排序方式: 共有22条查询结果,搜索用时 15 毫秒
1.
We describe the dielectric response of the semiconductor band edge in a dynamic density matrix model. Our treatment is based on a set of 3-relevant constitutive equations involving two-, four- and six-point density matrices. We demonstrate that under certain conditions all contributions to the third order susceptibility can be expressed in terms of excitonic and biexcitonic transitions. As a first application of these 3-relevant equations we investigate the influence of the biexciton on the optical Stark effect in CuCl. We calculate shifts and lineshapes. Our results turn out to be in excellent agreement with experiments as well as with other theoretical predictions. 相似文献
2.
We use tools of the equilibrium statistical mechanics of disordered systems to study analytically the statistical properties of an ecosystem composed of N species interacting via random mutual interactions, as well as via deterministic self-interactions of order p>/=2. We show that the main effect of increasing the order of the interactions among the species is to make the system less competitive, in the sense that the fraction of extinct species is greatly reduced. In addition, we find that for p>2 there is a threshold value which gives a lower bound to the concentration of the surviving species, preventing then the existence of rare species and, consequently, increasing the robustness of the ecosystem to external perturbations. 相似文献
3.
Yang Xu Christian Axt Minhang Song Reinhold Kneer Shuiqing Li 《Proceedings of the Combustion Institute》2021,38(3):4179-4188
This paper intensively investigated the ignition of turbulent coal flames in a novel fully-mixed tubular swirl burner. The Nikon D300s digital camera was used to capture the statistical ignition behavior of dispersed coal particle streams in different ambiences. Meanwhile, the combustion dynamics of individual coal particles were also recorded by means of high-speed photography. Two low-rank coal samples, Hulunbel lignite and Zhundong coal, were tested in this study. The ignition delay times of coal particles in the swirl burner were compared with those in a flat-flame burner. In contrast to previous work on a laminar flat-flame burner, the current experimental results show that the turbulent ambience significantly enhances the ignition of all coal samples, which is exceptionally pronounced under high temperature and low oxygen conditions. In addition, the sensitivity analysis suggests that both the enhanced heat and mass transfer contribute to the early ignition in turbulence. The effect of elevated mass transfer coefficient turns prominent in low oxygen fraction ambience, wherein the volatile barrier effect is suppressed by the enhanced mixing process. The combined effect of turbulence favors the shifting of ignition modes to the heterogeneous-dominant region. Last but not least, the ceased volatile flame that visualized in turbulent low oxygen ambience further confirms the important role of heterogeneous ignition. 相似文献
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K. Victor V. M. Axt G. Bartels A. Stahl K. Bott P. Thomas 《Zeitschrift für Physik B Condensed Matter》1995,98(2):197-205
We present the results of inelastic neutron scattering experiments for Cr2O3 carried out using the single-crystal time-of-flight spectrometer PRISMA at ISIS (U.K.) as well as the three-axis spectrometer TAS-1 at JAERI(J). The collected data are analysed on the basis of phenomenological shell models showing convincing agreement between calculation and experiment both for the frequencies and intensities. All together the dispersion relations of 12 out of 20 phonon branches along the three-fold axis are determined. By a comparison with the Cr2O3 iso-type sapphire (Al2O3) it is found that the dynamical response of the oxygen ion is only little affected by the ionic substitution. The chromiumd electrons find their expression mainly in a strengthening of the metal-oxygen bonding. The problem of ionic polarizabilities is addressed in detail both for the oxygen and chromium ions. 相似文献
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It is shown that lower bounds for the effective memory time induced by two-pair correlations can be estimated by monitoring changes of the shape of excitonic four-wave-mixing spectra. Experimentally we demonstrate a memory time of at least 540 fs for a ZnSe single quantum well. Microscopic calculations reveal that this lower bound is not sharp. Interactions retarded by more than 800 fs are shown to influence the dynamics, reflecting the presence of a long time tail in the memory kernel. 相似文献
8.
Allison M Fox Mike Anderson Corinne Reid Tim Smith Dorothy VM Bishop 《BMC neuroscience》2010,11(1):1-15
Background
We examined development of auditory temporal integration and inhibition by assessing electrophysiological responses to tone pairs separated by interstimulus intervals (ISIs) of 25, 50, 100, 200, 400, and 800 ms in 28 children aged 7 to 9 years, and 15 adults.Results
In adults a distinct neural response was elicited to tones presented at ISIs of 25 ms or longer, whereas in children this was only seen in response to tones presented at ISIs above 100 ms. In adults, late N1 amplitude was larger for the second tone of the tone pair when separated by ISIs as short as 100 ms, consistent with the perceptual integration of successive stimuli within the temporal window of integration. In contrast, children showed enhanced negativity only when tone pairs were separated by ISIs of 200 ms. In children, the amplitude of the P1 component was attenuated at ISIs below 200 ms, consistent with a refractory process.Conclusions
These results indicate that adults integrate sequential auditory information into smaller temporal segments than children. These results suggest that there are marked maturational changes from childhood to adulthood in the perceptual processes underpinning the grouping of incoming auditory sensory information, and that electrophysiological measures provide a sensitive, non-invasive method allowing further examination of these changes. 相似文献9.
We discuss the interaction of coherent electromagnetic fields with the semiconductor band edge in a dynamic density matrix model. Due to the influence of the Coulomb-interaction then-point density matrices are coupled in an infinite hierarchy of equations of motion. We show how this hierarchy is related to an expansion of the density matrices in terms of powers of the exciting field. We make use of the above results to set up a closed set of equations of motion involving two-, four-and six-point correlation functions, from which all third order contributions to the polarization can be calculated exactly. Comparison of our treatment of the hierarchy with the widely used RPA decoupling on the two-point level, gives interesting insight into the validity of the RPA. In particular we find, that a RPA-like factorization for two of the relevant density-matrices yields a solution of their respective equations of motion to lowest order in the electric field. 相似文献
10.
A. Krügel V.M. Axt T. Kuhn P. Machnikowski A. Vagov 《Applied physics. B, Lasers and optics》2005,81(7):897-904
The damping of Rabi oscillations in quantum dots as well as the renormalization of the carrier-light coupling, due to the
interaction with longitudinal acoustic phonons are studied as a function of temperature and laser pulse parameters. Numerical
results are obtained by using a correlation expansion within the density matrix theory. The observed features like a non-monotonous
dependence of the damping on the pulse duration are characteristic for the strongly non-Markovian nature of the phonon coupling
in these systems. The results can be well interpreted on the level of a perturbation expansion in the carrier-phonon interaction.
PACS 78.67.Hc; 63.20.Kr; 03.65.Yz 相似文献