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991.
P. Kratzer E. Penev M. Scheffler 《Applied Physics A: Materials Science & Processing》2002,75(1):79-88
We demonstrate how first-principles calculations using density-functional theory (DFT) can be applied to gain insight into
the molecular processes that rule the physics of materials processing. Specifically, we study the molecular beam epitaxy (MBE)
of arsenic compound semiconductors. For homoepitaxy of GaAs on GaAs (001), a growth model is presented that builds on results
of DFT calculations for molecular processes on the β2-reconstructed GaAs (001) surface, including adsorption, desorption,
surface diffusion, and nucleation. Kinetic Monte Carlo simulations on the basis of the calculated energetics enable us to
model MBE growth of GaAs from beams of Ga and As2 in atomistic detail. The simulations show that island nucleation is controlled by the reaction of As2 molecules with Ga adatoms on the surface. The analysis reveals that the scaling laws of standard nucleation theory for the
island density as a function of growth temperature are not applicable to GaAs epitaxy. We also discuss heteroepitaxy of InAs
on GaAs (001), and report first-principles DFT calculations for In diffusion on the strained GaAs substrate. In particular,
we address the effect of heteroepitaxial strain on the growth kinetics of coherently strained InAs islands. The strain field
around an island is found to cause a slowing down of material transport from the substrate towards the island, and thus helps
to achieve more homogeneous island sizes.
Received: 2 May 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002 相似文献
992.
R. Gerbaldo G. Ghigo G. Giunchi L. Gozzelino F. Laviano E. Mezzetti 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(3):297-300
We present results from an extended magneto-optical (MO) analysis of two samples cut from high-density pellets of MgB2. The first sample was studied in order to show that no matter how large the sample is and despite the bulk granularity, the
material enters into a critical state in a crystal-like fashion. The second sample was chosen for the quantitative analysis.
A numerical approach based on an inverted 2D Biot-Savart model was used to calculate the current paths across the homogeneous
polycrystalline bulk, as well as in the vicinity and across some morphological defects. Local current densities in the homogeneous
part were estimated as a function of the applied magnetic field at different temperatures, in three regimes: below full penetration,
at full penetration and above full penetration, respectively. A hypothesis of interpretation of the apparent absence of magnetic
granularity inside the polycrystalline microstructure is presented. It is related to a critical state likely reached by a
network of strongly coupled Josephson junctions.
Received 31 May 2001 and Received in final form 5 December 2001 相似文献
993.
994.
Th. Gerrits H. A. M. van den Berg J. Hohlfeld O. Gielkens L. Br Th. Rasing 《Journal of magnetism and magnetic materials》2002,240(1-3):283-286
We have measured the response of a 20×10 μm, 8 nm thin NiFe (80:20) permalloy film due to excitation by short in-plane magnetic field pulses. We will show that using a two-pulse-technique a complete control of the precessional motion of the magnetisation can be achieved on picosecond timescales. Furthermore, we will present numerical calculations which show that a complete suppression of magnetisation ringing after switching can only be realised by a cascade of short field pulses. 相似文献
995.
K. Weibert J. Main G. Wunner 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,19(3):379-388
In the periodic orbit quantization of physical systems, usually only the leading-order ? contribution to the density of states is considered. Therefore, by construction, the eigenvalues following from semiclassical
trace formulae generally agree with the exact quantum ones only to lowest order of ?. In different theoretical work the trace formulae have been extended to higher orders of ?. The problem remains, however, how to actually calculate eigenvalues from the extended trace formulae since, even with ? corrections included, the periodic orbit sums still do not converge in the physical domain. For lowest-order semiclassical trace formulae the convergence problem can be elegantly, and universally, circumvented by application of the
technique of harmonic inversion. In this paper we show how, for general scaling chaotic systems, also higher-order
? corrections to the Gutzwiller formula can be included in the harmonic inversion scheme, and demonstrate that corrected semiclassical
eigenvalues can be calculated despite the convergence problem. The method is applied to the open three-disk scattering system,
as a prototype of a chaotic system.
