(001)表面的扩散过程,研究了温度对空位扩散的影响.结果表明,当温度为1000K左右时,位于近表面第二层上的空位开始向表面运动;当温度在1400—2000K时,空位完全扩散到表面.这与实验结果和其他计算结果符合得很好.同时发现,温度为1400—1800K时,空位的扩散经历了两次迁移运动,其分别对应了均方位移图中的两个极大值.在不施加任何约束的条件下得到了空位的动态扩散路径,空位在金刚石近(001)表面的扩散势垒约为042eV.并探讨了一定温度下空位数目增多及其不同排列
关键词:
金刚石
空位
扩散
分子动力学 相似文献
215.
Potential for micromachined actuation of ultra-wide continuously tunable optoelectronic devices 总被引:1,自引:0,他引:1
H. Hillmer J. Daleiden C. Prott F. Römer S. Irmer V. Rangelov A. Tarraf S. Schüler M. Strassner 《Applied physics. B, Lasers and optics》2002,75(1):3-13
Tailored scaling represents a principle of success that, both in nature and in technology, allows the effectiveness of physical
effects to be enhanced. Mutation and selection in nature are imitated in technology, e.g. by model calculation and design.
Proper scaling of dimensions in natural photonic crystals and our fabricated artificial 1D photonic crystals (DBRs, distributed
Bragg reflectors) enable efficient diffractive interaction in a specific spectral range. For our optical microsystems we illustrate
that tailored miniaturization may also increase the mechanical stability and the effectiveness of spectral tuning by thermal
and electrostatic actuation, since the relative significance of the fundamental physical forces involved considerably changes
with scaling. These basic physical principles are rigorously applied in micromachined 1.55-μm vertical-resonator-based devices.
We modeled, implemented and characterized 1.55-μm micromachined optical filters and vertical-cavity surface-emitting laser
devices capable of wide, monotonic and kink-free tuning by a single control parameter. Tuning is achieved by mechanical actuation
of one or several air-gaps that are part of the vertical resonator including two ultra-highly reflective DBR mirrors of strong
refractive index contrast: (i) Δn=2.17 for InP/air-gap DBRs (3.5 periods) using GaInAs sacrificial layers and (ii) Δn=0.5
for Si3N4/SiO2 DBRs (12 periods) with a polymer sacrificial layer to implement the air-cavity. In semiconductor multiple air-gap filters,
a continuous tuning of >8% of the absolute wavelength is obtained. Varying the reverse voltage (U=0–5 V) between the membranes
(electrostatic actuation), a tuning range of >110 nm was obtained for a large number of devices. The correlation of the wavelength
and the applied voltage is accurately reproducible without any hysteresis. In two filters, tuning of 127 and 130 nm was observed
for about ΔU=7 V. The extremely wide tuning range and the very small voltage required are record values to the best of our
knowledge. For thermally actuated dielectric filters based on polymer sacrificial layers, Δλ/ΔU=-7 nm/V is found.
Received: 10 May 2002 / Published online: 8 August 2002 相似文献
216.
Electronic properties of model quantum-dot structures in zero and finite magnetic fields 总被引:1,自引:0,他引:1
H. Saarikoski E. Räsänen S. Siljamäki A. Harju M.J. Puska R.M. Nieminen 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(2):241-252
We have computed electronic structures and total energies of circularly confined two-dimensional quantum dots and their lateral
dimers in zero and finite uniform external magnetic fields using different theoretical schemes: the spin-density-functional
theory (SDFT), the current-and-spin-density-functional theory (CSDFT), and the variational quantum Monte Carlo (VMC) method.
The SDFT and CSDFT calculations employ a recently-developed, symmetry-unrestricted real-space algorithm allowing solutions
which break the spin symmetry. Results obtained for a six-electron dot in the weak confinement limit and in zero magnetic
field as well as in a moderate confinement and in finite magnetic fields enable us to draw conclusions about the reliability
of the more approximative SDFT and CSDFT schemes in comparison with the VMC method. The same is true for results obtained
for the two-electron quantum dot dimer as a function of inter-dot distance. The structure and role of the symmetry-breaking
solutions appearing in the SDFT and CSDFT calculations for the above systems are discussed.
Received 16 October 2001 and Received in final form 17 January 2002 相似文献
217.
A convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and its pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that particle deposition can be realized under a lower gas pressure with a CB nozzle. 相似文献
218.
Regina S. Burachik Rafail N. Gasimov Nergiz A. Ismayilova C. Yalçin. Kaya 《Journal of Global Optimization》2006,34(1):55-78
We study convergence properties of a modified subgradient algorithm, applied to the dual problem defined by the sharp augmented
Lagrangian. The primal problem we consider is nonconvex and nondifferentiable, with equality constraints. We obtain primal
and dual convergence results, as well as a condition for existence of a dual solution. Using a practical selection of the
step-size parameters, we demonstrate the algorithm and its advantages on test problems, including an integer programming and
an optimal control problem.
*Partially Supported by 2003 UniSA ITEE Small Research Grant Ero2.
Supported by CAPES, Brazil, Grant No. 0664-02/2, during her visit to the School of Mathematics and Statistics, UniSA. 相似文献
219.
Matthias Kriesell 《Graphs and Combinatorics》2002,18(1):133-146
Let k≤n be positive integers. A finite, simple, undirected graph is called k-critically n-connected, or, briefly, an (n,k)-graph, if it is noncomplete and n-connected and the removal of any set X of at most k vertices results in a graph which is not (n−|X|+1)-connected. We present some new results on the number of vertices of an (n,k)-graph, depending on new estimations of the transversal number of a uniform hypergraph with a large independent edge set.
Received: April 14, 2000 Final version received: May 8, 2001 相似文献
220.
In this paper, nonconvex multiobjective optimization problems are studied. New characterizations of a properly efficient solution in the sense of Geoffrion's are established in terms of the stability of one scalar optimization problem and the existence of an exact penalty function of a scalar constrained program, respectively. One of the characterizations is applied to derive necessary conditions for a properly efficient control-parameter pair of a nonconvex multiobjective discrete optimal control problem with linear constraints. 相似文献