Received 10 September 2001 and Received in final form 3 January 2002 相似文献
996.
The standard model on non-commutative space-time 总被引:1,自引:0,他引:1
X. Calmet B. Jurčo P. Schupp J. Wess M. Wohlgenannt 《The European Physical Journal C - Particles and Fields》2002,23(2):363-376
We consider the standard model on a non-commutative space and expand the action in the non-commutativity parameter . No new particles are introduced; the structure group is . We derive the leading order action. At zeroth order the action coincides with the ordinary standard model. At leading order
in we find new vertices which are absent in the standard model on commutative space-time. The most striking features are couplings
between quarks, gluons and electroweak bosons and many new vertices in the charged and neutral currents. We find that parity
is violated in non-commutative QCD. The Higgs mechanism can be applied. QED is not deformed in the minimal version of the
NCSM to the order considered.
Received: 29 November 2001 / Published online: 25 January 2002 相似文献
997.
A self-consistent model of the development of dendrites and partial discharges in a dielectric solid under a variable voltage
is suggested. Dendrites originate at sites with the enhanced local field strength and also where the dielectric is broken
down under the action of partial discharges. The numerical simulation is used to quantitatively describe the space-time dynamics
of the dendrites and partial discharges in epoxy resin for the tip-plane electrode configuration. The simulated data are compared
with electrical measurements of partial discharges and with optical images of dendrite growth under the same conditions. 相似文献
998.
The most promising next generation Image Velocimtry (IV) is the high-speed Dynamic PIV. It requires the development of innovative
high-speed video camera sensors. We started by specifying the required performance of these new sensors, for measurements
in air and water flows. These criteria founded on the most recent developments in PIV algorithms and incorporate results from
a large questionnaire survey of users of high-speed video cameras in Japan. The results suggest that the followings are required:
(1) frame rate of 1,000,000 fps, (2) pixel count of 1,000,000 pixels, (3) frame storage capacity of 100–200 frames for tracing
a single event and 10,000 frames for turbulent measurements, (4) gray levels of 4–8 bits for PTV; 12 bits for observation.
Finally, we reviewed the state of the art of high-speed video-image sensors. Currently the standard parallel-readout sensors
can operate at 1 Kfps with a pixel count of approximately 1 Kpixels. The In-situ Storage Image Sensor (ISIS) developed by
the authors has recently achieved frame rates of 1 Mfps. 相似文献
999.
M. W. Brenner K. A. Gridnev S. E. Belov W. Ershov E. Indola 《Physics of Atomic Nuclei》2002,65(4):612-615
The interaction of α particles above 5 MeV with a 2s-1d target is dominated by resonances. It cannot be described only in terms of a mean-field one-body potential. An analysis of the elastic α-particle scattering by 28Si encourages the comprehension of the resonance states to be mainly fragments of a mixed-parity band. In the present article, the angular distributions of particles scattered by 32S are analyzed in terms of such bands. The analysis of new data from an experiment made at Florida State University reveals the existence of states that do not belong to the above bands. This follows from a coupled-channel analysis of the elastic and inelastic (2+) cross sections. An α-particle structure at the nuclear surface is suggested. 相似文献
1000.
Switching kinetics of uniaxial ferroelastic ferroelectrics (FFs) in external electric and stress fields is studied using classical theory of nucleation and growth. The stage in which the polarization and deformation reversal involves the main body of the FF and the final stage (Ostwald ripening) of the FF switching are studied with allowance for the change in the repolarization and redeformation during the phase transition. The time dependences of the repolarization and redeformation are found, and equations are derived from which the polarization current and the deformation flux, as well as their time dependence, can be calculated. The calculated main characteristics of the FF switching are compared with the experimental data for switching of Rochelle salt single crystals. 相似文献